AllocationService is set here so Discovery Service can force shard reallocation immediately after node notifies master about leaving. This change was introduced in 0.20.4 and it eliminates one round of state publishing when a node leaves the cluster. Before this change, the master was first posting the change into list of available nodes and then on the next state update iteration was updating routing table. ZooKeeper discovery is using a different way of handling nodes leaving the cluster, so I am not really sure if this change would be useful or not in this case. I need to think about it.