When db_host is commented in publish, the worflow obviously fails on next action with:

(undercloud) [stack@undercloud ~]$ mistral execution-get d7cc2fab-2c53-4527-b835-5352b23a16ac
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Field              | Value                                                                                                                                                                                                     |
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ID                 | d7cc2fab-2c53-4527-b835-5352b23a16ac                                                                                                                                                                      |
| Workflow ID        | 3b22ad9a-b3ca-4290-b3df-91b740838719                                                                                                                                                                      |
| Workflow name      | tripleo.undercloud_backup.v1.backup                                                                                                                                                                       |
| Workflow namespace |                                                                                                                                                                                                           |
| Description        |                                                                                                                                                                                                           |
| Task Execution ID  | <none>                                                                                                                                                                                                    |
| Root Execution ID  | <none>                                                                                                                                                                                                    |
| State              | ERROR                                                                                                                                                                                                     |
| State info         | Failed to run task [error=Can not evaluate YAQL expression [expression=$.undercloud_db_host, error=u'undercloud_db_host', data={}], wf=tripleo.undercloud_backup.v1.backup, task=create_database_backup]: |
|                    | Traceback (most recent call last):                                                                                                                                                                        |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/task_handler.py", line 63, in run_task                                                                                                            |
|                    |     task.run()                                                                                                                                                                                            |
|                    |   File "/usr/lib/python2.7/site-packages/osprofiler/profiler.py", line 160, in wrapper                                                                                                                    |
|                    |     result = f(*args, **kwargs)                                                                                                                                                                           |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/tasks.py", line 453, in run                                                                                                                       |
|                    |     self._run_new()                                                                                                                                                                                       |
|                    |   File "/usr/lib/python2.7/site-packages/osprofiler/profiler.py", line 160, in wrapper                                                                                                                    |
|                    |     result = f(*args, **kwargs)                                                                                                                                                                           |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/tasks.py", line 485, in _run_new                                                                                                                  |
|                    |     self._schedule_actions()                                                                                                                                                                              |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/tasks.py", line 558, in _schedule_actions                                                                                                         |
|                    |     input_dict = self._get_action_input()                                                                                                                                                                 |
|                    |   File "/usr/lib/python2.7/site-packages/osprofiler/profiler.py", line 160, in wrapper                                                                                                                    |
|                    |     result = f(*args, **kwargs)                                                                                                                                                                           |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/tasks.py", line 589, in _get_action_input                                                                                                         |
|                    |     input_dict = self._evaluate_expression(self.task_spec.get_input(), ctx)                                                                                                                               |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/engine/tasks.py", line 615, in _evaluate_expression                                                                                                      |
|                    |     ctx_view                                                                                                                                                                                              |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/expressions/__init__.py", line 100, in evaluate_recursively                                                                                              |
|                    |     data[key] = _evaluate_item(data[key], context)                                                                                                                                                        |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/expressions/__init__.py", line 79, in _evaluate_item                                                                                                     |
|                    |     return evaluate(item, context)                                                                                                                                                                        |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/expressions/__init__.py", line 71, in evaluate                                                                                                           |
|                    |     return evaluator.evaluate(expression, context)                                                                                                                                                        |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/expressions/yaql_expression.py", line 159, in evaluate                                                                                                   |
|                    |     cls).evaluate(trim_expr, data_context)                                                                                                                                                                |
|                    |   File "/usr/lib/python2.7/site-packages/mistral/expressions/yaql_expression.py", line 113, in evaluate                                                                                                   |
|                    |     ", data=%s]" % (expression, str(e), data_context)                                                                                                                                                     |
|                    | YaqlEvaluationException: Can not evaluate YAQL expression [expression=$.undercloud_db_host, error=u'undercloud_db_host', data={}]                                                                         |
|                    |                                                                                                                                                                                                           |
| Created at         | 2019-03-21 12:42:08                                                                                                                                                                                       |
| Updated at         | 2019-03-21 12:42:09                                                                                                                                                                                       |
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
(undercloud) [stack@undercloud ~]$