307 lines
13 KiB
ReStructuredText
307 lines
13 KiB
ReStructuredText
.. _workflows:
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Workflows
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=========
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In OpenERP, a workflow is a technical artefact to manage a set of "things to do"
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associated to the records of some data model. The workflow provides a higher-
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level way to organize the things to do on a record.
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More specifically, a workflow is a directed graph where the nodes are called
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"activities" and the arcs are called "transitions".
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- Activities define work that should be done within the OpenERP server, such as
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changing the state of some records, or sending emails.
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- Transitions control how the workflow progresses from activity to activity.
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In the definition of a workflow, one can attach conditions, signals, and
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triggers to transitions, so that the behavior of the workflow depends on user
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actions (such as clicking on a button), changes to records, or arbitrary Python
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code.
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Basics
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------
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Defining a workflow with data files is straightforward: a record "workflow" is
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given together with records for the activities and the transitions. For
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instance, here is a simple sequence of two activities defined in XML::
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<record id="test_workflow" model="workflow">
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<field name="name">test.workflow</field>
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<field name="osv">test.workflow.model</field>
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<field name="on_create">True</field>
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</record>
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<record id="activity_a" model="workflow.activity">
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<field name="wkf_id" ref="test_workflow"/>
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<field name="flow_start">True</field>
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<field name="name">a</field>
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<field name="kind">function</field>
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<field name="action">print_a()</field>
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</record>
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<record id="activity_b" model="workflow.activity">
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<field name="wkf_id" ref="test_workflow"/>
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<field name="flow_stop">True</field>
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<field name="name">b</field>
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<field name="kind">function</field>
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<field name="action">print_b()</field>
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</record>
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<record id="trans_a_b" model="workflow.transition">
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<field name="act_from" ref="activity_a"/>
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<field name="act_to" ref="activity_b"/>
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</record>
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A worfklow is always defined with respect to a particular model (the model is
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given by the attribute ``osv`` on the model ``workflow``). Methods specified in
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the activities or transitions will be called on that model.
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In the example code above, a workflow called "test_workflow" is created. It is
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made up of two activies, named "a" and "b", and one transition, going from "a"
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to "b".
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The first activity has its attribute ``flow_start`` set to ``True`` so that
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OpenERP knows where to start the workflow traversal after it is instanciated.
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Because ``on_create`` is set to True on the workflow record, the workflow is
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instanciated for each newly created record. (Otherwise, the workflow should be
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instanciated by other means, such as from some module Python code.)
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When the workflow is instanciated, it begins with activity "a". That activity
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is of kind ``function``, which means that the action ``print_a()`` is a method
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call on the model ``test.workflow`` (the usual ``cr, uid, ids, context``
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arguments are passed for you).
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The transition between "a" and "b" does not specify any condition. This means
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that the workflow instance immediately goes from "a" to "b" after "a" has been
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processed, and thus also processes activity "b".
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Transitions
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-----------
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Transitions provide the control structures to orchestrate a workflow. When an
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activity is completed, the workflow engine tries to get across transitions
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departing from the completed activity, towards the next activities. In their
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simplest form (as in the example above), they link activities sequentially:
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activities are processed as soon as the activities preceding them are completed.
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Instead of running all activities in one fell swoop, it is also possible to wait
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on transitions, going through them only when some criteria are met. The criteria
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are the conditions, the signals, and the triggers. They are detailed in the
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following sections.
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Conditions
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''''''''''
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When an activity has been completed, its outgoing transitions are inspected to
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determine whether it is possible for the workflow instance to proceed through
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them and reach the next activities. When only a condition is defined (i.e., no
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signal or trigger is defined), the condition is evaluated by OpenERP, and if it
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evaluates to ``True``, the worklfow instance progresses through the transition.
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If the condition is not met, it will be reevaluated every time the associated
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record is modified, or by an explicit method call to do it.
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By default, the attribute ``condition`` (i.e., the expression to be evaluated)
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is just "True", which trivially evaluates to ``True``. Note that the condition
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may be several lines long; in that case, the value of the last one determines
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whether the transition can be taken.
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In the condition evaluation environment, several symbols are conveniently
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defined (in addition to the OpenERP ``safe_eval`` environment):
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- all the model column names, and
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- all the browse record's attributes.
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Signals
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'''''''
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In addition to a condition, a transition can specify a signal name. When such
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a signal name is present, the transition is not taken directly, even if the
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condition evaluates to ``True``. Instead the transition blocks, waiting to be
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woken up.
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In order to wake up a transition with a defined signal name, the signal must be
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sent to the workflow instance. A common way to send a signal is to use a button
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in the user interface, using the element ``<button/>`` with the signal name as
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the attribute ``name`` of the button. Once the button is clicked, the signal is
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sent to the workflow instance of the current record.
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.. note:: The condition is still evaluated when the signal is sent to the
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workflow instance.
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Triggers
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''''''''
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With conditions that evaluate to ``False``, transitions are not taken (and thus
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the activity it leads to is not processed immediately). Still, the workflow
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instance can get new chances to progress across that transition by providing
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so-called triggers. The idea is that when the condition is not satisfied,
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triggers are recorded in database. Later, it is possible to wake up
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specifically the workflow instances that installed those triggers, offering
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them to reevaluate their transition conditions. This mechanism makes it cheaper
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to wake up workflow instances by targetting just a few of them (those that have
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installed the triggers) instead of all of them.
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Triggers are recorded in database as record IDs (together with the model name)
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and refer to the workflow instance waiting for those records. The transition
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definition provides a model name (attribute ``trigger_model``) and a Python
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expression (attribute ``trigger_expression``) that evaluates to a list of record
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IDs in the given model. Any of those records can wake up the workflow instance
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they are associated with.
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.. note:: Note that triggers are not re-installed whenever the transition is
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re-tried.
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Splitting and joining transitions
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'''''''''''''''''''''''''''''''''
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When multiple transitions leave the same activity, or lead to the same activity,
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OpenERP provides some control over which transitions are actually taken, or how
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the reached activity will be processed. The attributes ``split_mode`` and
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``join_mode`` on the activity are used for such control. The possible values of
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those attributes are explained below.
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Activities
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----------
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While the transitions can be seen as the control structures of the workflows,
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activities are the places where everything happens, from changing record states
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to sending email.
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Different kinds of activities exist: ``Dummy``, ``Function``, ``Subflow``, and
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``Stop all``, each doing different things when the activity is processed. In
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addition to their kind, activies have other properties, detailed in the next
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sections.
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Flow start and flow stop
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''''''''''''''''''''''''
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The attribute ``flow_start`` is a boolean value specifying whether the activity
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is processed when the workflow is instanciated. Multiple activities can have
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their attribute ``flow_start`` set to ``True``. When instanciating a workflow
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for a record, OpenERP simply processes all of them, and evaluate all their
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outgoing transitions afterwards.
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The attribute ``flow_stop`` is a boolean value specifying whether the activity
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stops the workflow instance. A workflow instance is considered completed when
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all its activities with the attribute ``flow_stop`` set to ``True`` are
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completed.
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It is important for OpenERP to know when a workflow instance is completed. A
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workflow can have an activity that is actually another workflow (called a
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subflow); that activity is completed when the subflow is completed.
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Subflow
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'''''''
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An activity can embed a complete workflow, called a subflow (the embedding
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workflow is called the parent workflow). The workflow to instanciate is
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specified by attribute ``subflow_id``.
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.. note:: In the GUI, that attribute can not be set unless the kind of the
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activity is ``Subflow``.
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The activity is considered completed (and its outgoing transitions ready to be
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evaluated) when the subflow is completed (see attribute ``flow_stop`` above).
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Sending a signal from a subflow
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'''''''''''''''''''''''''''''''
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When a workflow is embedded in an activity (as a subflow) of a workflow, the
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sublow can send a signal from its own activities to the parent workflow by
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giving a signal name in the attribute ``signal_send``. OpenERP processes those
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activities by sending the value of ``signal_send`` prefixed by "subflow." to
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the parent workflow instance.
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In other words, it is possible to react and get transitions in the parent
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workflow as activities are executed in the sublow.
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Server actions
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''''''''''''''
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An activity can run a "Server Action" by specifying its ID in the attribute
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``action_id``.
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Python action
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'''''''''''''
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An activity can execute some Python code, given by the attribute ``action``.
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The evaluation environment is the same as the one explained in the section
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`Conditions`_.
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Split mode
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''''''''''
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After an activity has been processed, its outgoing transitions are evaluated.
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Normally, if a transition can be taken, OpenERP traverses it and proceed to the
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activity the transition leads to.
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Actually, when more than a single transition is leaving an activity, OpenERP may
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proceed or not, depending on the other transitions. That is, the conditions on
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the transitions can be combined together, and the combined result instructs
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OpenERP to traverse zero, one, or all the transitions. The way they are combined
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is controlled by the attribute ``split_mode``.
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There are three possible split modes: ``XOR``, ``OR`` and ``AND``.
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``XOR``
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When the transitions are combined with a ``XOR`` split mode, as soon as a
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transition has a satisfied condition, the transition is traversed and the
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others are skipped.
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``OR``
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With the ``OR`` mode, all the transitions with a satisfied condition are
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traversed. The remaining transitions will not be evaluated later.
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``AND``
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With the ``AND`` mode, OpenERP will wait for all outgoing transition
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conditions to be satisfied, then traverse all of them at once.
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Join mode
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'''''''''
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Just like outgoing transition conditions can be combined together to decide
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whether they can be traversed or not, incoming transitions can be combined
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together to decide if and when an activity may be processed. The attribute
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``join_mode`` controls that behavior.
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There are two possible join modes: ``XOR`` and ``AND``.
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``XOR``
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With the ``XOR`` mode, an incoming transition with a satisfied condition is
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traversed immediately, and enables the processing of the activity.
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``AND``
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With the ``AND`` mode, OpenERP will wait until all incoming transitions have
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been traversed before enabling the processing of the activity.
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Kinds
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'''''
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Activities can be of different kinds: ``dummy``, ``function``, ``subflow``, or
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``stopall``. The kind defines what type of work an activity can do.
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Dummy
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The ``dummy`` kind is for activities that do nothing, or for activities that
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only call a server action. Activities that do nothing can be used as hubs to
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gather/dispatch transitions.
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Function
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The ``function`` kind is for activities that only need to run some Python
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code, and possibly a server action.
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Stop all
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The ``stopall`` kind is for activities that will completely stop the
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workflow instance and mark it as completed. In addition they can also run
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some Python code.
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Subflow
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When the kind of the activity is ``subflow``, the activity embeds another
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workflow instance. When the subflow is completed, the activity is also
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considered completed.
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By default, the subflow is instanciated for the same record as the parent
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workflow. It is possible to change that behavior by providing Python code
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that returns a record ID (of the same data model as the subflow). The
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embedded subflow instance is then the one of the given record.
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