Standard capabilities are:

• store_objects - Backend can directly store & return objects. Whether the backend itself really can (like APCu) or the backend class takes care of this.
• compress - Can compress objects being stored. If the backend can do this, then we don't want the frontend to ever do it.
• array - Can return all elements in the cache as an associative array
• expire_ns - Backend supports storage TTLs on the namespace as a whole.
• expire_val - Backend supports storage TTLs on individual values stored within the namespace
• keepalive - When a value is accessed it's TTL can be reset to keep it alive in cache.