Introduction To Workflows

What Is A Workflow?

Workflow is the series of interactions that should happen to complete a task. Business organizations have many kinds of workflow.

For example, insurance companies process claims, delivery companies track shipments, and schools accept applications for admission.

All these tasks involve several people, sometimes take a long time, and vary significantly from organization to organization.

The goal of workflow software is to streamline and track workflow activity. Since different organizations have different workflow processes, workflow software must be flexible and easy to customize.

The workflow system inside of Plone is an example of a State Machine.

From Wikipedia:

A finite-state machine (FSM) or finite-state automaton (plural: automata), or simply a state machine, is a behavioral model used to design computer programs. It is composed of a finite number of states associated to transitions. A transition is a set of actions that starts from one state and ends in another (or the same) state. A transition is started by a trigger, and a trigger can be an event or a condition.
  • Any object controlled by workflow is always in precisely one state from each workflow in its chain.
  • The state in which an object is currently located controls what transitions are available to it
  • Any workflow can be diagrammed, showing the available states and the transitions between them
    • Diagrams like this can be of enormous help in understanding your workflow
    • You should always sketch up a diagram when you start figuring out the workflow you want
../_images/simple_workflow.png

What’s In a Workflow?

Workflows Control

  • What states and transitions are available
  • Which permissions will be managed (permissions not managed are left untouched from their current value by the workflow)
  • Which groups will be managed (see states below for more about this)
  • Which variables will be tracked by the workflow (values are set and stored every time a transition occurs)
  • What worklists will be generated (you can return lists of content matching values tracked by variables
  • What scripts are available to be used in conjunction with transitions
    • These are basic python scripts, and are not used much anymore now that events are available

States Control

  • What transitions are available out
  • What permissions are assigned to which roles locally to the object
  • What groups are assigned to which roles locally to the object
    • This is probably the least-used aspect of workflow
    • It can be spectacularly useful

Transitions Control

  • What state they will end in
  • What conditions or gaurds are required for the transition to be available
    • These can be permissions of the user, roles a user has, groups to which the user belongs, or even the boolean value of ‘TALES’ expressions
  • What scripts will be executed before and after the transition occurs (again, not used much now that we have events)
  • How the transition is triggered
    • This can be user-initiated or automatic
      • Automatic transitions happen when an object lands in a state from which they are a valid exit, and that object fulfills all conditions for the transition to be available.
      • If the conditions for the automated transition are not met, then the transition doesn’t happen
        • Updating the object to meet the conditions will not kick it off
        • You’ll have to back it out of the current state and re-do the transition that should have kicked it off

How Does Workflow Work In Plone?

The tool in Plone that handles all workflow is called portal_workflow

  • Types must be workflow aware
    • Types in Plone are made WorkflowAware by a base content mixin from CMFCore WorkflowAware (in CMFCatalogAware
  • Workflow is assigned by type
    • Each type gets a chain
    • A chain can have more than one workflow in it
  • portal_workflow is responsible for keeping track of all information about the workflow state of an object
    • A particular content object knows nothing about it’s own workflow state
    • queries about the workflow of an object must be addressed to portal_workflow
 >>> from plone import api
 >>> fpage = api.content.get("/front-page")
 >>> fpage.review_state
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
 AttributeError: review_state
 >>> api.content.get_state(fpage)
 'published'
 >>> wft = api.portal.get_tool('portal_workflow')
 >>> wft.getChainFor(fpage)
 ('simple_publication_workflow',)
 >>> wft.getTransitionsFor(fpage)
 ({'description': 'If you submitted the item by mistake or want to perform additional edits, this will take it back.', 'title': 'Member retracts submission', 'url': 'http://nohost/Plone/front-page/content_status_modify?workflow_action=retract', 'id': 'retract', 'title_or_id': 'Member retracts submission', 'name': 'Retract'}, {'description': 'Sending the item back will return the item to the original author instead of publishing it. You should preferably include a reason for why it was not published.', 'title': 'Reviewer sends content back for re-drafting', 'url': 'http://nohost/Plone/front-page/content_status_modify?workflow_action=reject', 'id': 'reject', 'title_or_id': 'Reviewer sends content back for re-drafting', 'name': 'Send back'})
>>> with api.env.adopt_user('content'):
...     contrib-page = api.content.create(container=api.portal.get(), type="Document", title="Content Contrib Page")
...     [i['id'] for i in wft.getTransitionsFor(api.content.get("/content-contrib-page")]
...
['submit']
>>> with api.env.adopt_roles(roles=['Manager',]):
...     [i['id'] for i in wft.getTransitionsFor(contrib-page)]
...
['submit', 'publish']
  • portal_workflow is security conscious, for all aspects of workflow it respects and validates the access levels of the current user
    • Users can only access the workflow information for which they have permissions
>>> with api.env.adopt_user('site-admin'):
...     wft.getTransitionsFor(fpage)
...
>>> from pprint import pprint
>>> pprint(wft.getTransitionsFor(fpage))
({'description': 'If you submitted the item by mistake or want to perform
                  additional edits, this will take it back.',
  'id': 'retract',
  'name': 'Retract',
  'title': 'Member retracts submission',
  'title_or_id': 'Member retracts submission',
  'url': 'Plone/front-page/content_status_modify?workflow_action=retract'},
 {'description': 'Sending the item back will return the item to the original
                  author instead of publishing it. You should preferably include
                  a reason for why it was not published.',
  'id': 'reject',
  'name': 'Send back',
  'title': 'Reviewer send content back for re-drafting',
  'title_or_id': 'Reviewer send content back for re-drafting',
  'url': 'Plone/front-page/content_status_modify?workflow_action=reject'})

Moving Content Through Workflows

  • As stated above, any object with workflow is always in exactly one state for each workflow in it’s chain.
  • When you initiate a transition, it is instantaneous.

  • What happens when this occurs?

    1. The BeforeTransitionEvent is notified, and any subscribers to that event are executed
    2. Any before script registered for the transition are executed.
    3. The transition takes place
    • values are set for the variables registered by the workflow
    • the new state of the object is set
    • the new set of permissions values for roles and groups are calculated and updated
      • first permissions are remapped
      • then group -> role mappings are changed
    • the object is re-indexed for all security related indexes.
    1. Any after script registered for the transition is executed
    2. The AfterTransitionEvent is notified, and any subscribers to that event are executed

In general, transitions are triggered by user action. This takes place when a user clicks on the state menu in the Plone UI and selects an available transition, or when the user presses save from the Change State dialog found in the folder listing view.

  • As stated above, automatic transitions are found as a result of undergoing manual transitions.
    • Step 3 above can actually be executed multiple times when a user triggers a transition.
    • Events and scripts are executed for each transition that happens
    • For this reason, when subscribing to workflow events, it’s a good idea to check which transition just happened before taking any actions in your handler:
def handleWorkflowTransition(ob, event):
    """ a handler meant to be used after a 'publish' transition """
    if event.transition != 'publish':
        return
    ...