.. _flow_control-feedback:

Feedback
--------

.. warning::

   This feature is still experimental. Please report issues and use cases on the
   :ref:`support-forum`

You can use ``Feedback`` connections as a simple why to take values from nodes
at the end of the workflow and bring them back to nodes at the beginning of the
workflow, effectively creating a feedback loop without using a
:ref:`flow_control-loop` with ``Shift`` terminals.

To mark a ``Connection`` as ``Feedback connection``, ``right-click`` the
connection that you want to use as the feedback and select
``Feedback connection``. The connection will now be dashed instead of solid.

.. figure:: ./images/feedback.png
   :align: center
   :width: 400

The sheet can now be run by pressing the |play| button. Every time the |play|
button is pressed, one iteration will complete. To automatically run the sheet,
press the |repeat| button. Press the |repeat| button again to stop iterating.

.. figure:: ./images/feedback.gif
   :align: center
   :width: 400

How it works
++++++++++++

When a connection is marked as a ``Feedback connection``, the connection is
removed from the `topological sort`_ and will not impact the order that the
nodes are executed in. However, when a node is executed, it will still try to
update the ``Feedback connection`` and use the value from the terminal of the
node that it is connected to.

Not all nodes can support this feature because on the first execution of the
sheet, the value from the ``Feedback connection`` could be undefined. Nodes
need to be either able to operate on (ignore) bad values or the value of the
``Feedback connection`` needs to be initialized to something that can be used.

For example, in the above workflow, the ``result`` terminals of the ``Sum``
nodes are initialized to ``0``. On the first execution of the sheet, the
``Feedback connection`` has a value of ``0``, which is an acceptable value
that the ``Sum`` node can operate on.

.. _topological sort: https://en.wikipedia.org/wiki/Topological_sorting

.. |play| image:: ../../../nodeworks/images/play.svg
.. |repeat| image:: ../../../nodeworks/images/repeat.svg