code: public synchronized void submitCancelTaskCommand(long jobId, int taskId, long workerId) {
      CancelTaskCommand.Builder cancelTaskCommand = CancelTaskCommand.newBuilder();
      cancelTaskCommand.setJobId(jobId);
      cancelTaskCommand.setTaskId(taskId);
      JobCommand.Builder command = JobCommand.newBuilder();
      command.setCancelTaskCommand(cancelTaskCommand);
      if (!mWorkerIdToPendingCommands.containsKey(workerId)) {
        mWorkerIdToPendingCommands.put(workerId, Lists.<JobCommand>newArrayList());
      }
      mWorkerIdToPendingCommands.get(workerId).add(command.build());
    }
docs: Submits a cancel-task command to a specified worker.
  
  @param jobId the job id
  @param taskId the task id
  @param workerId the worker id