ISSN: 2347-1697 International Journal of Informative & Futuristic Research (IJIFR) Volume - 4, Issue -4, December 2016 Continuous 40th Edition, Page No: 5920-5929 contrast, the latter approach is a pull (PL) operation that needs each cache node to pull the update messages from the server. [2] The source node may or may not maintain the status of its cache nodes. In the Stateful (SF) approach, the source records the information about the cache nodes as well as the data stored in them. In the Stateless (SL) approach server does not maintain the details of the cache nodes. As a substitute, it records an update history (of a reasonable length) and occasionally floods this statistics as an invalidation message, generating network communication overhead. [2] Lease protocol offer a strong consistency level and handles host and communication failures. Lease is a contact, gives its owner quantified rights over possessions for a limited period of time. During the term of lease the server cannot update the data without the consent of its holder. Once the approval for write is granted by the cache node it abolishes its local copy of data. Lease period can be for long or short durations.[5]TTR Mechanism provides a temporal consistency using a pull-based method. Here TTR value is calculated for each cached item and denotes when the server node should be communicated so as to revive the new value of cached data. For severe consistency requirements this value is kept small and vice-versa. Based on user requirements, TTR which can be adaptively varies can also be employed. There is another mechanism where the source node assigns each data a time-to-live field which indicates that until this value expires the cache node a node can rely on the value stored in the cache node. Once the TTL value expires the server node has to be contacted in order to check the consistency. Unlike Lease protocol there is no guarantee that the value will not be updated before the TTL expires. In this mechanism the server need not maintain the state of the cache nodes.[7] In Pull each read/client poll mechanism the cache node communicates the source node for each query it faces. This ensures a strong consistency but if the network between the cache node and source node is broken the read has to be blocked. Moreover this mechanism induces large round trip delay which will contradict the advantage of caching data close to users. In addition, massive network messages have to be flooded in the network to maintain the consistency requirements. [6] In server Invalidation scheme a notification is send to the cache nodes by the server node as and when data is updated. Thus only optimal network messages are floated for maintaining cache consistency. The disadvantage is that server node has to maintain the list of all the cache nodes that have read the object and has to maintain indefinitely. As the number of clients accessing a single data increases the resulting state space becomes huge and ever increasing. Another disadvantage is that for a data which is updated more often more invalidation messages need to be sent which will eventually lead to burst in network traffic. If cache nodes cannot be reached due to any network disconnectivity then source node has to block the data updation or violate consistency warranties. [7] In Invalidation Report (IR)-based Invalidation scheme the source node sends an invalidation report where the changed data items would be indicated. Thus there is no need of querying the server directly, rather the cache nodes can listen can listen to these IRs over wireless channel and use them to authenticate their local cache. The disadvantages of this scheme are
Neethu S. :: Enhanced Flexible Cache Consistency Maintenance Over Wireless Adhoc Networks
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