SingletonStore is a delegating cache store used for situations when only one instance should interact with the underlying store. The coordinator of the cluster will be responsible for the underlying CacheStore.
SingletonStore is a simply facade to a real CacheStore implementation. It always delegates reads to the real CacheStore.
Writes are delegated
only if this SingletonStore is currently the coordinator. This avoids having all stores in a cluster writing the same data to the same underlying store. Although not incorrect (e.g. a DB will just discard additional INSERTs for the same key, and throw an exception), this will avoid a lot of redundant work.
Whenever the current coordinator dies (or leaves), the second in line will take over. That SingletonStore will then pass writes through to its underlying CacheStore. Optionally, when a new coordinator takes over the Singleton, it can push the in-memory state to the cache cacheStore, within a time constraint.
@author Mircea Markus
@since 6.0