I like to run a single redis server process on each instance that's serving the webapp, and also have a shared redis cluster for shared cached state (like user sessions). That way there's a fast local cache for people to use for caching API calls and other things where duplication is preferable to latency.
If it's purely ephemeral and not replicated between instances what are you getting from redis that couldn't be done with SQLite or even a good old hashmap in a separate thread?
