Class DoubleBarrelLRUCache<TKey, TValue>

Simple concurrent LRU cache, using a "double barrel" approach where two ConcurrentHashMaps record entries.

At any given time, one hash is primary and the other is secondary. Get(TKey) first checks primary, and if that's a miss, checks secondary. If secondary has the entry, it's promoted to primary (NOTE: the key is cloned at this point). Once primary is full, the secondary is cleared and the two are swapped.

This is not as space efficient as other possible concurrent approaches (see LUCENE-2075): to achieve perfect LRU(N) it requires 2*N storage. But, this approach is relatively simple and seems in practice to not grow unbounded in size when under hideously high load.

@lucene.internal