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Writing a ParallelNode

Overview

This tutorial demonstrates the careBT ParallelNode. Therefore the SimpleParallel node is implemented which has three child nodes of the same type (AddTwoNumbersMultiTickAction). With the input parameters, the amount of ticks can be specified that the children take to complete. As the success_threshold is set to two, the SimpleParallel node succeeds in case that two nodes complete with SUCCESS or FIXED, or fails in case one child node completes with FAILURE or ABORTED. By 'playing around' with the input parameters different situation can be provoked.

Create the SimpleParallel node

Create a file named parallel.py with following content. Or use the provided file: :download:`parallel.py <../../carebt/examples/parallel.py>`

.. literalinclude:: ../../carebt/examples/parallel.py
    :language: python
    :lines: 15-
    :linenos:

The code explained

The AddTwoNumbersMultiTickAction node was already introduced in :doc:`Writing long running ActionNodes <longrun_actions>` and is just reused in this example.

The constructor (__init__) of the SimpleParallel needs to call the constructor (super().__init__) of the ParallelNode and passes the bt_runner, the success_threshold and the signature as arguments. The success_threshold is set to two, and the signature defines three input parameter.

.. literalinclude:: ../../carebt/examples/parallel.py
    :language: python
    :lines: 41-42

In the on_init function the three child nodes are added.

.. literalinclude:: ../../carebt/examples/parallel.py
    :language: python
    :lines: 44-47

Run the example

Start the Python interpreter and run the SimpleParallel node:

>>> import carebt
>>> from carebt.examples.parallel import SimpleParallel
>>> bt_runner = carebt.BehaviorTreeRunner()
>>> bt_runner.run(SimpleParallel, '2 4 6')
AddTwoNumbersMultiTickAction: (tick_count = 1/2)
AddTwoNumbersMultiTickAction: (tick_count = 1/4)
AddTwoNumbersMultiTickAction: (tick_count = 1/6)
AddTwoNumbersMultiTickAction: DONE 1 + 1 = 2
AddTwoNumbersMultiTickAction: (tick_count = 2/4)
AddTwoNumbersMultiTickAction: (tick_count = 2/6)
AddTwoNumbersMultiTickAction: (tick_count = 3/4)
AddTwoNumbersMultiTickAction: (tick_count = 3/6)
AddTwoNumbersMultiTickAction: DONE 2 + 2 = 4
AddTwoNumbersMultiTickAction: (tick_count = 4/6)
AddTwoNumbersMultiTickAction: on_abort
>>> bt_runner.run(SimpleParallel, '6 4 2')
AddTwoNumbersMultiTickAction: (tick_count = 1/6)
AddTwoNumbersMultiTickAction: (tick_count = 1/4)
AddTwoNumbersMultiTickAction: (tick_count = 1/2)
AddTwoNumbersMultiTickAction: (tick_count = 2/6)
AddTwoNumbersMultiTickAction: (tick_count = 2/4)
AddTwoNumbersMultiTickAction: DONE 3 + 3 = 6
AddTwoNumbersMultiTickAction: (tick_count = 3/6)
AddTwoNumbersMultiTickAction: (tick_count = 3/4)
AddTwoNumbersMultiTickAction: (tick_count = 4/6)
AddTwoNumbersMultiTickAction: DONE 2 + 2 = 4
AddTwoNumbersMultiTickAction: on_abort

In the first execution the first two nodes complete after two resp. four ticks and thus, the SimpleParallel node also completes. The third node is aborted. In the second execution the third and second node complete after two resp. four ticks. Again, the SimpleParallel node completes as two nodes succedded, and as a consequence the first node is aborted. When aborting a node the on_abort function of this node is called, this is why the message 'AddTwoNumbersMultiTickAction: on_abort' is printed on standard output (see implementation of AddTwoNumbersMultiTickAction).