@Test
  public void testClientUpdateMessageSerialization() throws Exception {
    // Update Value.
    clientPut(client1, KEY1, VALUE1);
    getLogWriter().fine(">>>>>>>> after clientPut(c1, k1, v1)");
    // Waiting in the client2 till it receives the event for the key.
    checkFromClient(client2);
    getLogWriter().fine("after checkFromClient(c2)");

    // performing check in the regionqueue of the server2
    checkFromServer(server2, KEY1);
    getLogWriter().fine("after checkFromServer(s2, k1)");

    // UPDATE.
    clientPut(client1, KEY1, VALUE1);
    getLogWriter().fine("after clientPut 2 (c1, k1, v1)");
    // Waiting in the client2 till it receives the event for the key.
    checkFromClient(client2);
    getLogWriter().fine("after checkFromClient 2 (c2)");

    // performing check in the regionqueue of the server2
    checkFromServer(server2, KEY1);
    getLogWriter().fine("after checkFromServer 2 (s2, k1)");

    getLogWriter().info("testClientUpdateMessageSerialization() completed successfully");
  }
This is to test the serialization mechanism of ClientUpdateMessage. Added after CQ support.
This could be done in different way, by overflowing the HARegion queue.
geode-cq/src/distributedTest/java/org/apache/geode/internal/cache/ha/HADispatcherDistributedTest.java