Context

I am storing a java.util.List inside ehcache.

Key(String) --> List<UserDetail>

The ordered List contains a Top 10 ranking of my most active users.

Problem

Concurrent 3rd party clients might be requesting for this list. I have a requirement to be as current as possible with regards to the ranking. Thus if the ranking is changed due the activities of users, the ordered List in the cache must not be left stale for very long. Once I've recalculated a new List, I want to replace the one in cache immediately.

Consider a busy scenario whereby multiple concurrent clients are requesting for the ranking; how can I replace the cache item in an fashion such that: Clients can continue to pull a possibly stale snapshot. They should never get a null value.

There will only be 1 server thread that writes to the cache.