Cooperative Semantic Caching (CoopSC)

Client side caching is a commonly used technique for reducing the response time of database queries. Semantic caching is a database caching approach in which results of old queries are cached and used for answering new queries. A new query will be split in a part that retrieves the portion of the result, which is available in a local cache (probe query) and a query that retrieves missing n-tuples for the database server (remainder query). This approach is especially suited for low-bandwidth environments or when database server is under heavy load. Semantic caching was successfully applied for optimizing the execution of queries on mobile clients or over lousy-coupled wide-area networks.Peer-to-Peer (P2P) networks have been applied successfully for improving the performance of cache systems. In consequence, this project aims to use a P2P approach in order to enhance the semantic caching technique.

In most applications, database servers are queried by multiple clients. When using the classic semantic caching approach, clients store and manage their own local caches independently. If the number of clients is high, the amount of data sent by database server and queries response times can rapidly increase even when caching is used. The performance can be further improved by allowing clients to share their entries in a cooperative way. Another limitation of existing semantic caching solutions is that they do not handle update queries. Modifications performed in the database are not propagated to cache entries stored by clients.

Therefore, the Cooperative Semantic Caching (CoopSC) approach will extend the general semantic caching mechanism by using a P2P (Peer-to-peer) approach in order to enable clients to share their local semantic caches in a cooperative manner. When executing a query, the content of both the local semantic cache and entries stored in caches of other clients can be used. A new query will be split into a probe, remote probes, and a remainder query. The probe retrieves the part of the answer which is available in the local cache. Remote probes retrieve those parts of the query which are available in caches of other clients. The remainder retrieves the missing n-tuples from the server. In order to execute the query rewriting, the cache entries of all clients will be indexed in a distributed data structure built on top of a P2P overlay that is formed by all clients which are interrogating a particular database server. Such an approach increases the performance of databases systems and presents economic advantages when used in a cloud-computing environment. Thus, CoopSC will use P2P technology in order to develop a cooperative caching solution. It will include a suitable and efficient mechanism for handling update queries, it will support select-project queries, where a query predicate is a n-dimensional range condition, it will design a heuristic to dynamically decide when using the cooperative cache is beneficial or not.