Workflows as State Machines

Workflows as State Machines

The workflow component is modelled after a Workflow net which is a subclass of a Petri net. By adding further restrictions you can get a state machine. The most important one being that a state machine cannot be in more than one place simultaneously. It is also worth noting that a workflow does not commonly have cyclic path in the definition graph, but it is common for a state machine.

Example of a State Machine

A pull request starts in an initial "start" state, a state for e.g. running tests on Travis. When this is finished, the pull request is in the "review" state, where contributors can require changes, reject or accept the pull request. At any time, you can also "update" the pull request, which will result in another Travis run.

../_images/pull_request.png

Below is the configuration for the pull request state machine.

  • YAML
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    # app/config/config.yml
    framework:
        workflows:
            pull_request:
                type: 'state_machine'
                supports:
                    - AppBundle\Entity\PullRequest
                places:
                    - start
                    - coding
                    - travis
                    - review
                    - merged
                    - closed
                transitions:
                    submit:
                        from: start
                        to: travis
                    update:
                        from: [coding, travis, review]
                        to: travis
                    wait_for_review:
                        from: travis
                        to: review
                    request_change:
                        from: review
                        to: coding
                    accept:
                        from: review
                        to: merged
                    reject:
                        from: review
                        to: closed
                    reopen:
                        from: closed
                        to: review
    
  • XML
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    <!-- app/config/config.xml -->
    <?xml version="1.0" encoding="utf-8" ?>
    <container xmlns="http://symfony.com/schema/dic/services"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:framework="http://symfony.com/schema/dic/symfony"
        xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd
            http://symfony.com/schema/dic/symfony http://symfony.com/schema/dic/symfony/symfony-1.0.xsd"
    >
    
        <framework:config>
            <framework:workflow name="pull_request" type="state_machine">
                <framework:marking-store type="single_state"/>
    
                <framework:support>AppBundle\Entity\PullRequest</framework:support>
    
                <framework:place>start</framework:place>
                <framework:place>coding</framework:place>
                <framework:place>travis</framework:place>
                <framework:place>review</framework:place>
                <framework:place>merged</framework:place>
                <framework:place>closed</framework:place>
    
                <framework:transition name="submit">
                    <framework:from>start</framework:from>
    
                    <framework:to>travis</framework:to>
                </framework:transition>
    
                <framework:transition name="update">
                    <framework:from>coding</framework:from>
                    <framework:from>travis</framework:from>
                    <framework:from>review</framework:from>
    
                    <framework:to>travis</framework:to>
                </framework:transition>
    
                <framework:transition name="wait_for_review">
                    <framework:from>travis</framework:from>
    
                    <framework:to>review</framework:to>
                </framework:transition>
    
                <framework:transition name="request_change">
                    <framework:from>review</framework:from>
    
                    <framework:to>coding</framework:to>
                </framework:transition>
    
                <framework:transition name="accept">
                    <framework:from>review</framework:from>
    
                    <framework:to>merged</framework:to>
                </framework:transition>
    
                <framework:transition name="reject">
                    <framework:from>review</framework:from>
    
                    <framework:to>closed</framework:to>
                </framework:transition>
    
                <framework:transition name="reopen">
                    <framework:from>closed</framework:from>
    
                    <framework:to>review</framework:to>
                </framework:transition>
    
            </framework:workflow>
    
        </framework:config>
    </container>
    
  • PHP
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    // app/config/config.php
    
    $container->loadFromExtension('framework', array(
        // ...
        'workflows' => array(
            'pull_request' => array(
              'type' => 'state_machine',
              'supports' => array('AppBundle\Entity\PullRequest'),
              'places' => array(
                'start',
                'coding',
                'travis',
                'review',
                'merged',
                'closed',
              ),
              'transitions' => array(
                'submit'=> array(
                  'from' => 'start',
                  'to' => 'travis',
                ),
                'update'=> array(
                  'from' => array('coding','travis','review'),
                  'to' => 'travis',
                ),
                'wait_for_review'=> array(
                  'from' => 'travis',
                  'to' => 'review',
                ),
                'request_change'=> array(
                  'from' => 'review',
                  'to' => 'coding',
                ),
                'accept'=> array(
                  'from' => 'review',
                  'to' => 'merged',
                ),
                'reject'=> array(
                  'from' => 'review',
                  'to' => 'closed',
                ),
                'reopen'=> array(
                  'from' => 'start',
                  'to' => 'review',
                ),
              ),
            ),
        ),
    ));
    

You can now use this state machine by getting the state_machine.pull_request service:

$stateMachine = $this->container->get('state_machine.pull_request');

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.