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Software Engineering Notes Compiled from

Software Engineering A Practitioner’s Approach APRACTITIONECH FIFTH EDITION

- Roger S. Pressman (For Private Circulation Only)

Compiled By :

Gaurav Sharma


INDEX

Chap. - 1 Chap. - 2 Chap. - 3 Chap. - 4 Chap. - 5 Chap. - 6 Chap. - 7 Chap. - 8 Chap. - 9 Chap. - 10 Chap. - 13 Chap. - 17 Chap. - 18 Chap. - 19

The Product The Process Project Management Software Process & Project Metrics Software Project Planning Risk Analysis & Management Project Scheduling & Tracking Software Quality Assurance Software Configuration Management System Engineering Design Concepts & Principles Software Testing Techniques Software Testing Strategies Technical Metrics For Software

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3 7 15 16 21 25 31 38 43 47 49 55 57 64


Chapter 1 : The Product 1. What are the Software Characteristics ? Software Characteristics: To gain an understanding of software (and ultimately an understanding of software engineering), it is important to examine the characteristics of software that make it different from other things that human beings build. When hardware is built, the human creative process (analysis, design, construction, testing) is ultimately translated into a physical form. If we build a new computer, our initial sketches, formal design drawings, and breadboarded prototype evolve into a physical product (chips, circuit boards, power supplies, etc.). Software is a logical rather than a physical system element. Therefore, software has characteristics that are considerably different than those of hardware: 1.

Software is developed or engineered, it is not manufactured in the classical sense. Although some similarities exist between software development and hardware manufacture, the two activities are fundamentally different. In both activities, high quality is achieved through good design, but the manufacturing phase for hardware can introduce quality problems that are nonexistent (or easily corrected) for software. Both activities are dependent on people, but the relationship between people applied and work accomplished is entirely different (see Chapter 7). Both activities require the construction of a "product" but the approaches are different.

Software costs are concentrated in engineering. This means that software projects cannot be managed as if they were manufacturing projects. 2.

Software doesn't "wear out." Figure 1.1 depicts failure rate as a function of time for hardware. The relationship, often called the "bathtub curve," indicates that hardware exhibits relatively high failure rates early in its life (these failures are often attributable to design or manufacturing defects); defects are corrected and the failure rate drops to a steady-state level (ideally, quite low) for some period of time. As time passes, however, the failure rate rises again as hardware components suffer from the cumulative affects of dust, vibration, abuse, temperature extremes, and many other environmental maladies. Stated simply, the hardware begins to wear out. Software is not susceptible to the environmental maladies that cause hardware to wear out. In theory, therefore, the failure rate curve for software should take the form of the “idealized curve� shown in Figure 1.2. Undiscovered defects will cause high failure rates early in the life of a program. However, these are corrected (ideally, without

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introducing other errors) and the curve flattens as shown. The idealized curve is a gross over simplification of actual failure models (see Chapter 8 for more information) for software. However, the implication is clear—software doesn't wear out. But it does deteriorate! This seeming contradiction can best be explained by considering the “actual curve” shown in Figure 1.2. During its life, software will undergo change (maintenance). As changes are made, it is likely that some new defects will be introduced, causing the failure rate curve to spike as shown in Figure 1.2. Before the curve can return to the original steady-state failure rate, another change is requested, causing the curve to spike again. Slowly, the minimum failure rate level begins to rise—the software is deteriorating due to change.

Another aspect of wear illustrates the difference between hardware and software. When a hardware component wears out, it is replaced by a spare part. There are no software spare parts. Every software failure indicates an error in design or in the process through which design was translated into machine executable code. Therefore, software maintenance involves considerably more complexity than hardware maintenance. 3.

Although the industry is moving toward component-based assembly, most software continues to be custom built. Consider the manner in which the control hardware for a computer-based product is designed and built. The design engineer draws a simple schematic of the digital circuitry, does some fundamental analysis to assure that proper function will be achieved, and then goes to the shelf where catalogs of digital components exist. Each integrated circuit (called an IC or a chip) has a part number, a defined and validated function, a well-define interface, and a standard set of integration guidelines. After each component is selected, it can be ordered off the shelf. As an engineering discipline evolves, a collection of standard design components is created. Standard screws and off-the-shelf integrated circuits are only two of thousands of standard components that are used by mechanical and electrical engineers as they design new systems. The reusable components have been created so that the engineer can concentrate on the truly innovative elements of a design, that is, the parts of the design that represent something new. In the hardware world, component reuse is a natural part of the engineering process. In the software world, it is something that has only begun to be achieved on a broad scale. A software component should be designed and implemented so that it can be reused in many different programs. In the 1960s, we built scientific subroutine libraries that were reusable in a broad array of engineering and scientific applications. These subroutine libraries reused well-defined algorithms in an effective manner but had a limited domain of application. Today, we have extended our view of reuse to encompass not only algorithms but also data structure. Modern reusable components encapsulate both data and the processing applied to the data, enabling the software engineer to create new applications from reusable parts. For example, today's graphical user interfaces are built using reusable components that enable the creation of graphics

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windows, pull-down menus, and a wide variety of interaction mechanisms. The data structure and processing detail required to build the interface are contained with a library of reusable components for interface construction.

2. What are Software Applications ? Software may be applied in any situation for which a pre-specified set of procedural steps (i.e., an algorithm) has been defined (notable exceptions to this rule are expert system software and neural network software). Information content and determinacy are important factors in determining the nature of a software application. Content refers to the meaning and form of incoming and outgoing information. For example, many business applications use highly structured input data (a database) and produce formatted “reports.� Software that controls an automated machine (e.g., a numerical control) accepts discrete data items with limited structure and produces individual machine commands in rapid succession. Information determinacy refers to the predictability of the order and timing of information. An engineering analysis program accepts data that have a predefined order, executes the analysis algorithm(s) without interruption, and produces resultant data in report or graphical format. Such applications are determinate. A multi-user operating system, on the other hand, accepts inputs that have varied content and arbitrary timing, executes algorithms that can be interrupted by external conditions, and produces output that varies as a function of environment and time. Applications with these characteristics are indeterminate. It is somewhat difficult to develop meaningful generic categories for software applications. As software complexity grows, neat compartmentalization disappears. The following software areas indicate the breadth of potential applications:

a.

System software.

System software is a collection of programs written to service other programs. Some system software (e.g., compilers, editors, and file management utilities) process complex, but determinate, information structures. Other systems applications (e.g., operating system components, drivers, telecommunications processors) process largely indeterminate data. In either case, the system software area is characterized by heavy interaction with computer hardware; heavy usage by multiple users; concurrent operation that requires scheduling, resource sharing, and sophisticated process management; complex data structures; and multiple external interfaces. b.

Real-time software.

Software that monitors/analyzes/controls real-world events as they occur is called real time. Elements of real-time software include a data gathering component that collects and formats information from an external environment, an analysis component that transforms information as required by the application, a control/output component that responds to the external environment, and a monitoring component that coordinates all other components so that real-time response (typically ranging from 1 millisecond to 1 second) can be maintained. c.

Business software.

Business information processing is the largest single softwareapplication area. Discrete "systems" (e.g., payroll, accounts receivable/payable, inventory) have evolved into management information system (MIS) software that accesses one or more large databases containing business information. Applications in this area restructure existing data in a way that facilitates business operations or management decision making. In addition to conventional data processing application, business

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software applications also encompass interactive computing (e.g., point of- sale transaction processing). d.

Engineering and scientific software.

Engineering and scientific software have been characterized by "number crunching" algorithms. Applications range from astronomy to volcanology, from automotive stress analysis to space shuttle orbital dynamics, and from molecular biology to automated manufacturing. However, modern applications within the engineering/scientific area are moving away from conventional numerical algorithms. Computer-aided design, system simulation, and other interactive applications have begun to take on real-time and even system software characteristics. e.

Embedded software.

Intelligent products have become commonplace in nearly every consumer and industrial market. Embedded software resides in read-only memory and is used to control products and systems for the consumer and industrial markets. Embedded software can perform very limited and esoteric functions (e.g., keypad control for a microwave oven) or provide significant function and control capability (e.g., digital functions in an automobile such as fuel control, dashboard displays, and braking systems). f.

Personal computer software.

The personal computer software market has burgeoned over the past two decades. Word processing, spreadsheets, computer graphics, multimedia, entertainment, database management, personal and business financial applications, external network, and database access are only a few of hundreds of applications. g.

Web-based software.

The Web pages retrieved by a browser are software that incorporates executable instructions (e.g., CGI, HTML, Perl, or Java), and data (e.g., hypertext and a variety of visual and audio formats). In essence, the network becomes a massive computer providing an almost unlimited software resource that can be accessed by anyone with a modem. h.

Artificial intelligence software.

Artificial intelligence (AI) software makes use of non-numerical algorithms to solve complex problems that are not amenable to computation or straightforward analysis. Expert systems, also called knowledgebased systems, pattern recognition (image and voice), artificial neural networks, theorem proving, and game playing are representative of applications within this category.

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Chapter 2 : The Process 1. Short Notes : a. THE LINEAR SEQUENTIAL MODEL : Sometimes called the classic life cycle or the waterfall model, the linear sequential model suggests a systematic, sequential approach5 to software development that begins at the system level and progresses through analysis, design, coding, testing, and support. Figure 2.4 illustrates the linear sequential model for software engineering. Modeled after a conventional engineering cycle, the linear sequential model encompasses the following activities: 

System / information engineering and modeling. Because software is always part of a larger system (or business), work begins by establishing requirements for all system elements and then allocating some subset of these requirements to software. This system view is essential when software must interact with other elements such as hardware, people, and databases. System engineering and analysis encompass requirements gathering at the system level with a small amount of top level design and analysis. Information engineering encompasses requirements gathering at the strategic business level and at the business area level. Software requirements analysis. The requirements gathering process is intensified and focused specifically on software. To understand the nature of the program(s) to be built, the software engineer ("analyst") must understand the information domain for the software, as well as required function, behavior, performance, and interface. Requirements for both the system and the software are documented and reviewed with the customer. Design. Software design is actually a multi-step process that focuses on four distinct attributes of a program: data structure, software architecture, interface representations, and procedural (algorithmic) detail. The design process translates requirements into a representation of the software that can be assessed for quality before coding begins. Like requirements, the design is documented and becomes part of the software configuration. Code generation. The design must be translated into a machine-readable form. The code generation step performs this task. If design is performed in a detailed manner, code generation can be accomplished mechanistically. Testing. Once code has been generated, program testing begins. The testing process focuses on the logical internals of the software, ensuring that all statements have been tested, and on the functional externals; that is, conducting tests to uncover errors and ensure that defined input will produce actual results that agree with required results. Support. Software will undoubtedly undergo change after it is delivered to the customer (a possible exception is embedded software). Change will occur because errors have been encountered, because the software must be adapted to accommodate changes in its external environment (e.g., a change required because of a new operating system or peripheral device), or because the customer requires functional or performance enhancements. Software support/maintenance reapplies each of the preceding phases to an existing program rather than a new one.

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The linear sequential model is the oldest and the most widely used paradigm for software engineering. Among the problems that are sometimes encountered when the linear sequential model is applied are: 1.

Real projects rarely follow the sequential flow that the model proposes. Although the linear model can accommodate iteration, it does so indirectly. As a result, changes can cause confusion as the project team proceeds.

2.

It is often difficult for the customer to state all requirements explicitly. The linear sequential model requires this and has difficulty accommodating the natural uncertainty that exists at the beginning of many projects.

3.

The customer must have patience. A working version of the program(s) will not be available until late in the project time-span. A major blunder, if undetected until the working program is reviewed, can be disastrous.

Each of these problems is real. However, the classic life cycle paradigm has a definite and important place in software engineering work. It provides a template into which methods for analysis, design, coding, testing, and support can be placed. The classic life cycle remains a widely used procedural model for software engineering. While it does have weaknesses, it is significantly better than a haphazard approach to software development.

b. THE PROTOTYPING MODEL : The prototyping paradigm (Figure 2.5) begins with requirements gathering. Developer and customer meet and define the overall objectives for the software, identify whatever requirements are known, and outline areas where further definition is mandatory. A "quick design" then occurs. The quick design focuses on a representation of those aspects of the software that will be visible to the customer/user (e.g., input approaches and output formats). The quick design leads to the construction of a prototype. The prototype is evaluated by the customer/user and used to refine requirements for the software to be developed. Iteration occurs as the prototype is tuned to satisfy the needs of the customer, while at the same time enabling the developer to better understand what needs to be done. Ideally, the prototype serves as a mechanism for identifying software requirements. If a working prototype is built, the developer attempts to use existing program fragments or applies tools (e.g., report generators, window managers) that enable working programs to be generated quickly.

The prototype can serve as "the first system." The one that Brooks recommends we throw away. But this may be an idealized view. It is true that both customers and developers like the prototyping paradigm. Users get a feel for the actual system and developers get to build something immediately. Yet, prototyping can also be problematic for the following reasons:

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1. The customer sees what appears to be a working version of the software, unaware that the prototype is held together “with chewing gum and baling wire,” unaware that in the rush to get it working no one has considered overall software quality or long-term maintainability. When informed that the product must be rebuilt so that high levels of quality can be maintained, the customer cries foul and demands that "a few fixes" be applied to make the prototype a working product. Too often, software development management relents. 2. The developer often makes implementation compromises in order to get a prototype working quickly. An inappropriate operating system or programming language may be used simply because it is available and known; an inefficient algorithm may be implemented simply to demonstrate capability. After a time, the developer may become familiar with these choices and forget all the reasons why they were inappropriate. The less-than-ideal choice has now become an integral part of the system. Although problems can occur, prototyping can be an effective paradigm for software engineering. The key is to define the rules of the game at the beginning; that is, the customer and developer must both agree that the prototype is built to serve as a mechanism for defining requirements. It is then discarded (at least in part) and the actual software is engineered with an eye toward quality and maintainability.

c. THE RAD MODEL : Rapid application development (RAD) is an incremental software development process model that emphasizes an extremely short development cycle. The RAD model is a “highspeed” adaptation of the linear sequential model in which rapid development is achieved by using component-based construction. If requirements are well understood and project scope is constrained, the RAD process enables a development team to create a “fully functional system” within very short time periods (e.g., 60 to 90 days). Used primarily for information systems applications, the RAD approach encompasses the following phases : Business modeling. The information flow among business functions is modeled in a way that answers the following questions: What information drives the business process? What information is generated? Who generates it? Where does the information go? Who processes it? Data modeling. The information flow defined as part of the business modeling phase is refined into a set of data objects that are needed to support the business. The characteristics (called attributes) of each object are identified and the relationships between these objects defined. Process modeling. The data objects defined in the data modeling phase are transformed to achieve the information flow necessary to implement a business function. Processing descriptions are created for adding, modifying, deleting, or retrieving a data object. Application generation. RAD assumes the use of fourth generation techniques. Rather than creating software using conventional third generation programming languages the RAD process works to reuse existing program components (when possible) or create reusable components (when necessary). In all cases, automated tools are used to facilitate construction of the software. Testing and turnover. Since the RAD process emphasizes reuse, many of the program components have already been tested. This reduces overall testing time. However, new components must be tested and all interfaces must be fully exercised.

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If a business application can be modularized in a way that enables each major function to be completed in less than three months (using the approach described previously), it is a candidate for RAD. Each major function can be addressed by a separate RAD team and then integrated to form a whole.

Like all process models, the RAD approach has drawbacks : •

For large but scalable projects, RAD requires sufficient human resources to create the right number of RAD teams.

RAD requires developers and customers who are committed to the rapid-fire activities necessary to get a system complete in a much abbreviated time frame. If commitment is lacking from either constituency, RAD projects will fail.

Not all types of applications are appropriate for RAD. If a system cannot be properly modularized, building the components necessary for RAD will be problematic. If high performance is an issue and performance is to be achieved through tuning the interfaces to system components, the RAD approach may not work.

RAD is not appropriate when technical risks are high. This occurs when a new application makes heavy use of new technology or when the new software requires a high degree of interoperability with existing computer programs.

d. THE EVOLUTIONARY SOFTWARE PROCESS MODEL : Evolutionary models are iterative. They are characterized in a manner that enables software engineers to develop increasingly more complete versions of the software.

i. The Incremental Model The incremental model combines elements of the linear sequential model (applied repetitively) with the iterative philosophy of prototyping. The incremental model applies linear sequences in a staggered fashion as calendar time progresses. Each linear sequence produces a deliverable “increment” of the software. It should be noted that the process flow for any increment can incorporate the prototyping paradigm. When an incremental model is used, the first increment is often a core product. That is, basic requirements are addressed,

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but many supplementary features (some known, others unknown) remain undelivered. The core product is used by the customer (or undergoes detailed review). As a result of use and/or evaluation, a plan is developed for the next increment. The plan addresses the modification of the core product to better meet the needs of the customer and the delivery of additional features and functionality. This process is repeated following the delivery of each increment, until the complete product is produced.

The incremental model focuses on the delivery of an operational product with each increment. Early increments are stripped down versions of the final product, but they do provide capability that serves the user and also provide a platform for evaluation by the user.

ii. The Spiral Model The spiral model is an evolutionary software process model that couples the iterative nature of prototyping with the controlled and systematic aspects of the linear sequential model. It provides the potential for rapid development of incremental versions of the software. Using the spiral model, software is developed in a series of incremental releases. During early iterations, the incremental release might be a paper model or prototype. During later iterations, increasingly more complete versions of the engineered system are produced. A spiral model is divided into a number of framework activities, also called task regions. Typically, there are between three and six task regions. Figure 2.8 depicts a spiral model that contains six task regions: • Customer communication tasks required to establish effective communication between developer and customer. • Planning tasks required to define resources, timelines, and other project related information. • Risk analysis tasks required to assess both technical and management risks. • Engineering tasks required to build one or more representations of the application. • Construction and release tasks required to construct, test, install, and provide user support (e.g., documentation and training).

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• Customer evaluation tasks required to obtain customer feedback based on evaluation of the software representations created during the engineering stage and implemented during the installation stage.

Each of the regions is populated by a set of work tasks, called a task set, that are adapted to the characteristics of the project to be undertaken. For small projects, the number of work tasks and their formality is low. For larger, more critical projects, each task region contains more work tasks that are defined to achieve a higher level of formality.

iii. The WINWIN Spiral Model The spiral model suggests a framework activity that addresses customer communication. The objective of this activity is to elicit project requirements from the customer. In an ideal context, the developer simply asks the customer what is required and the customer provides sufficient detail to proceed. Unfortunately, this rarely happens. In reality, the customer and the developer enter into a process of negotiation, where the customer may be asked to balance functionality, performance, and other product or system characteristics against cost and time to market. The best negotiations strive for a “win-win” result. That is, the customer wins by getting the system or product that satisfies the majority of the customer’s needs and the developer wins by working to realistic and achievable budgets and deadlines.

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Boehm’s WINWIN spiral model [BOE98] defines a set of negotiation activities at the beginning of each pass around the spiral. Rather than a single customer communication activity, the following activities are defined: 1. Identification of the system or subsystem’s key “stakeholders.” 2. Determination of the stakeholders’ “win conditions.” 3. Negotiation of the stakeholders’ win conditions to reconcile them into a set of win-win conditions for all concerned (including the software project team). Successful completion of these initial steps achieves a win-win result, which becomes the key criterion for proceeding to software and system definition. In addition to the emphasis placed on early negotiation, the WINWIN spiral model introduces three process milestones, called anchor points, that help establish the completion of one cycle around the spiral and provide decision milestones before the software project proceeds. In essence, the anchor points represent three different views of progress as the project traverses the spiral. The first anchor point, life cycle objectives (LCO), defines a set of objectives for each major software engineering activity. For example, as part of LCO, a set of objectives establishes the definition of top-level system/product requirements. The second anchor point, life cycle architecture (LCA), establishes objectives that must be met as the system and software architecture is defined. For example, as part of LCA, the software project team must demonstrate that it has evaluated the applicability of off-the-shelf and reusable software components and considered their impact on architectural decisions. Initial operational capability (IOC) is the third anchor point and represents a set of objectives associated with the preparation of the software for installation/distribution, site preparation prior to installation, and assistance required by all parties that will use or support the software.

e. COMPONENT-BASED DEVELOPMENT Object-oriented technologies provide the technical framework for a component-based process model for software engineering. The object oriented paradigm emphasizes the creation of classes that encapsulate both data and the algorithms used to manipulate the data. If properly designed and implemented, object-oriented classes are reusable across different applications and computer-based system architectures. The component-based development (CBD) model incorporates many of the characteristics of the spiral model. It is evolutionary in nature, demanding an iterative approach to the

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creation of software. However, the component-based development model composes applications from prepackaged software components (called classes). The engineering activity begins with the identification of candidate classes. This is accomplished by examining the data to be manipulated by the application and the algorithms that will be applied to accomplish the manipulation. Corresponding data and algorithms are packaged into a class.

The component-based development model leads to software reuse, and reusability provides software engineers with a number of measurable benefits. Classes created in past software engineering projects are stored in a class library or repository. Once candidate classes are identified, the class library is searched to determine if these classes already exist. If they do, they are extracted from the library and reused. If a candidate class does not reside in the library, it is engineered using object-oriented methods. The first iteration of the application to be built is then composed, using classes extracted from the library and any new classes built to meet the unique needs of the application. Process flow then returns to the spiral and will ultimately re-enter the component assembly iteration during subsequent passes through the engineering activity. The unified software development process is representative of a number of componentbased development models that have been proposed in the industry. Using the Unified Modeling Language (UML), the unified process defines the components that will be used to build the system and the interfaces that will connect the components. Using a combination of iterative and incremental development, the unified process defines the function of the system by applying a scenario-based approach (from the user point of view). It then couples function with an architectural framework that identifies the form the software will take.

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Chapter 3 : Project Management Concept 1. Explain W5HH Principle W5HH Principle : In an paper on software process and projects, Barry Boehm suggests an approach that addresses project objectives, milestones and schedules, responsibilities, management and technical approaches, and required resources. He calls it the WWWWWHH principle, after a series of questions that lead to a definition of key project characteristics and the resultant project plan: 

Why is the system being developed? The answer to this question enables all parties to assess the validity of business reasons for the software work. Stated in another way, does the business purpose justify the expenditure of people, time, and money? 

What will be done, by when? The answers to these questions help the team to establish a project schedule by identifying key project tasks and the milestones that are required by the customer. 

Who is responsible for a function? Earlier in this chapter, we noted that the role and responsibility of each member of the software team must be defined. The answer to this question helps accomplish this. 

Where are they organizationally located? Not all roles and responsibilities reside within the software team itself. The customer, users, and other stakeholders also have responsibilities. 

How will the job be done technically and managerially? Once product scope is established, a management and technical strategy for the project must be defined. 

How much of each resource is needed? The answer to this question is derived by developing estimates based on answers to earlier questions. Boehm’s W5HH principle is applicable regardless of the size or complexity of a software project. The questions noted provide an excellent planning outline for the project manager and the software team.

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Chapter 4 : Software Process & Project Metrics 1. Explain Measures, Metrics & Indicators Although the terms measure, measurement, and metrics are often used interchangeably, it is important to note the subtle differences between them. Measure can be used either as a noun or a verb. Within the software engineering context, a measure provides a quantitative indication of the extent, amount, dimension, capacity, or size of some attribute of a product or process. Measurement is the act of determining a measure. The IEEE Standard Glossary of Software Engineering Terms defines metric as “a quantitative measure of the degree to which a system, component, or process possesses a given attribute.� When a single data point has been collected (e.g., the number of errors uncovered in the review of a single module), a measure has been established. Measurement occurs as the result of the collection of one or more data points (e.g., a number of module reviews are investigated to collect measures of the number of errors for each). Software metric relates the individual measures in some way (e.g., the average number of errors found per review or the average number of errors found per person-hour expended on reviews. A software engineer collects measures and develops metrics so that indicators will be obtained. An indicator is a metric or combination of metrics that provide insight into the software process, a software project, or the product itself [RAG95]. An indicator provides insight that enables the project manager or software engineers to adjust the process, the project, or the process to make things better. For example, four software teams are working on a large software project. Each team must conduct design reviews but is allowed to select the type of review that it will use. Upon examination of the metric, errors found per person-hour expended, the project manager notices that the two teams using more formal review methods exhibit an errors found per person-hour expended that is 40 percent higher than the other teams. Assuming all other parameters equal, this provides the project manager with an indicator that formal review methods may provide a higher return on time investment than another, less formal review approach. She may decide to suggest that all teams use the more formal approach. The metric provides the manager with insight. And insight leads to informed decision making.

2. Software Measurement : Measurements in the physical world can be categorized in two ways: direct measures (e.g., the length of a bolt) and indirect measures (e.g., the "quality" of bolts produced, measured by counting rejects). Software metrics can be categorized similarly. Direct measures of the software engineering process include cost and effort applied. Direct measures of the product include lines of code (LOC) produced, execution speed, memory size, and defects reported over some set period of time. Indirect measures of the product include functionality, quality, complexity, efficiency, reliability, maintainability, and many other "–abilities".

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The cost and effort required to build software, the number of lines of code produced, and other direct measures are relatively easy to collect, as long as specific conventions for measurement are established in advance. However, the quality and functionality of software or its efficiency or maintainability are more difficult to assess and can be measured only indirectly.

a. Size-Oriented Metrics : Size-oriented software metrics are derived by normalizing quality and/or productivity measures by considering the size of the software that has been produced. If a software organization maintains simple records, a table of size-oriented measures, such as the one shown in Figure 4.4, can be created.

The table lists each software development project that has been completed over the past few years and corresponding measures for that project. Referring to the table entry (Figure 4.4) for project alpha: 12,100 lines of code were developed with 24 person-months of effort at a cost of $168,000. It should be noted that the effort and cost recorded in the table represent all software engineering activities (analysis, design, code, and test), not just coding. Further information for project alpha indicates that 365 pages of documentation were developed, 134 errors were recorded before the software was released, and 29 defects were encountered after release to the customer within the first year of operation. Three people worked on the development of software for project alpha. In order to develop metrics that can be assimilated with similar metrics from other projects, we choose lines of code as our normalization value. From the rudimentary data contained in the table, a set of simple size-oriented metrics can be developed for each project: • Errors per KLOC (thousand lines of code). • Defects4 per KLOC. • $ per LOC. • Page of documentation per KLOC. In addition, other interesting metrics can be computed: • Errors per person-month. • LOC per person-month. • $ per page of documentation.

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b. Function-Oriented Metrics : Function-oriented software metrics use a measure of the functionality delivered by the application as a normalization value. Since ‘functionality’ cannot be measured directly, it must be derived indirectly using other direct measures. Function-oriented metrics were first proposed by Albrecht, who suggested a measure called the function point. Function points are derived using an empirical relationship based on countable (direct) measures of software's information domain and assessments of software complexity. Function points are computed by completing the table shown in Figure 4.5.

Five information domain characteristics are determined and counts are provided in the appropriate table location. Information domain values are defined in the following manner : Number of user inputs. Each user input that provides distinct application oriented data to the software is counted. Inputs should be distinguished from inquiries, which are counted separately. Number of user outputs. Each user output that provides application oriented information to the user is counted. In this context output refers to reports, screens, error messages, etc. Individual data items within a report are not counted separately. Number of user inquiries. An inquiry is defined as an on-line input that results in the generation of some immediate software response in the form of an on-line output. Each distinct inquiry is counted. Number of files. Each logical master file (i.e., a logical grouping of data that may be one part of a large database or a separate file) is counted. Number of external interfaces. All machine readable interfaces (e.g., data files on storage media) that are used to transmit information to another system are counted. Once these data have been collected, a complexity value is associated with each count. Organizations that use function point methods develop criteria for determining whether a particular entry is simple, average, or complex. Nonetheless, the determination of complexity is somewhat subjective. To compute function points (FP), the following relationship is used: FP = count total _ [0.65 + 0.01 _ ÎŁ(Fi)] (4-1) where count total is the sum of all FP entries obtained from Figure 4.5.

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The Fi (i = 1 to 14) are "complexity adjustment values" based on responses to the following questions : 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Does the system require reliable backup and recovery? Are data communications required? Are there distributed processing functions? Is performance critical? Will the system run in an existing, heavily utilized operational environment? Does the system require on-line data entry? Does the on-line data entry require the input transaction to be built over multiple screens or operations? Are the master files updated on-line? Are the inputs, outputs, files, or inquiries complex? Is the internal processing complex? Is the code designed to be reusable? Are conversion and installation included in the design? Is the system designed for multiple installations in different organizations? Is the application designed to facilitate change and ease of use by the user?

Each of these questions is answered using a scale that ranges from 0 (not important or applicable) to 5 (absolutely essential). The constant values in Equation (4-1) and the weighting factors that are applied to information domain counts are determined empirically. Once function points have been calculated, they are used in a manner analogous to LOC as a way to normalize measures for software productivity, quality, and other attributes: • Errors per FP. • Defects per FP. • $ per FP. • Pages of documentation per FP. • FP per person-month.

3. Case Study : Computing Function Point – Case Study Diagram (optional) 1.

Direct Measures : Measurement Parameter

Count

No. of User Inputs No. of User Outputs No. of User Inquiries No. of Files No. of External Interfaces

4 2 3 2 2

Weighting Factor

X X X X X

Product

Simple

Average

Complex

3 4 3 7 5

4 5 4 10 7

6 7 6 15 10

19

= = = = =

16 10 12 20 14

Count-Total

72


2. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Indirect Measures : Does the system require reliable backup and recovery? Are data communications required? Are there distributed processing functions? Is performance critical? Will the system run in an existing, heavily utilized operational environment? Does the system require on-line data entry? Does the on-line data entry require the input transaction to be built over multiple screens or operations? Are the master files updated on-line? Are the inputs, outputs, files, or inquiries complex? Is the internal processing complex? Is the code designed to be reusable? Are conversion and installation included in the design? Is the system designed for multiple installations in different organizations? Is the application designed to facilitate change and ease of use by the user?

5 3 2 4 5 3 3 3 5 2 3 5 4 3

∑(Fi) = 50 FP = Count-Total * [ 0.65 + 0.01 * ∑(Fi) ] FP = = = = ~

72 * [ 0.65 + 0.01 * 50 ] 72 * [ 0.65 + 0.50 ] 72 * 1.15 82.8 83 Function Point = 83

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Chapter 5 : Software Project Planning 1. COCOMO : COCOMO was first published in 1981 Barry W. Boehm's Book Software engineering economics as a model for estimating effort, cost, and schedule for software projects. It drew on a study of 63 projects at TRW Aerospace where Barry Boehm was Director of Software Research and Technology in 1981. The study examined projects ranging in size from 2000 to 100,000 lines of code, and programming languages ranging from assembly to PL/I. These projects were based on the waterfall model of software development which was the prevalent software development process in 1981. References to this model typically call it COCOMO 81. In 1997 COCOMO II was developed and finally published in 2001 in the book Software Cost Estimation with COCOMO II. COCOMO II is the successor of COCOMO 81 and is better suited for estimating modern software development projects. It provides more support for modern software development processes and an updated project database. The need for the new model came as software development technology moved from mainframe and overnight batch processing to desktop development, code reusability and the use of off-the-shelf software components. This article refers to COCOMO 81. COCOMO consists of a hierarchy of three increasingly detailed and accurate forms. 

Basic COCOMO It the first level and is good for quick, early, rough order of magnitude estimates of software costs, but its accuracy is limited due to its lack of factors to account for difference in project attributes (Cost Drivers).

Intermediate COCOMO It takes the Cost Drivers into account

Advance / Detailed COCOMO It additionally accounts for the influence of individual project phases.

Basic COCOMO :

Basic COCOMO is a static, single-valued model that computes software development effort (and cost) as a function of program size expressed in estimated lines of code. COCOMO applies to three classes of software projects:  

Organic projects - are relatively small, simple software projects in which small teams with good application experience work to a set of less than rigid requirements. Semi-detached projects - are intermediate (in size and complexity) software projects in which teams with mixed experience levels must meet a mix of rigid and less than rigid requirements.

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Embedded projects - are software projects that must be developed within a set of tight hardware, software, and operational constraints.

The basic COCOMO equations take the form b b

E=ab(KLOC) D=cb(E)db P=E/D

where E is the effort applied in person-months, D is the development time in chronological months, KLOC is the estimated number of delivered lines of code for the project (expressed in thousands), and P is the number of people required. The coefficients ab, bb, cb and db are given in the following table. Software project

ab

bb

cb

db

Organic Semi-detached Embedded

2.4 3.0 3.6

1.05 1.12 1.20

2.5 2.5 2.5

0.38 0.35 0.32

Basic COCOMO is good for quick, early, rough order of magnitude estimates of software costs, but it does not account for differences in hardware constraints, personnel quality and experience, use of modern tools and techniques, and other project attributes known to have a significant influence on software costs, which limits its accuracy.

Intermediate COCOMO :

Intermediate COCOMO computes software development effort as function of program size and a set of "cost drivers" that include subjective assessment of product, hardware, personnel and project attributes. This extension considers a set of four "cost drivers", each with a number of subsidiary attributes. 

Product attributes o Required software reliability o Size of application database o Complexity of the product Hardware attributes o Run-time performance constraints o Memory constraints o Volatility of the virtual machine environment o Required turnabout time Personnel attributes o Analyst capability o Software engineering capability o Applications experience o Virtual machine experience o Programming language experience Project attributes o Use of software tools o Application of software engineering methods o Required development schedule

Each of the 15 attributes receives a rating on a six-point scale that ranges from "very low" to "extra high" (in importance or value). An effort multiplier from the table below applies to the

22


rating. The product of all effort multipliers results in an effort adjustment factor (EAF). Typical values for EAF range from 0.9 to 1.4. The Intermediate Cocomo formula now takes the form: (b )

E=ai(KLoC) i .EAF where E is the effort applied in person-months, KLoC is the estimated number of thousands of delivered lines of code for the project, and EAF is the factor calculated above. The coefficient ai and the exponent bi are given in the next table. Software project

ai

bi

Organic

3.2

1.05

Semi-detached

3.0

1.12

Embedded

2.8

1.20

The Development time D calculation uses E in the same way as in the Basic COCOMO.



Advance / Detailed COCOMO :

Detailed COCOMO - incorporates all characteristics of the intermediate version with an assessment of the cost driver's impact on each step (analysis, design, etc.) of the software engineering process.

4. Case Study : COCOMO – Case Study Given : modules with sizes : 4k, 2k, 1k, 2k, 3k To find : a) Overall cost & schedule estimates b) Cost & schedules for different phases / components Solution : Basic COCOMO Equations : E = Ab ( KLOC ) D = Cb ( E )

Bb

Efforts in Person-Months

Db

Development time in Months

N=E/D

No. of required Persons

S/W Project

Ab

Bb

Cb

Db

Organic Semi-Detached Embedded

2.4 3.0 3.5

1.05 1.12 1.2

2.5 2.5 2.5

0.38 0.35 0.32

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a) Overall cost & schedule estimates : Here, we will take the Project to be Organic. Total Lines of Code = 4k + 2k + 1k + 2k + 3k = 12k E = 2.4 ( 12 )

1.05

D = 2.5 ( 32.61 )

0.38

N = 32.61 / 9.3975

- ( 12 KLOC)

= 32.61

Persons-Months

= 9.3975

Months

= 3.47 ► 4

Persons

( Optional : Assume Salary = 2000, Then Cost Of Project = E X Salary = 32.61 X 2000 = Rs. 65220 )

b) Cost & schedules for different phases / components :

Module 1 (4k)

E = 3.0 (4)

1.12

= 14.17 Persons-Months D = 2.5(14.17)0.35 = 6.32 Months

N = 14.17 6.32 = 2.24 ► 3 Persons

Module 2 (2k) 1.12

E = 3.0 (2) = 6.52

Persons-Months D = 2.5(6.52)0.35 = 4.81 Months

N = 6.52 4.81 = 1.35 ► 2 Persons

Module 3 (1k) 1.12

E = 3.0 (1) = 3.0

Persons-Months D = 2.5(3.0)0.35 = 3.67 Months

N = 3.0 3.67 = 0.81 ► 1 Persons

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Module 4 (2k) 1.12

E = 3.0 (2) = 6.52

Persons-Months D = 2.5(6.52)0.35 = 4.81 Months

N = 6.52 4.81 = 1.35 ► 2 Persons

Module 5 (3k)

E = 3.0 (3)

1.12

= 3.42 Persons-Months D = 2.5(3.42)0.35 = 3.84 Months

N = 3.42 3.84 = 0.89 ► 1 Persons


Chapter 6 : Risk Analysis & Management 1. Explain Software Risks Risk is defined as "The possibility of suffering harm or loss; danger." Even if we're not familiar with the formal definition, most of us have an innate sense of risk. We are aware of the potential dangers that permeate even simple daily activities, from getting injured when crossing the street to having a heart attack because our cholesterol level is too high. Although we prefer not to dwell on the myriad of hazards that surround us, these risks shape many of our behaviors. Experience (or a parent) has taught us to look both ways before stepping off the curb and most of us at least think twice before ordering a steak. Indeed, we manage personal risks every day. Risks in Software Project Management Unlike the hazards of daily living, the dangers in the young and emerging field of software engineering must often be learned without the benefit of lifelong exposure. A more deliberate approach is required. Such an approach involves studying the experiences of successful project managers as well as keeping up with the leading writers and thinkers in the field. One such writer in the area of risk is Dr. Barry W. Boehm. In his article "Software Risk Management: Principles and Practices" he lists the following top 10 software risk items: 1. Personnel Shortfalls 2. Unrealistic schedules and budgets 3. Developing the wrong functions and properties 4. Developing the wrong user interface 5. Gold-plating 6. Continuing stream of requirements changes 7. Shortfalls in externally furnished components 8. Shortfalls in externally performed tasks 9. Real-time performance shortfalls 10. Straining computer-science capabilities There is a general agreement that risk always involves two characteristics :  

Uncertainty—the risk may or may not happen; that is, there are no 100% probable risks. Loss—if the risk becomes a reality, unwanted consequences or losses will occur.

When risks are analyzed, it is important to quantify the level of uncertainty and the degree of loss associated with each risk. To accomplish this, different categories of risks are considered. 

Project risks threaten the project plan. That is, if project risks become real, it is likely that project schedule will slip and that costs will increase. Project risks identify potential budgetary, schedule, personnel (staffing and organization), resource, customer, and requirements problems and their impact on a software project.

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Ex : project complexity, size, and the degree of structural uncertainty can be defined as project (and estimation) risk factors. 

Technical risks threaten the quality and timeliness of the software to be produced. If a technical risk becomes a reality, implementation may become difficult or impossible. Technical risks identify potential design, implementation, interface, verification, and maintenance problems. In addition, specification ambiguity, technical uncertainty, technical obsolescence, and "leading-edge" technology are also risk factors. Technical risks occur because the problem is harder to solve than we thought it would be.

Business risks threaten the viability of the software to be built. Business risks often jeopardize the project or the product. Candidates for the top five business risks are (1) building a excellent product or system that no one really wants (market risk), (2) building a product that no longer fits into the overall business strategy for the company (strategic risk), (3) building a product that the sales force doesn't understand how to sell, (4) losing the support of senior management due to a change in focus or a change in people (management risk), (5) losing budgetary or personnel commitment (budget risks).

It is extremely important to note that simple categorization won't always work. Some risks are simply unpredictable in advance. Another general categorization of risks is: 

Known risks are those that can be uncovered after careful evaluation of the project plan, the business and technical environment in which the project is being developed, and other reliable information sources (e.g., unrealistic delivery date, lack of documented requirements or software scope, poor development environment).

Predictable risks are extrapolated from past project experience (e.g., staff turnover, poor communication with the customer, dilution of staff effort as ongoing maintenance requests are serviced).

Unpredictable risks are the joker in the deck. They can and do occur, but they are extremely difficult to identify in advance.

2. Explain Reactive Vs Proactive Risk Strategies Reactive Risk Strategy : A reactive strategy monitors the project for likely risks. Resources are set aside to deal with them, should they become actual problems. More commonly, the software team does nothing about risks until something goes wrong. Then, the team flies into action in an attempt to correct the problem rapidly. This is often called a fire fighting mode. When this fails, “crisis management” takes over and the project is in realjeopardy.

Proactive Risk Strategy : It is a considerably more intelligent strategy for risk management. A proactive strategy begins long before technical work is initiated. Potential risks are identified, their probability and impact are assessed, and they are ranked by importance. Then, the software team establishes a plan for managing risk. The primary objective is to avoid risk, but because not all risks can be avoided, the team works to develop a contingency plan that will enable it to respond in a controlled and effective manner.

3. Explain Risk Projection

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Risk Projection : Risk projection, also called risk estimation, attempts to rate each risk in two ways—the likelihood or probability that the risk is real and the consequences of the problems associated with the risk, should it occur. The project planner, along with other managers and technical staff, performs four risk projection activities : (1) establish a scale that reflects the perceived likelihood of a risk, (2) delineate the consequences of the risk, (3) estimate the impact of the risk on the project and the product, (4) note the overall accuracy of the risk projection so that there will be no misunderstandings.

Developing a Risk Table : A risk table provides a project manager with a simple technique for risk projection.

A project team begins by listing all risks (no matter how remote) in the first column of the table. This can be accomplished with the help of the risk item checklists referenced in Section 6.3. Each risk is categorized in the second column (e.g., PS implies a project size risk, BU implies a business risk). The probability of occurrence of each risk is entered in the next column of the table. The probability value for each risk can be estimated by team members individually. Individual team members are polled in round-robin fashion until their assessment of risk probability begins to converge. Next, the impact of each risk is assessed. Each risk component is assessed using the characterization presented in Figure 6.1, and an impact category is determined. The categories for each of the four risk components—performance, support, cost, and schedule—are averaged to determine an overall impact value.

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Once the first four columns of the risk table have been completed, the table is sorted by probability and by impact. High-probability, high-impact risks percolate to the top of the table, and low-probability risks drop to the bottom. This accomplishes first-order risk prioritization. The project manager studies the resultant sorted table and defines a cutoff line. The cutoff line (drawn horizontally at some point in the table) implies that only risks that lie above the line will be given further attention. Risks that fall below the line are re-evaluated to accomplish second-order prioritization. Referring to Figure 6.3, risk impact and probability have a distinct influence on management concern. A risk factor that has a high impact but a very low probability of occurrence should not absorb a significant amount of management time. However, high-impact risks with moderate to high probability and low-impact risks with high probability should be carried forward into the risk analysis steps that follow. All risks that lie above the cutoff line must be managed. The column labeled RMMM contains a pointer into a Risk Mitigation, Monitoring and Management Plan or alternatively, a collection of risk information sheets developed for all risks that lie above the cutoff.

4. Explain Risk Mitigation, Monitoring, Management Risk Projection : All of the risk analysis activities have a single goal—to assist the project team in developing a strategy for dealing with risk. An effective strategy must consider three issues: • risk avoidance • risk monitoring • risk management and contingency planning If a software team adopts a proactive approach to risk, avoidance is always the best strategy. This is achieved by developing a plan for risk mitigation. For example, assume that high staff turnover is noted as a project risk, r1. Based on past history and management intuition, the likelihood, l1, of high turnover is estimated to be 0.70 (70 percent, rather high) and the impact, x1, is projected at level 2. That is, high turnover will have a critical impact on project cost and schedule. Mitigation : To mitigate this risk, project management must develop a strategy for reducing turnover. Among the possible steps to be taken are • Meet with current staff to determine causes for turnover (e.g., poor working conditions, low pay, competitive job market). • Mitigate those causes that are under our control before the project starts. • Once the project commences, assume turnover will occur and develop techniques to ensure continuity when people leave. • Organize project teams so that information about each development activity is widely dispersed. • Define documentation standards and establish mechanisms to be sure that documents are developed in a timely manner. • Conduct peer reviews of all work (so that more than one person is "up to speed”). • Assign a backup staff member for every critical technologist.

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Monitoring : As the project proceeds, risk monitoring activities commence. The project manager monitors factors that may provide an indication of whether the risk is becoming more or less likely. In the case of high staff turnover, the following factors can be monitored: • General attitude of team members based on project pressures. • The degree to which the team has jelled. • Interpersonal relationships among team members. • Potential problems with compensation and benefits. • The availability of jobs within the company and outside it. In addition to monitoring these factors, the project manager should monitor the effectiveness of risk mitigation steps. For example, a risk mitigation step noted here called for the definition of documentation standards and mechanisms to be sure that documents are developed in a timely manner. This is one mechanism for ensuring continuity, should a critical individual leave the project. The project manager should monitor documents carefully to ensure that each can stand on its own and that each imparts information that would be necessary if a newcomer were forced to join the software team somewhere in the middle of the project. Management : Risk management and contingency planning assumes that mitigation efforts have failed and that the risk has become a reality. Continuing the example, the project is well underway and a number of people announce that they will be leaving. If the mitigation strategy has been followed, backup is available, information is documented, and knowledge has been dispersed across the team. In addition, the project manager may temporarily refocus resources (and readjust the project schedule) to those functions that are fully staffed, enabling newcomers who must be added to the team to “get up to speed.” Those individuals who are leaving are asked to stop all work and spend their last weeks in “knowledge transfer mode.” This might include video-based knowledge capture, the development of “commentary documents,” and/or meeting with other team members who will remain on the project. RMMM Plan : A risk management strategy can be included in the software project plan or the risk management steps can be organized into a separate Risk Mitigation, Monitoring and Management Plan. The RMMM plan documents all work performed as part of risk analysis and is used by the project manager as part of the overall project plan. Some software teams do not develop a formal RMMM document. Rather, each risk is documented individually using a risk information sheet (RIS) . Once RMMM has been documented and the project has begun, risk mitigation and monitoring steps commence. As we have already discussed, risk mitigation is a problem avoidance activity. Risk monitoring is a project tracking activity with three primary objectives: (1) to assess whether predicted risks do, in fact, occur; (2) to ensure that risk aversion steps defined for the risk are being properly applied; (3) to collect information that can be used for future risk analysis. In many cases, the problems that occur during a project can be traced to more than one risk. Another job of risk monitoring is to attempt to allocate origin (what risk(s) caused which problems throughout the project).

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30


Chapter 7 : Project Scheduling & Tracking 1. Explain Task Set Selector Different software models offer different paradigms for software development. Regardless of whether a software team chooses a linear sequential paradigm, an iterative paradigm, an evolutionary paradigm, a concurrent paradigm or some permutation, the process model is populated by a set of tasks that enable a software team to define, develop, and ultimately support computer software. No single set of tasks is appropriate for all projects. The set of tasks that would be appropriate for a large, complex system would likely be perceived as overkill for a small, relatively simple software product. Therefore, an effective software process should define a collection of task sets, each designed to meet the needs of different types of projects. A task set is a collection of software engineering work tasks, milestones, and deliverables that must be accomplished to complete a particular project. The task set to be chosen must provide enough discipline to achieve high software quality. But, at the same time, it must not burden the project team with unnecessary work. Task sets are designed to accommodate different types of projects and different degrees of rigor. Although it is difficult to develop a comprehensive taxonomy of software project types, most software organizations encounter the following projects: 1. Concept development projects that are initiated to explore some new business concept or application of some new technology. 2. New application development projects that are undertaken as a consequence of a specific customer request. 3. Application enhancement projects that occur when existing software undergoes major modifications to function, performance, or interfaces that are observable by the end-user. 4. Application maintenance projects that correct, adapt, or extend existing software in ways that may not be immediately obvious to the end-user. 5. Reengineering projects that are undertaken with the intent of rebuilding an existing (legacy) system in whole or in part. Even within a single project type, many factors influence the task set to be chosen. When taken in combination, these factors provide an indication of the degree of rigor with which the software process should be applied.

Degree of Rigor : Even for a project of a particular type, the degree of rigor with which the software process is applied may vary significantly. The degree of rigor is a function of many project characteristics. As an example, small, non-business-critical projects can generally be addressed with somewhat less rigor than large, complex business-critical applications. It should be noted, however, that all projects must be conducted in a manner that results in timely, high-quality deliverables. Four different degrees of rigor can be defined:

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Casual.

All process framework activities (Chapter 2) are applied, but only a minimum task set is required. In general, umbrella tasks will be minimized and documentation requirements will be reduced. All basic principles of software engineering are still applicable.

Structured.

Strict.

The process framework will be applied for this project. Framework activities and related tasks appropriate to the project type will be applied and umbrella activities necessary to ensure high quality will be applied. SQA, SCM, documentation, and measurement tasks will be conducted in a streamlined manner.

The full process will be applied for this project with a degree of discipline that will ensure high quality. All umbrella activities will be applied and robust work products will be produced.

Quick reaction.

The process framework will be applied for this project, but because of an emergency situation only those tasks essential to maintaining good quality will be applied. “Back-filling” (i.e., developing a complete set of documentation, conducting additional reviews) will be accomplished after the application/product is delivered to the customer.

Defining Adaptation Criteria Adaptation criteria are used to determine the recommended degree of rigor with which the software process should be applied on a project. Eleven adaptation criteria are defined for software projects: • Size of the project • Number of potential users • Mission criticality • Application longevity • Stability of requirements • Ease of customer/developer communication • Maturity of applicable technology • Performance constraints • Embedded and non-embedded characteristics • Project staff • Reengineering factors Each of the adaptation criteria is assigned a grade that ranges between 1 and 5, where 1 represents a project in which a small subset of process tasks are required and overall methodological and documentation requirements are minimal, and 5 represents a project in which a complete set of process tasks should be applied and overall methodological and documentation requirements are substantial.

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Computing a Task Set Selector Value To select the appropriate task set for a project, the following steps should be conducted: 1. Review each of the adaptation criteria in Section 7.3.2 and assign the appropriate grades (1 to 5) based on the characteristics of the project. These grades should be entered into Table 7.1. 2. Review the weighting factors assigned to each of the criteria. The value of a weighting factor ranges from 0.8 to 1.2 and provides an indication of the relative importance of a particular adaptation criterion to the types of software developed within the local environment. If modifications are required to better reflect local circumstances, they should be made. 3. Multiply the grade entered in Table 7.1 by the weighting factor and by the entry point multiplier for the type of project to be undertaken. The entry point multiplier takes on a value of 0 or 1 and indicates the relevance of the adaptation criterion to the project type. The result of the product grade x weighting factor x entry point multiplier is placed in the Product column of Table 7.1 for each adaptation criteria individually. 4. Compute the average of all entries in the Product column and place the result in the space marked task set selector (TSS). This value will be used to help select the task set that is most appropriate for the project.

Interpreting the TSS Value and Selecting the Task Set Once the task set selector is computed, the following guidelines can be used to select the appropriate task set for a project: Task set selector value Degree of rigor TSS < 1.2 casual 1.0 < TSS < 3.0 structured TSS > 2.4 strict Table 7.2 illustrates how TSS might be computed for a hypothetical project. The project manager selects the grades shown in the Grade column. The project type is new application development. Therefore, entry point multipliers are selected from the NDev column. The entry in the Product column is computed using Grade x Weight x NewDev entry point multiplier

33


The value of TSS (computed as the average of all entries in the product column) is 2.8. Using the criteria discussed previously, the manager has the option of using either the structured or the strict task set. The final decision is made once all project factors have been considered.

2. Explain Scheduling or Tracking & Scheduling Risk Projection : Scheduling of a software project does not differ greatly from scheduling of any multitask engineering effort. Therefore, generalized project scheduling tools and techniques can be applied with little modification to software projects. Program evaluation and review technique (PERT) and critical path method (CPM) are two project scheduling methods that can be applied to software development. Both techniques are driven by information already developed in earlier project planning activities: • Estimates of effort • A decomposition of the product function • The selection of the appropriate process model and task set • Decomposition of tasks Interdependencies among tasks may be defined using a task network. Tasks, sometimes called the project work breakdown structure (WBS), are defined for the product as a whole or for individual functions. Both PERT and CPM provide quantitative tools that allow the software planner to (1) determine the critical path—the chain of tasks that determines the duration of the project; (2) establish “most likely” time estimates for individual tasks by applying statistical models; (3) calculate “boundary times” that define a time "window" for a particular task. Boundary time calculations can be very useful in software project scheduling. Slippage in the design of one function, for example, can retard further development of other functions. Riggs describes important boundary times that may be discerned from a PERT or CPM network: (1) the earliest time that a task can begin when all preceding tasks are completed in the shortest possible time, (2) the latest time for task initiation before the minimum project completion time is delayed, (3) the earliest finish—the sum of the earliest start and the task duration, (4) the latest finish— the latest start time added to task duration, (5) the total float—the amount of surplus time or leeway allowed in scheduling tasks so that the network critical path is maintained on schedule. Boundary time calculations lead to a determination of critical path and provide the manager with a quantitative method for evaluating progress as tasks are completed. Both PERT and CPM have been implemented in a wide variety of automated tools that are available for the personal computer. Such tools are easy to use and make the scheduling methods described previously available to every software project manager.

34


Timeline Charts When creating a software project schedule, the planner begins with a set of tasks (the work breakdown structure). If automated tools are used, the work breakdown is input as a task network or task outline. Effort, duration, and start date are then input for each task. In addition, tasks may be assigned to specific individuals. As a consequence of this input, a timeline chart, also called a Gantt chart, is generated. A timeline chart can be developed for the entire project. Alternatively, separate charts can be developed for each project function or for each individual working on the project. Figure 7.4 illustrates the format of a timeline chart. It depicts a part of a software project schedule that emphasizes the concept scoping task (Section 7.5) for a new word-processing (WP) software product. All project tasks (for concept scoping) are listed in the left-hand column. The horizontal bars indicate the duration of each task. When multiple bars occur at the same time on the calendar, task concurrency is implied. The diamonds indicate milestones. Once the information necessary for the generation of a timeline chart has been input, the majority of software project scheduling tools produce project tablesâ&#x20AC;&#x201D;a tabular listing of all project tasks, their planned and actual start- and end-dates, and a variety of related information (Figure 7.5). Used in conjunction with the timeline chart, project tables enable the project manager to track progress.

35


Tracking the Schedule The project schedule provides a road map for a software project manager. If it has been properly developed, the project schedule defines the tasks and milestones that must be tracked and controlled as the project proceeds. Tracking can be accomplished in a number of different ways: • Conducting periodic project status meetings in which each team member reports progress and problems. • Evaluating the results of all reviews conducted throughout the software engineering process. • Determining whether formal project milestones (the diamonds shown in Figure 7.4) have been accomplished by the scheduled date. • Comparing actual start-date to planned start-date for each project task listed in the resource table (Figure 7.5). • Meeting informally with practitioners to obtain their subjective assessment of progress to date and problems on the horizon. • Using earned value analysis to assess progress quantitatively. In reality, all of these tracking techniques are used by experienced project managers. Control is employed by a software project manager to administer project resources, cope with problems, and direct project staff. If things are going well (i.e., the project is on schedule and within budget, reviews indicate that real progress is being made and milestones are being reached), control is light. But when problems occur, the project manager must exercise control to reconcile them as quickly as possible. After a problem has been diagnosed,10 additional resources may be focused on the problem area: staff may be redeployed or the project schedule can be redefined. When faced with severe deadline pressure, experienced project managers sometimes use a project scheduling and control technique called time-boxing. The time-boxing strategy recognizes that the complete product may not be deliverable by the predefined deadline. Therefore, an incremental software paradigm is chosen and a schedule is derived for each incremental delivery. The tasks associated with each increment are then time-boxed. This means that the schedule for each task is adjusted by working backward from the delivery date

36


for the increment. A “box” is put around each task. When a task hits the boundary of its time box (plus or minus 10 percent), work stops and the next task begins. The initial reaction to the time-boxing approach is often negative: “If the work isn’t finished, how can we proceed?” The answer lies in the way work is accomplished. By the time the time-box boundary is encountered, it is likely that 90 percent of the task has been completed.11 The remaining 10 percent, although important, can (1) be delayed until the next increment or (2) be completed later if required. Rather than becoming “stuck” on a task, the project proceeds toward the delivery date.

37


Chapter 8 : Software Quality Assurance 1. Short Note : Software Quality Assurance A high-quality software is our most important goal. But how do we define quality? Many definitions of software quality have been proposed in the literature. For our purposes, software quality is defined as Conformance to explicitly stated functional and performance requirements, explicitly documented development standards, and implicit characteristics that are expected of all professionally developed software. There is little question that this definition could be modified or extended. In fact, a definitive definition of software quality could be debated endlessly. For the purposes of this book, the definition serves to emphasize three important points: 1.

Software requirements are the foundation from which quality is measured. Lack of conformance to requirements is lack of quality.

2. Specified standards define a set of development criteria that guide the manner in which software is engineered. If the criteria are not followed, lack of quality will almost surely result. 2. A set of implicit requirements often goes unmentioned (e.g., the desire for ease of use and good maintainability). If software conforms to its explicit requirements but fails to meet implicit requirements, software quality is suspect.

SQA Activities : Software quality assurance is composed of a variety of tasks associated with two different constituencies—the software engineers who do technical work and an SQA group that has responsibility for quality assurance planning, oversight, record keeping, analysis, & reporting. Software engineers address quality (and perform quality assurance and quality control activities) by applying solid technical methods and measures, conducting formal technical reviews, and performing well-planned software testing. The charter of the SQA group is to assist the software team in achieving a high quality end product. The Software Engineering Institute [PAU93] recommends a set of SQA activities that address quality assurance planning, oversight, record keeping, analysis, and reporting. These activities are performed (or facilitated) by an independent SQA group that: 

Prepares an SQA plan for a project.

The plan is developed during project planning and is reviewed by all interested parties. Quality assurance activities performed by the software engineering team and the SQA group are governed by the plan. The plan identifies • evaluations to be performed • audits and reviews to be performed • standards that are applicable to the project • procedures for error reporting and tracking • documents to be produced by the SQA group • amount of feedback provided to the software project team

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Participates in the development of the project’s software process description.

The software team selects a process for the work to be performed. The SQA group reviews the process description for compliance with organizational policy, internal software standards, externally imposed standards (e.g., ISO-9001), and other parts of the software project plan. 

Reviews software engineering activities to verify compliance with the defined software process.

The SQA group identifies, documents, and tracks deviations from the process and verifies that corrections have been made. 

Audits designated software work products to verify compliance with those defined as part of the software process.

The SQA group reviews selected work products; identifies, documents, and tracks deviations; verifies that corrections have been made; and periodically reports the results of its work to the project manager. 

Ensures that deviations in software work and work products are documented and handled according to a documented procedure.

Deviations may be encountered in the project plan, process description, applicable standards, or technical work products. 

Records any noncompliance and reports to senior management.

Noncompliance items are tracked until they are resolved. In addition to these activities, the SQA group coordinates the control and management of change and helps to collect and analyze software metrics.

2. Short Note : Software Reliability, Availability, Safety There is no doubt that the reliability of a computer program is an important element of its overall quality. If a program repeatedly and frequently fails to perform, it matters little whether other software quality factors are acceptable. Software reliability, unlike many other quality factors, can be measured directed and estimated using historical and developmental data. Software reliability is defined in statistical terms as "the probability of failure-free operation of a computer program in a specified environment for a specified time". To illustrate, program X is estimated to have a reliability of 0.96 over eight elapsed processing hours. In other words, if program X were to be executed 100 times and require eight hours of elapsed processing time (execution time), it is likely to operate correctly (without failure) 96 times out of 100. Whenever software reliability is discussed, a pivotal question arises: What is meant by the term failure? In the context of any discussion of software quality and reliability, failure is nonconformance to software requirements. Yet, even within this definition, there are gradations. Failures can be only annoying or catastrophic. One failure can be corrected within seconds while another requires weeks or even months to correct. Complicating the issue even further, the correction of one failure may in fact result in the introduction of other errors that ultimately result in other failures.

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Measures of Reliability and Availability : Most hardware-related reliability models are predicated on failure due to wear rather than failure due to design defects. In hardware, failures due to physical wear (e.g., the effects of temperature, corrosion, shock) are more likely than a design-related failure. Unfortunately, the opposite is true for software. In fact, all software failures can be traced to design or implementation problems; wear (see Chapter 1) does not enter into the picture. There has been debate over the relationship between key concepts in hardware reliability and their applicability to software. Although an irrefutable link has yet be established, it is worthwhile to consider a few simple concepts that apply to both system elements. If we consider a computer-based system, a simple measure of reliability is meantimebetween-failure (MTBF), where MTBF = MTTF + MTTR The acronyms MTTF and MTTR are mean-time-to-failure and mean-time-to-repair, respectively. Many researchers argue that MTBF is a far more useful measure than defects/KLOC or defects/FP. Stated simply, an end-user is concerned with failures, not with the total error count. Because each error contained within a program does not have the same failure rate, the total error count provides little indication of the reliability of a system. For example, consider a program that has been in operation for 14 months. Many errors in this program may remain undetected for decades before they are discovered. The MTBF of such obscure errors might be 50 or even 100 years. Other errors, as yet undiscovered, might have a failure rate of 18 or 24 months. Even if every one of the first category of errors (those with long MTBF) is removed, the impact on software reliability is negligible. In addition to a reliability measure, we must develop a measure of availability. Software availability is the probability that a program is operating according to requirements at a given point in time and is defined as Availability = [MTTF/(MTTF + MTTR)] x 100% The MTBF reliability measure is equally sensitive to MTTF and MTTR. The availability measure is somewhat more sensitive to MTTR, an indirect measure of the maintainability of software. Software Safety : Before software was used in safety critical systems, they were often controlled by conventional (nonprogrammable) mechanical and electronic devices. System safety techniques are designed to cope with random failures in these [nonprogrammable] systems. Human design errors are not considered since it is assumed that all faults caused by human errors can be avoided completely or removed prior to delivery and operation. When software is used as part of the control system, complexity can increase by an order of magnitude or more. Subtle design faults induced by human errorâ&#x20AC;&#x201D;something that can be uncovered and eliminated in hardware-based conventional controlâ&#x20AC;&#x201D; become much more difficult to uncover when software is used. Software safety is a software quality assurance activity that focuses on the identification and assessment of potential hazards that may affect software negatively and cause an entire system to fail. If hazards can be identified early in the software engineering process, software design features can be specified that will either eliminate or control potential hazards.

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A modeling and analysis process is conducted as part of software safety. Initially, hazards are identified and categorized by criticality and risk. For example, some of the hazards associated with a computer-based cruise control for an automobile might be • causes uncontrolled acceleration that cannot be stopped • does not respond to depression of brake pedal (by turning off) • does not engage when switch is activated • slowly loses or gains speed Once these system-level hazards are identified, analysis techniques are used to assign severity and probability of occurrence. To be effective, software must be analyzed in the context of the entire system. For example, a subtle user input error (people are system components) may be magnified by a software fault to produce control data that improperly positions a mechanical device. If a set of external environmental conditions are met (and only if they are met), the improper position of the mechanical device will cause a disastrous failure. Analysis techniques such as fault tree analysis, real-time logic, or petri net models can be used to predict the chain of events that can cause hazards and the probability that each of the events will occur to create the chain. Once hazards are identified and analyzed, safetyrelated requirements can be specified for the software. That is, the specification can contain a list of undesirable events and the desired system responses to these events. The role of software in managing undesirable events is then indicated. Although software reliability and software safety are closely related to one another, it is important to understand the subtle difference between them. Software reliability uses statistical analysis to determine the likelihood that a software failure will occur. However, the occurrence of a failure does not necessarily result in a hazard or mishap. Software safety examines the ways in which failures result in conditions that can lead to a mishap. That is, failures are not considered in a vacuum, but are evaluated in the context of an entire computer-based system.

3. Short Note : Software Quality Assurance Plan The SQA Plan provides a road map for instituting software quality assurance. Developed by the SQA group, the plan serves as a template for SQA activities that are instituted for each software project. A standard for SQA plans has been recommended by the IEEE. Initial sections describe the purpose and scope of the document and indicate those software process activities that are covered by quality assurance. All documents noted in the SQA Plan are listed and all applicable standards are noted. The management section of the plan describes SQA’s place in the organizational structure, SQA tasks and activities and their placement throughout the software process, and the organizational roles and responsibilities relative to product quality. The documentation section describes (by reference) each of the work products produced as part of the software process. These include: • project documents (e.g., project plan) • models (e.g., ERDs, class hierarchies) • technical documents (e.g., specifications, test plans) • user documents (e.g., help files) In addition, this section defines the minimum set of work products that are acceptable to achieve high quality. The standards, practices, and conventions section lists all applicable standards and practices that are applied during the software process (e.g., document standards, coding standards, and review guidelines). In addition, all project, process, and (in some instances) product metrics that are to be collected as part of software engineering

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work are listed. The reviews and audits section of the plan identifies the reviews and audits to be conducted by the software engineering team, the SQA group, and the customer. It provides an overview of the approach for each review and audit. The test section references the Software Test Plan and Procedure. It also defines test record-keeping requirements. Problem reporting and corrective action defines procedures for reporting, tracking, and resolving errors and defects, and identifies the organizational responsibilities for these activities. The remainder of the SQA Plan identifies the tools and methods that support SQA activities and tasks; references software configuration management procedures for controlling change; defines a contract management approach; establishes methods for assembling, safeguarding, and maintaining all records; identifies training required to meet the needs of the plan; and defines methods for identifying, assessing, monitoring, and controlling risk.

2. Statistical SQA Plan : Data Collection for Statistical SQA :

Si = the number of serious errors Mi = the number of moderate errors Ti = the number of minor errors PS = size of the product (LOC, design statements, pages of documentation) at the ith step Ws, Wm, Wt

- (weighting factors for serious, moderate, and trivial errors) By Default : Ws=10, Wm =3 , Wt =1

At each step in the software process, a phase index, PIi, is computed: PIi = ws (Si/Ei) + wm (Mi/Ei) + wt (Ti/Ei) The error index is computed by calculating the cumulative effect on each PIi EI = =

Summation {(i x PIi)/PS} (PI1 + 2PI2 + 3PI3 + . . . iPIi)/PS

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Chapter 9 : Software configuration Management 1. Short Note : Software Configuration Management Software configuration management (SCM) is an umbrella activity that is applied throughout the software process. Because change can occur at any time, SCM activities are developed to (1) (2) (3) (4)

identify change, control change, ensure that change is being properly implemented, report changes to others who may have an interest.

It is important to make a clear distinction between software support and software configuration management. Support is a set of software engineering activities that occur after software has been delivered to the customer and put into operation. Software configuration management is a set of tracking and control activities that begin when a software engineering project begins and terminate only when the software is taken out of operation. The output of the software process is information that may be divided into three broad categories: (1) computer programs (both source level and executable forms); (2) documents that describe the computer programs (targeted at both technical practitioners and users), (3) data (contained within the program or external to it). The items that comprise all information produced as part of the software process are collectively called a software configuration. As the software process progresses, the number of software configuration items (SCIs) grows rapidly. A System Specification spawns a Software Project Plan and Software Requirements Specification (as well as hardware related documents). These in turn spawn other documents to create a hierarchy of information. If each SCI simply spawned other SCIs, little confusion would result. Unfortunately, another variable enters the process—change. Change may occur at any time, for any reason. Software configuration management is a set of activities that have been developed to manage change throughout the life cycle of computer software. SCM can be viewed as a software quality assurance activity that is applied throughout the software process. Any discussion of SCM introduces a set of complex questions: • How does an organization identify and manage the many existing versions of a program (and its documentation) in a manner that will enable change to be accommodated efficiently? • How does an organization control changes before and after software is released to a customer? • Who has responsibility for approving and ranking changes? • How can we ensure that changes have been made properly? • What mechanism is used to appraise others of changes that are made?

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These questions lead us to the definition of five SCM tasks: identification, version control, change control, configuration auditing, and reporting.

A. IDENTIFICATION OF OBJECTS IN THE SOFTWARE CONFIGURATION : To control and manage software configuration items, each must be separately named and then organized using an object-oriented approach. Two types of objects can be identified : basic objects and aggregate objects. A basic object is a "unit of text" that has been created by a software engineer during analysis, design, code, or test. For example, a basic object might be a section of a requirements specification, a source listing for a component, or a suite of test cases that are used to exercise the code. An aggregate object is a collection of basic objects and other aggregate objects. Each object has a set of distinct features that identify it uniquely: a name, a description, a list of resources, and a "realization." The object name is a character string that identifies the object unambiguously. The object description is a list of data items that identify • the SCI type (e.g., document, program, data) represented by the object • a project identifier • change and/or version information

B. VERSION CONTROL : Version control combines procedures and tools to manage different versions of configuration objects that are created during the software process version control in the context of SCM: “Configuration management allows a user to specify alternative configurations of the software system through the selection of appropriate versions. This is supported by associating attributes with each software version, and then allowing a configuration to be specified [and constructed] by describing the set of desired attributes.” These "attributes" mentioned can be as simple as a specific version number that is attached to each object or as complex as a string of Boolean variables (switches) that indicate specific types of functional changes that have been applied to the system. One representation of the different versions of a system is the evolution graph presented in Figure 9.3. Each node on the graph is an aggregate object, that is, a complete version of the software. Each version of the software is a collection of SCIs (source code, documents, data), and each version may be composed of different variants.

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C. CHANGE CONTROL : Change control is vital. But the forces that make it necessary also make it annoying. We worry about change because a tiny perturbation in the code can create a big failure in the product. But it can also fix a big failure or enable wonderful new capabilities. We worry about change because a single rogue developer could sink the project; yet brilliant ideas originate in the minds of those rogues, and a burdensome change control process could effectively discourage them from doing creative work. For a large software engineering project, uncontrolled change rapidly leads to chaos. For such projects, change control combines human procedures and automated tools to provide a mechanism for the control of change. The change control process is illustrated schematically in Figure 9.5. A change request is submitted and evaluated to assess technical merit, potential side effects, overall impact on other configuration objects and system functions, and the projected cost of the change. The results of the evaluation are presented as a change report, which is used by a change control authority (CCA)â&#x20AC;&#x201D;a person or group who makes a final decision on the status and priority of the change. An engineering change order (ECO) is generated for each approved change. The ECO describes the change to be made, the constraints that must be respected, and the criteria for review and audit. The object to be changed is "checkedout" of the project database, the change is made, and appropriate SQA activities are applied. The object is then "checked in" to the database and appropriate version control mechanisms are used to create the next version of the software.

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The "check-in" and "check-out" process implements two important elements of change controlâ&#x20AC;&#x201D;access control and synchronization control. Access control governs which software engineers have the authority to access and modify a particular configuration object. Synchronization control helps to ensure that parallel changes, performed by two different people, don't overwrite one another. Based on an approved change request and ECO, a software engineer checks out a configuration object. An access control function ensures that the software engineer has authority to check out the object, and synchronization control locks the object in the project database so that no updates can be made to it until the currently checked out version has been replaced. Note that other copies can be checked-out, but other updates cannot be made. A copy of the base-lined object, called the extracted version, is modified by the software engineer. After appropriate SQA and testing, the modified version of the object is checked in and the new baseline object is unlocked.

D. CONFIGURATION AUDIT A software configuration audit complements the formal technical review by assessing a configuration object for characteristics that are generally not considered during review. The audit asks and answers the following questions: 1. Has the change specified in the ECO been made? Have any additional modifications been incorporated? 2. Has a formal technical review been conducted to assess technical correctness? 3. Has the software process been followed and have software engineering standards been properly applied? 4. Has the change been "highlighted" in the SCI? Have the change date and change author been specified? Do the attributes of the configuration object reflect the change? 5. Have SCM procedures for noting the change, recording it, and reporting it been followed? 6. Have all related SCIs been properly updated? In some cases, the audit questions are asked as part of a formal technical review. However, when SCM is a formal activity, the SCM audit is conducted separately by the quality assurance group.

E. STATUS REPORTING Configuration status reporting (sometimes called status accounting) is an SCM task that answers the following questions: (1) What happened? (2) Who did it? (3) When did it happen? (4) What else will be affected? The flow of information for configuration status reporting (CSR) is illustrated in Figure 9.5. Each time an SCI is assigned new or updated identification, a CSR entry is made. Each time a change is approved by the CCA (i.e., an ECO is issued), a CSR entry is made. Each time a configuration audit is conducted, the results are reported

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Chapter 10 : System Engineering 1. Short Note : Computer Based System â&#x20AC;&#x153;A set or arrangement of elements that are organized to accomplish some predefined goal by processing information.â&#x20AC;? The goal may be to support some business function or to develop a product that can be sold to generate business revenue. To accomplish the goal, a computer-based system makes use of a variety of system elements: Software. Computer programs, data structures, and related documentation that serve to effect the logical method, procedure, or control that is required. Hardware. Electronic devices that provide computing capability, the interconnectivity devices (e.g., network switches, telecommunications devices) that enable the flow of data, and electromechanical devices (e.g., sensors, motors, pumps) that provide external world function. People. Users and operators of hardware and software. Database. A large, organized collection of information that is accessed via software. Documentation. Descriptive information (e.g., hardcopy manuals, on-line help files, Web sites) that portrays the use and/or operation of the system. Procedures. The steps that define the specific use of each system element or the procedural context in which the system resides. One complicating characteristic of computer-based systems is that the elements constituting one system may also represent one macro element of a still larger system. The macro element is a computer-based system that is one part of a larger computer-based system. As an example, we consider a "factory automation system" that is essentially a hierarchy of systems. At the lowest level of the hierarchy we have a numerical control machine, robots, and data entry devices. Each is a computer based system in its own right. The elements of the numerical control machine include electronic and electromechanical hardware (e.g., processor and memory, motors, sensors), software (for communications, machine control, interpolation), people (the machine operator), a database (the stored NC program), documentation, and procedures. At the next level in the hierarchy, a manufacturing cell is defined. The manufacturing cell is a computer-based system that may have elements of its own (e.g., computers, mechanical fixtures) and also integrates the macro elements that we have called numerical control machine, robot, and data entry device. To summarize, the manufacturing cell and its macro elements each are composed of system elements with the generic labels: software, hardware, people, database, procedures, and documentation. In some cases, macro elements may share a generic element. For example, the robot and the NC machine both might be managed by a single operator (the people element). In other cases, generic elements are exclusive to one system.

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Chapter 13 : Design Concepts & Principles 1. Explain Effective Modular Design Alsoâ&#x20AC;Ś Explain Coupling and/or Cohesion Modularity has become an accepted approach in all engineering disciplines. A modular design reduces complexity, facilitates change (a critical aspect of software maintainability), and results in easier implementation by encouraging parallel development of different parts of a system.

1) Functional Independence The concept of functional independence is a direct outgrowth of modularity and the concepts of abstraction and information hiding. Functional independence is achieved by developing modules with "single-minded" function and an "aversion" to excessive interaction with other modules. Stated another way, we want to design software so that each module addresses a specific sub-function of requirements and has a simple interface when viewed from other parts of the program structure. It is fair to ask why independence is important. Software with effective modularity, that is, independent modules, is easier to develop because function may be compartmentalized and interfaces are simplified (consider the ramifications when development is conducted by a team). Independent modules are easier to maintain (and test) because secondary effects caused by design or code modification are limited, error propagation is reduced, and reusable modules are possible. To summarize, functional independence is a key to good design, and design is the key to software quality. Independence is measured using two qualitative criteria: cohesion and coupling. Cohesion is a measure of the relative functional strength of a module. Coupling is a measure of the relative interdependence among modules.

2) Cohesion Cohesion is a natural extension of the information hiding concept. A cohesive module performs a single task within a software procedure, requiring little interaction with procedures being performed in other parts of a program. Stated simply, a cohesive module should (ideally) do just one thing. Cohesion may be represented as a "spectrum." We always strive for high cohesion, although the mid-range of the spectrum is often acceptable. The scale for cohesion is nonlinear. That is, low-end cohesiveness is much "worse" than middle range, which is nearly as "good" as high-end cohesion. In practice, a designer need not be concerned with categorizing cohesion in a specific module. Rather, the overall concept should be understood and low levels of cohesion should be avoided when modules are designed.

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At the low (undesirable) end of the spectrum, we encounter a module that performs a set of tasks that relate to each other loosely, if at all. Such modules are termed coincidentally cohesive. A module that performs tasks that are related logically (e.g., a module that produces all output regardless of type) is logically cohesive. When a module contains tasks that are related by the fact that all must be executed with the same span of time, the module exhibits temporal cohesion. As an example of low cohesion, consider a module that performs error processing for an engineering analysis package. The module is called when computed data exceed prespecified bounds. It performs the following tasks for subsequent processing : (1) (2) (3) (4) (5)

computes supplementary data based on original computed data, produces an error report (with graphical content) on the user's workstation, performs follow-up calculations requested by the user, updates a database, enables menu selection

Although the preceding tasks are loosely related, each is an independent functional entity that might best be performed as a separate module. Combining the functions into a single module can serve only to increase the likelihood of error propagation when a modification is made to one of its processing tasks. Moderate levels of cohesion are relatively close to one another in the degree of module independence. When processing elements of a module are related and must be executed in a specific order, procedural cohesion exists. When all processing elements concentrate on one area of a data structure, communicational cohesion is present. High cohesion is characterized by a module that performs one distinct procedural task. As we have already noted, it is unnecessary to determine the precise level of cohesion. Rather it is important to strive for high cohesion and recognize low cohesion so that software design can be modified to achieve greater functional independence.

3) Coupling Coupling is a measure of interconnection among modules in a software structure. Coupling depends on the interface complexity between modules, the point at which entry or reference is made to a module, and what data pass across the interface. In software design, we strive for lowest possible coupling. Simple connectivity among modules results in software that is easier to understand and less prone to a "ripple effect", caused when errors occur at one location and propagate through a system.

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Figure 13.6 provides examples of different types of module coupling. Modules a and d are subordinate to different modules. Each is unrelated and therefore no direct coupling occurs. Module c is subordinate to module a and is accessed via a conventional argument list, through which data are passed. As long as a simple argument list is present (i.e., simple data are passed; a one-to-one correspondence of items exists), low coupling (called data coupling) is exhibited in this portion of structure. A variation of data coupling, called stamp coupling, is found when a portion of a data structure (rather than simple arguments) is passed via a module interface. This occurs between modules b and a. At moderate levels, coupling is characterized by passage of control between modules. Control coupling is very common in most software designs and is shown in Figure 13.6 where a â&#x20AC;&#x153;control flagâ&#x20AC;? (a variable that controls decisions in a subordinate or superordinate module) is passed between modules d and e. Relatively high levels of coupling occur when modules are tied to an environment external to software. For example, I/O couples a module to specific devices, formats, and communication protocols. External coupling is essential, but should be limited to a small number of modules with a structure. High coupling also occurs when a number of modules reference a global data area. Common coupling, as this mode is called, is shown in Figure 13.6. Modules c, g, and k each access a data item in a global data area (e.g., a disk file or a globally accessible memory area). Module c initializes the item. Later module g re-computes and updates the item. Let's assume that an error occurs and g updates the item incorrectly. Much later in processing module, k reads the item, attempts to process it, and fails, causing the software to abort. The apparent cause of abort is module k; the actual cause, module g. Diagnosing problems in structures with considerable common coupling is time consuming and difficult. However, this does not mean that the use of global data is necessarily "bad." It does mean that a software designer must be aware of potential consequences of common coupling and take special care to guard against them. The highest degree of coupling, content coupling, occurs when one module makes use of data or control information maintained within the boundary of another module. Secondarily, content coupling occurs when branches are made into the middle of a module. This mode of coupling can and should be avoided. The coupling modes just discussed occur because of design decisions made when structure was developed. Variants of external coupling, however, may be introduced during coding. For example, compiler coupling ties source code to specific (and often nonstandard) attributes of a compiler; operating system (OS) coupling ties design and resultant code to operating system "hooks" that can create havoc when OS changes occur.

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1. Explain Design Heuristic For Effective Modularity Once program structure has been developed, effective modularity can be achieved by applying the various design concepts. The program structure can be manipulated according to the following set of heuristics : 1. Evaluate the "first iteration" of the program structure to reduce coupling and improve cohesion. Once the program structure has been developed, modules may be exploded or imploded with an eye toward improving module independence. An exploded module becomes two or more modules in the final program structure. An imploded module is the result of combining the processing implied by two or more modules. An exploded module often results when common processing exists in two or more modules and can be redefined as a separate cohesive module. When high coupling is expected, modules can sometimes be imploded to reduce passage of control, reference to global data, and interface complexity.

2. Attempt to minimize structures with high fan-out; strive for fan-in as depth increases. The structure shown inside the cloud in Figure 13.7 does not make effective use of factoring. All modules are â&#x20AC;&#x153;pancakedâ&#x20AC;? below a single control module. In general, a more reasonable distribution of control is shown in the upper structure. The structure takes an oval shape, indicating a number of layers of control and highly utilitarian modules at lower levels. 3. Keep the scope of effect of a module within the scope of control of that module. The scope of effect of module e is defined as all other modules that are affected by a decision made in module e. The scope of control of module e is all modules that are subordinate and ultimately subordinate to module e. Referring to Figure 13.7, if module e makes a decision that affects module r, we have a violation of this heuristic, because module r lies outside the scope of control of module e. 4. Evaluate module interfaces to reduce complexity and redundancy and improve consistency. Module interface complexity is a prime cause of software errors. Interfaces should be designed to pass information simply and should be consistent with the function of a module.

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Interface inconsistency (i.e., seemingly unrelated data passed via an argument list or other technique) is an indication of low cohesion. The module in question should be reevaluated. 5. Define modules whose function is predictable, but avoid modules that are overly restrictive. A module is predictable when it can be treated as a black box; that is, the same external data will be produced regardless of internal processing details.7 Modules that have internal "memory" can be unpredictable unless care is taken in their use. A module that restricts processing to a single sub-function exhibits high cohesion and is viewed with favor by a designer. However, a module that arbitrarily restricts the size of a local data structure, options within control flow, or modes of external interface will invariably require maintenance to remove such restrictions. 6. Strive for â&#x20AC;&#x153;controlled entryâ&#x20AC;? modules by avoiding "pathological connections." This design heuristic warns against content coupling. Software is easier to understand and therefore easier to maintain when module interfaces are constrained and controlled. Pathological connection refers to branches or references into the middle of a module.

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Chapter 17 : Software Testing Technique 1. Explain White-box Technique White-box testing, sometimes called glass-box testing, is a test case design method that uses the control structure of the procedural design to derive test cases. Using white-box testing methods, the software engineer can derive test cases that (1) guarantee that all independent paths within a module have been exercised at least once, (2) exercise all logical decisions on their true and false sides, (3) execute all loops at their boundaries and within their operational bounds, (4) exercise internal data structures to ensure their validity. We need White-box testing because : ď&#x201A;ˇ

Logic errors and incorrect assumptions are inversely proportional to the probability that a program path will be executed. Errors tend to creep into our work when we design and implement function, conditions, or controls that are out of the mainstream. Everyday processing tends to be well understood (and well scrutinized), while "special case" processing tends to fall into the cracks.

ď&#x201A;ˇ

We often believe that a logical path is not likely to be executed when, in fact, it may be executed on a regular basis. The logical flow of a program is sometimes counterintuitive, meaning that our unconscious assumptions about flow of control and data may lead us to make design errors that are uncovered only once path testing commences.

ď&#x201A;ˇ

Typographical errors are random. When a program is translated into programming language source code, it is likely that some typing errors will occur. Many will be uncovered by syntax and type checking mechanisms, but others may go undetected until testing begins. It is as likely that a typo will exist on an obscure logical path as on a mainstream path.

Each of these reasons provides an argument for conducting white-box tests. Black-box testing, no matter how thorough, may miss the kinds of errors noted here. White-box testing is far more likely to uncover them.

2. Explain Cyclomatic Complexity Cyclomatic complexity is a software metric that provides a quantitative measure of the logical complexity of a program. When used in the context of the basis path testing method, the value computed for cyclomatic complexity defines the number of independent paths in the basis set of a program and provides us with an upper bound for the number of tests that must be conducted to ensure that all statements have been executed at least once. An independent path is any path through the program that introduces at least one new set of processing statements or a new condition. When stated in terms of a flow graph, an independent path must move along at least one edge that has not been traversed before the path is defined.

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For example, a set of independent paths for the flow graph illustrated in Figure 17.2B is path 1: 1-11 path 2: 1-2-3-4-5-10-1-11 path 3: 1-2-3-6-8-9-10-1-11 path 4: 1-2-3-6-7-9-10-1-11 Note that each new path introduces a new edge. The path 1-2-3-4-5-10-1-2-3-6-8-9-10-1-11 is not considered to be an independent path because it is simply a combination of already specified paths and does not traverse any new edges. Paths 1, 2, 3, and 4 constitute a basis set for the flow graph in Figure 17.2B. That is, if tests can be designed to force execution of these paths (a basis set), every statement in the program will have been guaranteed to be executed at least one time and every condition will have been executed on its true and false sides. It should be noted that the basis set is not unique. In fact, a number of different basis sets can be derived for a given procedural design. How do we know how many paths to look for? The computation of cyclomatic complexity provides the answer. Cyclomatic complexity has a foundation in graph theory and provides us with an extremely useful software metric. Complexity is computed in one of three ways: 1. The number of regions of the flow graph correspond to the cyclomatic complexity. 2. Cyclomatic complexity, V(G), for a flow graph, G, is defined as V(G) = E _ N + 2 where E is the number of flow graph edges, N is the number of flow graphnodes. 3. Cyclomatic complexity, V(G), for a flow graph, G, is also defined as V(G) = P + 1 where P is the number of predicate nodes contained in the flow graph G.

Referring once more to the flow graph in Figure 17.2B, the cyclomatic complexity can be computed using each of the algorithms just noted:

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1. The flow graph has four regions. 2. V(G) = 11 edges _ 9 nodes + 2 = 4. 3. V(G) = 3 predicate nodes + 1 = 4. Therefore, the cyclomatic complexity of the flow graph in Figure 17.2B is 4. More important, the value for V(G) provides us with an upper bound for the number of independent paths that form the basis set and, by implication, an upper bound on the number of tests that must be designed and executed to guarantee coverage of all program statements.

3. Explain Black-box Testing

(Never asked so far… )

Black-box testing, also called behavioral testing, focuses on the functional requirements of the software. That is, black-box testing enables the software engineer to derive sets of input conditions that will fully exercise all functional requirements for a program. Black-box testing is not an alternative to white-box techniques. Rather, it is a complementary approach that is likely to uncover a different class of errors than white-box methods. Black-box testing attempts to find errors in the following categories: (1) incorrect or missing functions, (2) interface errors, (3) errors in data structures or external data base access, (4) behavior or performance errors, (5) initialization and termination errors. Unlike white-box testing, which is performed early in the testing process, black-box testing tends to be applied during later stages of testing. Because black-box testing purposely disregards control structure, attention is focused on the information domain. Tests are designed to answer the following questions: • How is functional validity tested? • How is system behavior and performance tested? • What classes of input will make good test cases? • Is the system particularly sensitive to certain input values? • How are the boundaries of a data class isolated? • What data rates and data volume can the system tolerate? • What effect will specific combinations of data have on system operation? By applying black-box techniques, we derive a set of test cases that satisfy the following Criteria : (1) test cases that reduce, by a count that is greater than one, the number of additional test cases that must be designed to achieve reasonable testing. (2) test cases that tell us something about the presence or absence of classes of errors, rather than an error associated only with the specific test at hand.

Black-box testing methods :     

Graph-Based Testing Methods Equivalence Partitioning Boundary Value Analysis Comparison Testing Orthogonal Array Testing

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Chapter 18 : Software Testing Strategies 1. Explain Software Testing Strategies A strategy for software testing integrates software test case design methods into a wellplanned series of steps that result in the successful construction of software. The strategy provides a road map that describes the steps to be conducted as part of testing, when these steps are planned and then undertaken, and how much effort, time, and resources will be required. Therefore, any testing strategy must incorporate test planning, test case design, test execution, and resultant data collection and evaluation. A software testing strategy should be flexible enough to promote a customized testing approach. At the same time, it must be rigid enough to promote reasonable planning and management tracking as the project progresses. The following issues must be addressed if a successful software testing strategy is to be implemented: Specify product requirements in a quantifiable manner long before testing commences. Although the overriding objective of testing is to find errors, a good testing strategy also assesses other quality characteristics such as portability, maintainability, and usability. These should be specified in a way that is measurable so that testing results are unambiguous. State testing objectives explicitly. The specific objectives of testing should be stated in measurable terms. For example, test effectiveness, test coverage, mean time to failure, the cost to find and fix defects, remaining defect density or frequency of occurrence, and test work-hours per regression test all should be stated within the test plan. Understand the users of the software and develop a profile for each user category. Use-cases that describe the interaction scenario for each class of user can reduce overall testing effort by focusing testing on actual use of the product. Develop a testing plan that emphasizes “rapid cycle testing.” It is recommended that a software engineering team “learn to test in rapid cycles (2 percent of project effort) of customer-useful, at least field ‘trialable,’ increments of functionality and/or quality improvement.” The feedback generated from these rapid cycle tests can be used to control quality levels and the corresponding test strategies. Build “robust” software that is designed to test itself. Software should be designed in a manner that uses anti-bugging techniques. That is, software should be capable of diagnosing certain classes of errors. In addition, the design should accommodate automated testing and regression testing. Use effective formal technical reviews as a filter prior to testing. Formal technical reviews can be as effective as testing in uncovering errors. For this reason, reviews can reduce the amount of testing effort that is required to produce high-quality software. Conduct formal technical reviews to assess the test strategy and test cases themselves. Formal technical reviews can uncover inconsistencies, omissions, and outright errors in the testing approach. This saves time and also improves product quality. Develop a continuous improvement approach for the testing process. The test strategy should be measured. The metrics collected during testing should be used as part of a statistical process control approach for software testing.

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3. UNIT TESTING Unit testing focuses verification effort on the smallest unit of software designâ&#x20AC;&#x201D;the software component or module. Using the component-level design description as a guide, important control paths are tested to uncover errors within the boundary of the module. The relative complexity of tests and uncovered errors is limited by the constrained scope established for unit testing. The unit test is white-box oriented, and the step can be conducted in parallel for multiple components. The module interface is tested to ensure that information properly flows into and out of the program unit under test. The local data structure is examined to ensure that data stored temporarily maintains its integrity during all steps in an algorithm's execution. Boundary conditions are tested to ensure that the module operates properly at boundaries established to limit or restrict processing. All independent paths (basis paths) through the control structure are exercised to ensure that all statements in a module have been executed at least once. And finally, all error handling paths are tested.

Because a component is not a stand-alone program, driver and/or stub software must be developed for each unit test. The unit test environment is illustrated in Figure 18.5. In most applications a driver is nothing more than a "main program" that accepts test case data, passes such data to the component (to be tested), and prints relevant results. Stubs serve to replace modules that are subordinate (called by) the component to be tested. A stub or "dummy subprogram" uses the subordinate module's interface, may do minimal data manipulation, prints verification of entry, and returns control to the module undergoing testing. Unit testing is simplified when a component with high cohesion is designed. When only one function is addressed by a component, the number of test cases is reduced and errors can be more easily predicted and uncovered.

3. INTEGRATION TESTING : Integration testing is a systematic technique for constructing the program structure while at the same time conducting tests to uncover errors associated with interfacing. The objective is to take unit tested components and build a program structure that has been dictated by design. There is often a tendency to attempt non-incremental integration; that is, to construct the program using a "big bang" approach. All components are combined in advance. The entire program is tested as a whole. And chaos usually results! A set of errors is encountered. Correction is difficult because isolation of causes is complicated by the vast expanse of the entire program. Once these errors are corrected, new ones appear and the process continues in a seemingly endless loop.

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Incremental integration is the antithesis of the big bang approach. The program is constructed and tested in small increments, where errors are easier to isolate and correct; interfaces are more likely to be tested completely; and a systematic test approach may be applied.

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Top-down Integration

Top-down integration testing is an incremental approach to construction of program structure. Modules are integrated by moving downward through the control hierarchy, beginning with the main control module (main program). Modules subordinate (and ultimately subordinate) to the main control module are incorporated into the structure in either a depthfirst or breadth-first manner.

The integration process is performed in a series of five steps: 1. The main control module is used as a test driver and stubs are substituted for all components directly subordinate to the main control module. 2. Depending on the integration approach selected (i.e., depth or breadth first), subordinate stubs are replaced one at a time with actual components. 3. Tests are conducted as each component is integrated. 4. On completion of each set of tests, another stub is replaced with the real component. 5. Regression testing may be conducted to ensure that new errors have not been introduced. The process continues from step 2 until the entire program structure is built. The top-down integration strategy verifies major control or decision points early in the test process. In a wellfactored program structure, decision making occurs at upper levels in the hierarchy and is therefore encountered first. If major control problems do exist, early recognition is essential. If depth-first integration is selected, a complete function of the software may be implemented and demonstrated.

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Bottom-up Integration

Bottom-up integration testing, as its name implies, begins construction and testing with atomic modules (i.e., components at the lowest levels in the program structure). Because components are integrated from the bottom up, processing required for components subordinate to a given level is always available and the need for stubs is eliminated.

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A bottom-up integration strategy may be implemented with the following steps: 1. Low-level components are combined into clusters (sometimes called builds) that perform a specific software sub-function. 2. A driver (a control program for testing) is written to coordinate test case input and output. 3. The cluster is tested. 4. Drivers are removed and clusters are combined moving upward in the program structure. Integration follows the pattern illustrated in Figure 18.7. Components are combined to form clusters 1, 2, and 3. Each of the clusters is tested using a driver (shown as a dashed block). Components in clusters 1 and 2 are subordinate to Ma. Drivers D1 and D2 are removed and the clusters are interfaced directly to Ma. Similarly, driver D3 for cluster 3 is removed prior to integration with module Mb. Both Ma and Mb will ultimately be integrated with component Mc, and so forth.

As integration moves upward, the need for separate test drivers lessens. In fact, if the top two levels of program structure are integrated top down, the number of drivers can be reduced substantially and integration of clusters is greatly simplified.

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Regression Testing

Each time a new module is added as part of integration testing, the software changes. New data flow paths are established, new I/O may occur, and new control logic is invoked. These changes may cause problems with functions that previously worked flawlessly. In the context of an integration test strategy, regression testing is the re-execution of some subset of tests that have already been conducted to ensure that changes have not propagated unintended side effects. In a broader context, successful tests (of any kind) result in the discovery of errors, and errors must be corrected. Whenever software is corrected, some aspect of the software configuration (the program, its documentation, or the data that support it) is changed. Regression testing is the activity that helps to ensure that changes (due to testing or for other reasons) do not introduce unintended behavior or additional errors.

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Regression testing may be conducted manually, by re-executing a subset of all test cases or using automated capture/playback tools. Capture/playback tools enable the software engineer to capture test cases and results for subsequent playback and comparison. The regression test suite (the subset of tests to be executed) contains three different classes of test cases: • A representative sample of tests that will exercise all software functions. • Additional tests that focus on software functions that are likely to be affected by the change. • Tests that focus on the software components that have been changed. As integration testing proceeds, the number of regression tests can grow quite large. Therefore, the regression test suite should be designed to include only those tests that address one or more classes of errors in each of the major program functions. It is impractical and inefficient to re-execute every test for every program function once a change has occurred.

Smoke Testing

Smoke testing is an integration testing approach that is commonly used when “shrinkwrapped” software products are being developed. It is designed as a pacing mechanism for time-critical projects, allowing the software team to assess its project on a frequent basis. In essence, the smoke testing approach encompasses the following activities: 1. Software components that have been translated into code are integrated into a “build.” A build includes all data files, libraries, reusable modules, and engineered components that are required to implement one or more product functions. 2. A series of tests is designed to expose errors that will keep the build from properly performing its function. The intent should be to uncover “show stopper” errors that have the highest likelihood of throwing the software project behind schedule. 3. The build is integrated with other builds and the entire product (in its current form) is smoke tested daily. The integration approach may be top down or bottom up. The daily frequency of testing the entire product may surprise some readers. However, frequent tests give both managers and practitioners a realistic assessment of integration testing progress. “The smoke test should exercise the entire system from end to end. It does not have to be exhaustive, but it should be capable of exposing major problems. The smoke test should be thorough enough that if the build passes, you can assume that it is stable enough to be tested more thoroughly.

3. VALIDATION TESTING At the culmination of integration testing, software is completely assembled as a package, interfacing errors have been uncovered and corrected, and a final series of software tests—validation testing—may begin.

Validation Test Criteria

Software validation is achieved through a series of black-box tests that demonstrate conformity with requirements.

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After each validation test case has been conducted, one of two possible conditions exist: (1) The function or performance characteristics conform to specification and are accepted or (2) A deviation from specification is uncovered and a deficiency list is created. Deviation or error discovered at this stage in a project can rarely be corrected prior to scheduled delivery. It is often necessary to negotiate with the customer to establish a method for resolving deficiencies.

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Configuration Review

An important element of the validation process is a configuration review. The intent of the review is to ensure that all elements of the software configuration have been properly developed, are cataloged, and have the necessary detail to bolster the support phase of the software life cycle. The configuration review is sometimes called an Audit.

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Alpha and Beta Testing

The alpha test is conducted at the developer's site by a customer. The software is used in a natural setting with the developer "looking over the shoulder" of the user and recording errors and usage problems. Alpha tests are conducted in a controlled environment. The beta test is conducted at one or more customer sites by the end-user of the software. Unlike alpha testing, the developer is generally not present. Therefore, the beta test is a "live" application of the software in an environment that cannot be controlled by the developer. The customer records all problems (real or imagined) that are encountered during beta testing and reports these to the developer at regular intervals. As a result of problems reported during beta tests, software engineers make modifications and then prepare for release of the software product to the entire customer base.

2. SYSTEM TESTING A classic system testing problem is "finger-pointing." This occurs when an error is uncovered, and each system element developer blames the other for the problem. Rather than indulging in such nonsense, the software engineer should anticipate potential interfacing problems and (1) design error-handling paths that test all information coming from other elements of the system, (2) conduct a series of tests that simulate bad data or other potential errors at the software interface, (3) record the results of tests to use as "evidence" if finger-pointing does occur, (4) participate in planning and design of system tests to ensure that software is adequately tested. System testing is actually a series of different tests whose primary purpose is to fully exercise the computer-based system. Although each test has a different purpose, all work to verify that system elements have been properly integrated and perform allocated functions.

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Recovery Testing

Recovery testing is a system test that forces the software to fail in a variety of ways and verifies that recovery is properly performed. If recovery is automatic (performed by the system itself), re-initialization, check-pointing mechanisms, data recovery, and restart are evaluated for correctness. If recovery requires human intervention, the mean-time-to-repair (MTTR) is evaluated to determine whether it is within acceptable limits.

Security Testing

Security testing attempts to verify that protection mechanisms built into a system will, in fact, protect it from improper penetration. During security testing, the tester plays the role(s) of the individual who desires to penetrate the system. Anything goes! The tester may attempt to acquire passwords through external clerical means; may attack the system with custom software designed to breakdown any defenses that have been constructed; may overwhelm the system, thereby denying service to others; may purposely cause system errors, hoping to penetrate during recovery; may browse through insecure data, hoping to find the key to system entry. Given enough time and resources, good security testing will ultimately penetrate a system. The role of the system designer is to make penetration cost more than the value of the information that will be obtained.

Stress Testing

Stress tests are designed to confront programs with abnormal situations. In essence, the tester who performs stress testing asks: "How high can we crank this up before it fails?" Stress testing executes a system in a manner that demands resources in abnormal quantity, frequency, or volume. For example, (1) special tests may be designed that generate ten interrupts per second, when one or two is the average rate, (2) input data rates may be increased by an order of magnitude to determine how input functions will respond, (3) test cases that require maximum memory or other resources are executed, (4) test cases that may cause thrashing in a virtual operating system are designed, (5) test cases that may cause excessive hunting for disk-resident data are created. Essentially, the tester attempts to break the program.

Performance Testing

For real-time and embedded systems, software that provides required function but does not conform to performance requirements is unacceptable. Performance testing is designed to test the run-time performance of software within the context of an integrated system. Performance testing occurs throughout all steps in the testing process. Even at the unit level, the performance of an individual module may be assessed as white-box tests are conducted. However, it is not until all system elements are fully integrated that the true performance of a system can be ascertained.

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Chapter 19 : Technical Metrics For Software 1. Explain McCallâ&#x20AC;&#x2122;s Software Quality Factors McCallâ&#x20AC;&#x2122;s Quality Factors The factors that affect software quality can be categorized in two broad groups: (1) factors that can be directly measured (e.g., defects per function-point) (2) factors that can be measured only indirectly (e.g., usability or maintainability). Referring to the factors noted in the figure, McCall and his colleagues provide the following descriptions:

Correctness. The extent to which a program satisfies its specification and fulfills the customer's mission objectives. Reliability. The extent to which a program can be expected to perform its intended function with required precision. [It should be noted that other, more complete definitions of reliability have been proposed. Efficiency. The amount of computing resources and code required by a program to perform its function. Integrity. Extent to which access to software or data by unauthorized persons can be controlled. Usability. Effort required to learn, operate, prepare input, and interpret output of a program. Maintainability. Effort required to locate and fix an error in a program. Flexibility. Effort required to modify an operational program.

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Testability. Effort required to test a program to ensure that it performs its intended function. Portability. Effort required to transfer the program from one hardware and/or software system environment to another. Reusability. Extent to which a program [or parts of a program] can be reused in other applications â&#x20AC;&#x201D; related to the packaging and scope of the functions that the program performs. Interoperability. Effort required to couple one system to another.

It is difficult, and in some cases impossible, to develop direct measures of these quality factors. Therefore, a set of metrics are defined and used to develop expressions for each of the factors according to the following relationship: Fq = c1 _ m1 + c2 _ m2 + . . . + cn _ mn where Fq is a software quality factor, cn are regression coefficients, mn are the metrics that affect the quality factor. The following metrics are used in the grading scheme: Auditability. The ease with which conformance to standards can be checked. Accuracy. The precision of computations and control. Communication commonality. The degree to which standard interfaces, protocols, and bandwidth are used. Completeness. The degree to which full implementation of required function has been achieved. Conciseness. The compactness of the program in terms of lines of code. Consistency. The use of uniform design and documentation techniques throughout the software development project. Data commonality. The use of standard data structures and types throughout the program. Error tolerance. The damage that occurs when the program encounters an error. Execution efficiency. The run-time performance of a program. Expandability. The degree to which architectural, data, or procedural design can be extended. Generality. The breadth of potential application of program components.

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Hardware independence. The degree to which the software is decoupled from the hardware on which it operates. Instrumentation. The degree to which the program monitors its own operation and identifies errors that do occur. Modularity. The functional independence of program components. Operability. The ease of operation of a program. Security. The availability of mechanisms that control or protect programs and data. Self-documentation. The degree to which the source code provides meaningful documentation. Simplicity. The degree to which a program can be understood without difficulty. Software system independence. The degree to which the program is independent of non-standard programming language features, operating system characteristics, and other environmental constraints. Traceability. The ability to trace a design representation or actual program component back to requirements. Training. The degree to which the software assists in enabling new users to apply the system.

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These notes are for quick & selective study. For complete understanding you must refer Software Engineering A Practitioner’s Approach APRACTITIONECH FIFTH EDITION

- Roger S. Pressman

All The Best… - Gaurav Sharma

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Software engineering notes