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Overview
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========
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BPMN and SpiffWorkflow
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----------------------
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.. sidebar:: BPMN Resources
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This guide is a mere introduction to BPMN.
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For more serious modeling, we recommend looking for more comprehensive
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resources. We have used the `books by Bruce Silver <https://www.amazon.com/Bruce-Silver/e/B0062AXUFY/ref=dp_byline_cont_pop_book_1>`_
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as a guide for our BPMN modeling.
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.. image:: figures/bpmnbook.jpg
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:align: center
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Business Process Model and Notation (BPMN) is a diagramming language for
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specifying business processes. BPMN links the realms of business and IT, and
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creates a common process language that can be shared between the two.
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BPMN describes details of process behaviors efficiently in a diagram. The
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meaning is precise enough to describe the technical details that control
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process execution in an automation engine. SpiffWorkflow allows you to create
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code to directly execute a BPMN diagram.
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When using SpiffWorkflow, a client can manipulate the BPMN diagram and still
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have their product work without a need for you to edit the Python code,
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improving response and turnaround time.
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Today, nearly every process modeling tool supports BPMN in some fashion making
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it a great tool to learn and use.
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To use SpiffWorkflow, you need at least a basic understanding of BPMN.
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This page offers a brief overview. There are many resources for additional
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information about BPMN.
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.. sidebar:: BPMN Modelers
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There are a number of modelers in existence, and any BPMN compliant modeler should work.
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SpiffWorkflow has some basic support for the free Camunda modeler, to use it's form building
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capabilities, but we intend to encapsulate this support in an extension module and remove
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it from the core library eventually. It does help for making some examples and demonstrating
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how one might implement user tasks in an online environment.
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In these examples and throughout the documentation we use the
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`BPMN.js <https://bpmn.io/toolkit/bpmn-js/>`_ BPMN Modeler.
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A Simple Workflow
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-----------------
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All BPMN models have a start event and at least one end event. The start event
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is represented with a single thin border circle. An end event is represented
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by a single thick border circle.
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The following example also has one task, represented by the rectangle with curved corners.
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.. figure:: figures/simplestworkflow.png
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:scale: 25%
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:align: center
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A simple workflow.
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The sequence flow is represented with a solid line connector. When the node at
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the tail of a sequence flow completes, the node at the arrowhead is enabled to start.
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A More Complicated Workflow
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---------------------------
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.. figure:: figures/ExclusiveGateway.png
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:scale: 25%
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:align: center
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A workflow with a gateway
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In this example, the diamond shape is called a gateway. It represents a branch
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point in our flow. This gateway is an exclusive data-based gateway (also
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called an XOR gateway). With an exclusive gateway, you must take one path or
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the other based on some data condition. BPMN has other gateway types.
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The important point is that we can use a gateway to add a branch in the
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workflow **without** creating an explicit branch in our Python code.
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Events
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------
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In the above simple workflows, all of the transitions are deterministic and we
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have direct connections between tasks. We need to handle the cases where an event
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may or may not happen and link these events in different parts of the workflow.
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BPMN has a comprehensive suite of event elements that can used for this purpose.
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SpiffWorkflow does not support every single BPMN event type, but it can handle
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many of them.
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.. figure:: figures/events.png
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:scale: 25%
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:align: center
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A workflow containing events
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We've already seen plain Start and End Events. BPMN also include the concepts
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of Intermediate Events (standalone events that may be Throwing or Catching) as well
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as Boundary Events (which can only be Caught).
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All Start Events are inherently Catching Events (a workflow can be initiated if a
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particular event occurs) and all End Events are Throwing Events (they can convey
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the final state of a workflow or path to other tasks and workflows).
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If an Intermediate Throwing Event is added to a flow, the event it represents
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will be generated and the flow will continue immediately. If an Intermediate
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Catching Event is added to a flow, the workflow will wait to catch the event it
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represents before advancing.
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A Boundary Event represents an event that may be caught only while a particular task
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is being executed and comes in two types: Interrupting (in which case the task it is
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attached to will be cancelled if the event is received) or Non-Interrupting (in
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which case the task will continue). In both cases, flows may emanate from the
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Boundary Event, which will trigger those paths if the events occur while the task
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is being executed.
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