Values that must expire could perhaps use a truncated time value as part of the key? It's not as flexible as regular expiration, because the entries would only

Values that must expire could perhaps use a truncated time value as part of the key? It's not as flexible as regular expiration, because the entries would only expire when you cross the point in time where the truncated time value changes, which could be a problem in a lot of applications (sudden surges of requests every x seconds)