Windows Server AppFabric uses expiration and eviction to maintain the size of its cache and available memory on each cache host.
Eviction is usually dependent upon two levels of thresholds called Watermarks. When the memory usage of cache on a particular host reaches its Low Watermark (LWM) then the expired cache items are taken off from the cache. However, when the memory usage of the cache reaches High Watermark (HWM) then cache items are evicted based on LRU, regardless of their expiry status.
Note
The Windows Server AppFabric cache's eviction is based on a Least Recently Used, or LRU, policy.
This eviction continues until cache usage goes under the lower watermark levels. When cache usage is above watermark levels, subsequent caching requests are routed to other hosts in the cache cluster.
Eviction will also occur when the amount of available physical memory on the Windows Server AppFabric cache host is critically low.