private void updateBackupMasters() throws InterruptedIOException {
    backupMasters =
      ImmutableList.copyOf(MasterAddressTracker.getBackupMastersAndRenewWatch(watcher));
  }
@param watcher ZK watcher
@param sn      ServerName
@param master  In an instance of a Master.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ActiveMasterManager.java