Currently 6 types of web caches are implemented, including the base class Http/Cache. Its five derived subclasses implement 5 types of cache consistency algorithms: Plain old TTL, adaptive TTL, Omniscient TTL, Hierarchical multicast invalidation, and hierarchical multicast invalidation plus direct request.
In the following we'll only describe the base class Http/Cache, because all the subclasses involves discussion of cache consistency algorithms and it does not seem to be appropriate here.