.. _bskPrinciples-7:

Advanced: Redirecting Module Output to Stand-Alone Message
==========================================================

.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/e_938BFwtiI" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

.. sidebar:: Source Code

    The python code shown below can be downloaded :download:`here </../../docs/source/codeSamples/bsk-7.py>`.

Consider a more advanced Basilisk simulation setup where you have two modules that both need to write to the same
stand-alone message. The motivation here is to simultaneously run two or more flight guidance algorithm modules,
but only one gets executed depending on the flight mode. Regardless of which guidance module is executed, the guidance
output message must be fed to the same control module. This cannot be accomplished if the third module subscribes either
to the output message of module 1 or 2. To avoid trying to re-subscribe to different module output messages when switching
flight modes, we can choose to have both modules 1 and 2 write to the same stand-alone message as illustrated below.

.. image:: ../../_images/static/qs-bsk-7a.svg
   :align: center

The benefit is that the 3rd module can subscribe its input message to this one stand-alone message. To be clear, this
sample application assumes either module 1 or 2 is executed, but not both. Otherwise, one would overwrite the others'
message output.

The sample simulation script creates two modules which have their individual output messages redirected to a
stand-alone message.

.. image:: ../../_images/static/qs-bsk-7.svg
   :align: center

In the following sample code, two modules are created. To create a stand-alone message, assume a message
of type ``SomeMsg`` needs to be created. This is done using::

    standAloneMsg = messaging.SomeMsg()

To redirect the output of a module ``someModule`` to this stand-alone message, simply set::

    someModule.dataOutMsg = standAloneMsg

.. literalinclude:: ../../codeSamples/bsk-7.py
   :language: python
   :linenos:
   :lines: 18-

.. note::

    In C++, we are setting ``standAloneMsg`` equal to ``someModule.dataOutMsg``. Recording either one will
    give the same result. Be sure to record ``standAloneMsg`` to ensure you capture the redirected message output.

To see the message states of both the module internal message objects and the stand-alone messages, the sample script
shows how to use ``.read()`` to read the current state of the message object. This will return a copy of the message
payload structure.

After executing the script you should see the following terminal output:

.. code-block::

    source/codeSamples % python bsk-7.py
    BSK_INFORMATION: Variable dummy set to 0.000000 in reset.
    BSK_INFORMATION: Variable dummy set to 0.000000 in reset.
    BSK_INFORMATION: Module ID 1 ran Update at 0.000000s
    BSK_INFORMATION: Module ID 2 ran Update at 0.000000s
    BSK_INFORMATION: Module ID 1 ran Update at 1.000000s
    BSK_INFORMATION: Module ID 2 ran Update at 1.000000s
    mod1.dataOutMsg:
    [2.0, 0.0, 0.0]
    cppMsg:
    [2.0, 0.0, 0.0]
    mod2.dataOutMsg:
    [2.0, 0.0, 0.0]
    cppMsg:
    [2.0, 0.0, 0.0]