AllocationModule manages several modules related to the allocation process.
RoutingAllocation keeps a shard which routing allocation has faild
AllocationCommand taken from the Deciders
RoutingAllocation keep the state of the current allocation of shards and holds the
AllocationDeciders which are responsible for the current routing state.
RoutingAllocation
RerouteExplanation explanations.
RoutingAllocation holds a list of started shards within a cluster