private void startLocalCluster() {
   synchronized (lock) {
     if (localCluster == null) {
       org.apache.flink.configuration.Configuration configuration =
           new org.apache.flink.configuration.Configuration();
       configuration.setInteger(
           ConfigConstants.TASK_MANAGER_NUM_TASK_SLOTS, env.getParallelism() * 2);
       localCluster = new LocalFlinkMiniCluster(configuration, false);
       localCluster.start();
     }
     localClusterUsers++;
   }
 }