@Override
    public void doHandleChildChange(String parentPath, List<String> currentChildren)
        throws Exception {
      synchronized (mutex) {
        if (currentChildren == null) {
          LOG.warn("handleChildChange on path " + parentPath + " was invoked with NULL list of children");
        } else {
          LOG.info("ParticipantChangeHandler::handleChildChange - Path: {} Current Children: {} ", parentPath, currentChildren);
          mutex.notify();
        }
      }
    }
Listener for changes in children of LOCK
children are the ephemeral nodes created to acquire the lock
samza-core/src/main/java/org/apache/samza/zk/ZkDistributedLock.java