CQRS Documents by Greg Young Impedance Mismatch Using events as a storage mechanism also offers very different properties when compared to a typical relational model when the impedance mismatch that exists between a typical relational model and the object oriented domain model is analyzed. Scott Ambler describes the problem in an essay on agiledata.org as “Why does this impedance mismatch exist? The object-oriented paradigm is based on proven software engineering principles. The relational paradigm, however, is based on proven mathematical principles. Because the underlying paradigms are different the two technologies do not work together seamlessly. The impedance mismatch becomes apparent when you look at the preferred approach to access: With the object paradigm you traverse objects via their relationships whereas with the relational paradigm you join the data rows of tables. This fundamental difference results in a non-ideal combination of object and relational technologies, although when have you ever used two different things together without a few hitches?” (Ambler) The impedance mismatch between the domain model and the relational database has a large cost associated with it. There are many tools that aim to help minimize the effects of the impedance mismatch such as Object Relational Mappers (ORM). They tend to work well in most situations but there is still a fairly large cost associated to the impedance mismatch even when using tools such as ORMs. The cost is that a developer really needs to be intimately with both the relational model and the object oriented model. They also need to be familiar with the many subtle differences between the two models. Scott identifies this with “To succeed using objects and relational databases together you need to understand both paradigms, and their differences, and then make intelligent tradeoffs based on that knowledge.” (Ambler) Some of these subtle differences can be found in Wikipedia under the “Object-Relational Impedance Mismatch” page but to include some of the major differences. Declarative vs. imperative interfaces — Relational thinking tends to use data as interfaces, not behavior as interfaces. It thus has a declarative tilt in design philosophy in contrast to OO's behavioral tilt. (Some relational proponents propose using triggers, stored procedures, etc. to provide complex behavior, but this is not a common viewpoint.) (Object-Relational Impedance Mismatch)
Structure vs. behaviour — OO primarily focuses on ensuring that the structure of the program is reasonable (maintainable, understandable, extensible, reusable, safe), whereas relational systems focus on what kind of behaviour the resulting run-time system has (efficiency, adaptability, fault-tolerance, liveness, logical integrity, etc.). Object-oriented methods generally assume that the primary user of the object-oriented code and its interfaces are the application developers. In relational systems, the endusers' view of the behaviour of the system is sometimes considered to be more important. However, relational queries and "views" are common techniques to re-represent information in application- or http://cqrsinfo.com
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