public void start() throws KeeperException, IOException {
    watcher.registerListener(this);
    // Add a ServerListener to check if a server is draining when it's added.
    serverManager.registerListener(new ServerListener() {
      @Override
      public void serverAdded(ServerName sn) {
        if (drainingServers.contains(sn)) {
          serverManager.addServerToDrainList(sn);
        }
      }
    });
    List<String> servers =
      ZKUtil.listChildrenAndWatchThem(watcher, watcher.getZNodePaths().drainingZNode);
    add(servers);
  }
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