PagePool and its derived classes are used by servers to generate page
information (name, size, modification time, lifetime, etc.), by caches
to describe which pages are in storage, and by clients to generate a
request stream. Figure provides an overview of
the class hierarchy here.
Among these, class PagePool/Client is mostly used by caches to store pages and other cache-related information; other three classes are used by servers and clients. In the following we describe these classes one by one.