code: public void enqueue(final ExecutableFlow exflow, final ExecutionReference ref)
        throws ExecutorManagerException {
      if (hasExecution(exflow.getExecutionId())) {
        final String errMsg = "Flow already in queue " + exflow.getExecutionId();
        throw new ExecutorManagerException(errMsg);
      }
  
      final Pair<ExecutionReference, ExecutableFlow> pair =
          new Pair<>(ref, exflow);
      try {
        this.queuedFlowMap.put(exflow.getExecutionId(), pair);
        this.queuedFlowList.put(pair);
      } catch (final InterruptedException e) {
        final String errMsg = "Failed to insert flow " + exflow.getExecutionId();
        logger.error(errMsg, e);
        throw new ExecutorManagerException(errMsg);
      }
    }
docs: <pre>
  Helper method to have a single point of insertion in the queued flows
  
  @param exflow
  flow to be enqueued
  @param ref
  reference to be enqueued
  @throws ExecutorManagerException
  case 1: if blocking queue put method fails due to
  InterruptedException
  case 2: if there already an element with
  same execution Id
  </pre>