@Override
  public void nodeDeleted(final String path) {
    if (path.startsWith(watcher.getZNodePaths().drainingZNode)) {
      final ServerName sn = ServerName.valueOf(ZKUtil.getNodeName(path));
      LOG.info("Draining RS node deleted, removing from list [" + sn + "]");
      remove(sn);
    }
  }
Starts the tracking of draining RegionServers.
<p>
All Draining RSs will be tracked after this method is called.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/DrainingServerTracker.java