Mistral on-success, on-error and on-complete

I spent a bit of time today looking into the subtleties of the Mistral task properties on-success, on-complete and on-error when used with the fail engine commands.

As an upcoming docs patch explains, these are similar to the Python try, except, finally blocks. Meaning, that it would look like the following.

    # on-success
    # on-error
    # on-complete

I was looking to see how the Mistral engine command would work in combination with these. In TripleO we want to mark a workflow as failed if it sends a Zaqar message with the value {"status": "FAILED"}. So our task would look a bit like this...

        action: zaqar.queue_post
          queue_name: <% $.queue_name %>
              status: <% $.get('status', 'SUCCESS') %>
          - fail: <% $.get('status') = "FAILED" %>

This task uses the zaqar.queue_post action to send a message containing the status. Once it is complete it will fail the workflow if the status is equal to "FAILED". Then in the mistral execution-list the workflow will show as failed. This is good, because we want to surface the best error in the execution list.

However, if the zaqar.queue_post action fails then we want to surface that error instead. At the moment it will still be possible to see it in the list of action executions. However, looking at the workflow executions it isn't obvious where the problem was.

Changing the above example to on-success solves that. We only want to manually mark the workflow as having failed if the Zaqar message was sent with the FAILED status. Otherwise if the message fails to send, the workflow will error anyway with a more detailed error.

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