Per-thread service that caches page instances for the duration of the request, and is also responsible for tracking the active page (the page which will ultimately render the response).
Since {@link org.apache.tapestry5.internal.structure.Page} is internal, most user-code should use the{@link org.apache.tapestry5.services.ComponentSource} service instead.
Starting in 5.2, page instances are shared (with externalized mutable state), not pooled, but the cache is still useful for managing the page's {@linkplain PageLifecycleListener lifecycle}.