Deferred - A deferred cache save indicates that a cache item may not be persisted immediately by the pool. A Pool object MAY delay persisting a deferred cache item in order to take advantage of bulk-set operations supported by some storage engines. A Pool MUST ensure that any deferred cache items are eventually persisted and data is not lost, and MAY persist them before a Calling Library requests that they be persisted. When a Calling Library invokes the commit() method all outstanding deferred items MUST be persisted. An Implementing Library MAY use whatever logic is appropriate to determine when to persist deferred items, such as an object destructor, persisting all on save(), a timeout or max-items check or any other appropriate logic. Requests for a cache item that has been deferred MUST return the deferred but not-yet-persisted item.