This class implements the functionality required to use simple key/value stores for hierarchical cache groups. key/value stores like memcache or APC do not support wildcards in keys, therefore a cache group cannot be cleared like "a|*" - which is no problem to filesystem and RDBMS implementations.

This implementation is based on the concept of invalidation. While one specific cache can be identified and cleared, any range of caches cannot be identified. For this reason each level of the cache group hierarchy can have its own value in the store. These values are nothing but microtimes, telling us when a particular cache group was cleared for the last time. These keys are evaluated for every cache read to determine if the cache has been invalidated since it was created and should hence be treated as inexistent.

Although deep hierarchies are possible, they are not recommended. Try to keep your cache groups as shallow as possible. Anything up 3-5 parents should be ok. So »a|b|c« is a good depth where »a|b|c|d|e|f|g|h|i|j|k« isn't. Try to join correlating cache groups: if your cache groups look somewhat like »a|b|$page|$items|$whatever« consider using »a|b|c|$page-$items-$whatever« instead.