Caching over the Entire User-to-Data Path in the Internet

A Web client request traverses four types of Web caches, before the Web server as the origin of the requested document is reached. This client-to-server path is continued to the backend DB server if timely and transaction-consistent data is needed to generate the document. Web caching typically supports access to single Web objects kept ready somewhere in caches up to the server, whereas database caching, applied in the remaining path to the DB data, allows declarative query processing in the cache. Optimization issues in Web caches concern management of documents decomposed into templates and fragments to support dynamic Web documents with reduced network bandwidth usage and server interaction. When fragment-enabled caching of fine-grained objects can be performed in proxy caches close to the client, user-perceived delays may become minimal. On the other hand, database caching uses a full-fledged DBMS as cache manager to adaptively maintain sets of records from a remote database and to evaluate queries on them. Using so-called cache groups, we introduce the new concept of constraint-based database caching. These cache groups are constructed from parameterized cache constraints, and their use is based on the key concepts of value completeness and predicate completeness. We show how cache constraints affect the correctness of query evaluations in the cache and which optimizations they allow. Cache groups supporting practical applications must exhibit controllable load behavior for which we identify necessary conditions. Finally, we comment on future research problems.