public void subscribeToProcessorChange(IZkChildListener listener) {
    LOG.info("Subscribing for child change at:" + keyBuilder.getProcessorsPath());
    zkClient.subscribeChildChanges(keyBuilder.getProcessorsPath(), listener);
    metrics.subscriptions.inc();
  }
subscribe to the changes in the list of processors in ZK
@param listener - will be called when a processor is added or removed.
samza-core/src/main/java/org/apache/samza/zk/ZkUtils.java