.. _workflows: Workflows ========= A workflow is a directed graph where the nodes are called "activities" and the arcs are called "transitions". - Activities define work that should be done within the OpenERP server, such as changing the state of some records, or sending mails. - Transitions control how the workflow will go from activities to activities. When defining a workflow, one can attach conditions, signals, and triggers to transitions, so that the behavior of the workflow can depend on user actions (such as clicking on a button), changes to records, or arbitrary Python code. Basics ------ Defining a workflow with data files is straightforward: a record "workflow" is needed together with records for the activities and the transitions. For instance here is a simple sequence of two activities defined in XML:: test.workflow test.workflow.model True True a function print_a() True b function print_b() A worfklow is always defined with respect to a particular model (the model is given through the ``osv`` attribute on the ``workflow`` model). Methods specified in the activities or transitions will be called on that model. In the example code above, a workflow called "test_workflow" is created. It is made up of two activies, named "a" and "b", and one transition, going from "a" to "b". The first activity has its ``flow_start`` attribute set to True so that OpenERP knows where to start the workflow when it is instanciated. Because ``on_create`` is set to True on the workflow record, the workflow is instanciated for each newly created record. (Otherwise, the workflow should be created by other means, such as from some module Python code.) When the workflow is instanciated, it will start by the "a" activity. That activity is of kind ``function`` which means the action ``print_a()`` is a method to be called on the ``test.workflow`` model (the usual ``cr, uid, ids, context`` arguments are passed for you). The transition between "a" and "b" does not specify any conditions. This means the workflow instance will immediately progress from "a" to "b" (after "a" has been processed), and thus also process the "b" activity. Transitions ----------- Transitions provide the control structures to orchestrate a workflow. In their simplest form they just link activities from one to the others (as in the example above), and activities are processed as soon as the activities preceding them are completed. But instead of running all activities in one fell swoop, it is also possible to block on transitions, going through them only when some criteria are met. Such criteria are the conditions, the signals, and the triggers. They are detailed in the next sections. Conditions '''''''''' When an activity has been completed, its outgoing transitions will be inspected to see if it is possible for the workflow instance to proceed through them and reach the next activities. When only a condition is defined (i.e. no signal or trigger is defined), the condition is evaluated by OpenERP, and if it evaluates to ``True``, the worklfow instance will go through. By default, the ``condition`` attribute (i.e. the expression to be evaluated) is just "True", which will trivially evaluate to ``True``. Actually, the condition can be several lines long, and the value of the last one will be used to test if the transition can be taken. In the condition evaluation environment, several symbols are conveniently defined: - The database cursor (``cr``), - the user ID (``uid``), the record ID tied to the workflow instance (``id``), - the user ID wrapped in a list (``ids``), - the model name (``model``), - the model instance (``obj``), - all the model column names, - and all the record (the one obtained by browsing the provided ID) attributes. Signals ''''''' Triggers '''''''' When an activity is completed, the workflow engine will try to get across transitions departing from the completed activity, towards the next activities. To get across a transition, its associated condition should evaluate to True. If the condition evaluates to False, the transition is not taken (and thus the activity it leads to will not be processed). Still, the workflow instance can get new chances to progress across that transition by providing so-called triggers. The idea is that when the condition fails, triggers (actually just model name/record IDs pairs) are recorded in database. Later, it is possible to wake-up specifically the workflow instances that installed those triggers, offering them a new chance to evaluation their transition conditions. This mechnism makes it cheaper to wake-up workflow instances by targetting just a few of them (those that have installed the triggers) instead of all of them. On each transition, in addition to a condition, records can be defined as a trigger. The records will be defined as triggers as the transition is tried withing a workflow, after the condition has failed. The actual records are stored as model name and record ids. The model name is defined by the trigger_model attribute of the transition while the record IDs are retrived by evaluating the trigger_expression (also defined on the transition). - I think the triggers are never deleted from the database. They are: they are 'on delete cascade' on both the workflow instance and the workitem. - Are those triggers re-installed whenever the transition is tried ? Nope.