A subclass of ReplicaPlacementPolicyDefault, ReplicaPlacementPolicyNodeGroup was developed along with unit tests to support the four-layer hierarchical topology.
The replica placement strategy used in ReplicaPlacementPolicyNodeGroup virtualization is almost the same as the original one. The differences are:
1. The 3rd replica will be off node group of the 2nd replica
2. If there is no local node available, the 1st replica will be placed on a node in the local node group.