The key is calculated in lock-step with the object that’s represented in the content. This is commonly done by making a timestamp part of the key, so for example [class]/[id]-[timestamp], like todos/5-20110218104500 or projects/15-20110218104500, which is what Active Record in Rails does by default when you call #cache_key.
When the key changes, you simply write the new content to this new key. So if you update the todo, the key changes from todos/5-20110218104500 to todos/5-20110218105545, and thus the new content is written based on the updated object.
This generates a lot of cache garbage. Once we’ve updated the todo, the old cache will never get read again. The beauty of that system is that you just don’t care. Memcached will automatically evict the oldest keys first when it runs out of space. It can do this because it keeps track of when everything was last read.
