Add porting guide

docs/index.rst

@@ -44,6 +44,7 @@ Examples of this could include:
    execution_environments
    remote_jobs
    modules
+   porting_guides/porting_guide
 
 
 Indices and tables

docs/porting_guides/porting_guide.rst

@@ -0,0 +1,12 @@
+*****************************
+Ansible Runner Porting Guides
+*****************************
+
+This section lists porting guides that can help you when upgrading to newer
+versions of ``ansible-runner``.
+
+
+.. toctree::
+   :maxdepth: 1
+
+   porting_guide_v2.5

docs/porting_guides/porting_guide_v2.5.rst

@@ -0,0 +1,45 @@
+********************************
+Ansible Runner 2.5 Porting Guide
+********************************
+
+This section discusses the behavioral changes between ``ansible-runner`` version 2.4 and version 2.5.
+
+.. contents:: Topics
+
+Changes to the Python Interface
+===============================
+
+:ref:`Remote job execution <remote_jobs>` could experience problems when transmitting job arguments
+to a worker node with an older version of ``ansible-runner``. If the older worker received a job
+argument that it did not understand, like a new API value, that worker would terminate abnormally.
+To address this, a two-stage plan was devised:
+
+#. Modify the streaming job arguments so that only job arguments that have non-default values are
+   streamed to the worker node. A new API job argument will not be problematic for older workers
+   unless that argument is actually used.
+#. Have the worker node fail gracefully on unrecognized job arguments.
+
+The first step of this process is implemented in this version of ``ansible-runner``. The second
+step will be completed in a future version.
+
+For this change, it was necessary to modify how the ``run()`` and ``run_async()`` functions
+of the :ref:`Python API <python_interface>` are implemented. The API function arguments are now completely
+defined in the ``RunnerConfig`` object, and both functions now take an optional ``config`` parameter.
+
+For backward compatibility, keyword arguments to the ``run()/run_async()`` API functions are passed
+along to the ``RunnerConfig`` object initialization for you. Alternatively, you may choose to use
+the more up-to-date signature of those API functions where you pass in a manually created ``RunnerConfig``
+object. For example:
+
+.. code-block:: python
+
+    import ansible_runner
+    config = ansible_runner.RunnerConfig('private_data_dir': '/tmp/demo', 'playbook': 'test.yml')
+    r = ansible_runner.interface.run(config=config)
+
+The above is identical to the more familiar usage of the API:
+
+.. code-block:: python
+
+    import ansible_runner
+    r = ansible_runner.interface.run('private_data_dir': '/tmp/demo', 'playbook': 'test.yml')