If all you're doing is keeping a copy of objects in a memory store for quick retrieval you might want to implement a read-through caching pattern using Memcache

If all you're doing is keeping a copy of objects in a memory store for quick retrieval you might want to implement a read-through caching pattern using Memcached (or Redis but I'd turn any persistence off). Most chat clients don't allow for editing of posts, so you could just use Redis as your persistent storage using lists and RPUSH.