@Override
  public void nodeChildrenChanged(final String path) {
    if (path.equals(watcher.getZNodePaths().drainingZNode)) {
      try {
        final List<String> newNodes =
          ZKUtil.listChildrenAndWatchThem(watcher, watcher.getZNodePaths().drainingZNode);
        add(newNodes);
      } catch (KeeperException e) {
        abortable.abort("Unexpected zk exception getting RS nodes", e);
      } catch (IOException e) {
        abortable.abort("Unexpected zk exception getting RS nodes", e);
      }
    }
  }
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