Publish/output in case of task/workflow failure

https://blueprints.launchpad.net/mistral/+spec/mistral-publish-on-error

Currently it is not possible to provide any reasonable output in case of a task or workflow failure. Implementing this would greatly simplify error handling in workflows.

Problem description

This blueprint is a proposal to introduce two new attributes, publish-on-error for tasks and output-on-error for workflows for this purpose.

Use Cases

• As a user, I would like to define a workflow with a generic error handler task.

• As a user, I would like to simplify error handling in my complex workflow system.

Proposed change

To solve the problem I would like to introduce the following new task and workflow attributes:

• Task - publish-on-error: Any data structure arbitrarily containing YAQL

  expressions that defines output of a task to be published into workflow context if it goes into error state.

• Workflow - output-on-error: Any data structure arbitrarily containing YAQL

  expressions that defines output of a workflow to be returned if it goes into error state.

Example workbook:

workflows:
    main:
        tasks:
            task_1:
                workflow: sub-workflow
                publish-on-error:
                    failure_cause: <% task(task_1).result.failure_cause %>
                    detailed_cause: <% task(task_1).result.detailed_cause %>
                on-error:
                    - error-handler
            task_2:
                ...
                publish-on-error:
                    failure_cause: <% task(task_2).result.failure_cause %>
                    detailed_cause: <% task(task_2).result.detailed_cause %>
                on-error:
                    - error-handler

            error-handler:
                action: send_email
                input:
                    body: |
                        <% $.failure_cause %>
                        Details:
                        <% $.detailed_cause %>

    sub-workflow:
        output:
            result: <% $.result %>
        output-on-error:
            failure_cause: <% $.failure_cause %>
            detailed_cause: <% $.detailed_cause %>

        tasks:
            task1:
                ...
                publish-on-error:
                    failure_cause: "Failure in sub-workflow.task1!"
                    detailed_cause: <% task(task1).result %>
                on-success:
                    - task2
            task2:
                ...
                publish-on-error:
                    failure_cause: "Failure in sub-workflow.task2!"
                    detailed_cause: <% task(task2).result %>

Alternatives

N/A

Data model impact

Two new fields introduced: * Task spec - publish-on-error * Workflow spec - output-on-error

REST API impact

None

End user impact

Workflow language additions that allow to handle errors in a more flexible way. Existing workflows will work without any change.

Performance Impact

None

Deployer impact

None

Implementation

Assignee(s)

Primary assignee:

István Imre <istvan.imre@nokia.com>

Other contributors:

Endre János Kovács <endre.kovacs@nokia.com>

Work Items

• add publish-on-error to task spec

• add output-on-error to workflow spec

• documentation

Dependencies

None

Testing

• new engine test for the two new attributes

References

None