those that have been provided to at least one request and have not yet
 been released. Once all consumers of a resource have released that resource, the resource then
 goes to cache. If the resource is ever returned to a new consumer from cache, it is re-added to
 the active resources. If the resource is evicted from the cache, its resources are recycled and
 re-used if possible and the resource is discarded. There is no strict requirement that
 consumers release their resources so active resources are held