But the comparisons are not done there. An object cache can be implemented with various storage engines just as you can store your food in various storage devices. You can put your haul in the refrigerator, the freezer, a cabinet, the counter, on shelves, in the pantry, in the basement, etc. You make these decisions based on what storage options you have, how full the storage devices are, your access to additional storage devices, etc. When deciding on a storage engine for your application, you mull over these same decisions. Just like it makes good sense to put your eggs in the refrigerator instead of the cupboard, it might make more sense to hold your page cache in Varnish vs. Memcached; however, sometimes you do not have a refrigerator at your disposal, you have to improvise and store your eggs in a cooler full of ice.