private void initialize() {
    int replicaCount = 12;
    List<Port> portList = Collections.singletonList(new Port(PORT, PortType.PLAINTEXT));
    List<String> mountPaths = Collections.singletonList("mockMountPath");
    datanodes = new ArrayList<>(Arrays.asList(new MockDataNodeId(portList, mountPaths, "dc-0"),
        new MockDataNodeId(portList, mountPaths, "dc-1"), new MockDataNodeId(portList, mountPaths, "dc-2"),
        new MockDataNodeId(portList, mountPaths, "dc-3")));
    mockPartition = new MockPartitionId();
    populateReplicaList(replicaCount, ReplicaState.STANDBY);
    localDcName = datanodes.get(0).getDatacenterName();
    mockClusterMap = new MockClusterMap(false, datanodes, 1, Collections.singletonList(mockPartition), localDcName);
  }
Initialize 4 DCs, each DC has 1 data node, which has 3 replicas.
ambry-router/src/test/java/com/github/ambry/router/OperationTrackerTest.java