Workflow Component

This component provides a workflow engine written as a PHP library.

Instead of modeling a workflow as a Petri net or trying to enumerate workflow patterns, the library consider a workflow as a simple directed graph: vertices model nodes and edges model transitions.

Nodes

A node represents a point in a life cycle. The Node class implements the concept. A node is referenced by a unique name across the workflow. The constraint is the responsibility of NodeMap class.

Transitions

A transition is a link between nodes. The Transition class implements the concept. At creation, a transition is given a specification object implementing the SpecificationInterface. the specification is used as a business rule to decide where to advance in the workflow.

Tokens

A token is a simple string used to initialize the workflow in a particular node. The idea is to consider the token as a thing placed at the center of a node. When workflow engine is on, the token is moving from node to node.

Events

An event is an object created each time a token arrives at a node. The Event class implements the concept. This class extends the Event class from the Symfony EventDispatcher component. You can write listeners or subscribers to implement any business behaviour.

Usage

Let's say you are writing a blog engine in PHP and you want to implement the following workflow:

• an article begins its existence as a draft
• when ready, the article gets published
• if controversial, the article is deleted
• when too old, the article is archived

First of all, you need to write classes implementing SpecificationInterface for every business rule:

namespace BlogEngine\Domain\Specification;

use Alterway\Component\Workflow\ContextInterface;
use Alterway\Component\Workflow\SpecificationInterface;

class DraftableArticleSpecification implements SpecificationInterface
{
    public function isSatisfiedBy(ContextInterface $context)
    {
        // an article can always be drafted
        return true;
    }
}

class PublishableArticleSpecification implements SpecificationInterface
{
    public function isSatisfiedBy(ContextInterface $context)
    {
        // an article needs two reviews to be published
        return 1 < count($context->get('article')->getReviews());
    }
}

class DeletableArticleSpecification implements SpecificationInterface
{
    public function isSatisfiedBy(ContextInterface $context)
    {
        // an article can always be deleted if requested
        return 'delete' === $context->get('action');
    }
}

class ArchivableArticleSpecification implements SpecificationInterface
{
    public function isSatisfiedBy(ContextInterface $context)
    {
        // an article needs to be one month old to be archived
        $publishedAtPlusOneMonth = clone $context->get('publishedAt');
        $publishedAtPlusOneMonth->modify('+1 month');

        return 'archive' === $context->get('action') && $publishedAtPlusOneMonth < $context->get('now');
    }
}

Then, you can use the Builder class and the specifications to describe the workflow:

namespace BlogEngine\Domain\Service;

use Alterway\Component\Workflow\Builder;
use Alterway\Component\Workflow\ContextInterface;
use BlogEngine\Domain\Event\ArticleSubscriber;
use BlogEngine\Domain\Specification\DraftableArticleSpecification;
use BlogEngine\Domain\Specification\PublishableArticleSpecification;
use BlogEngine\Domain\Specification\DeletableArticleSpecification;
use BlogEngine\Domain\Specification\ArchivableArticleSpecification;
use BlogEngine\Util\Context;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;

class ArticleService
{
    private $workflow;

    public function __construct(EventDispatcherInterface $eventDispatcher)
    {
        $this->workflow = (new Builder($eventDispatcher))
            ->open('article.draft', new DraftableArticleSpecification())
            ->link('article.draft', 'article.published', new PublishableArticleSpecification())
            ->link('article.published', 'article.deleted', new DeletableArticleSpecification())
            ->link('article.published', 'article.archived', new ArchivableArticleSpecification())
            ->getWorkflow();

        $eventDispatcher->addSubscriber(new ArticleSubscriber());
    }

    public function create(Article $article)
    {
        $this->advance($article, new Context());
    }

    public function publish(Article $article)
    {
        $context = new Context();
        $context->set('article', $article);

        $this->advance($article, $context);
    }

    public function delete(Article $article)
    {
        $context = new Context();
        $context->set('action', 'delete');

        $this->advance($article, $context);
    }

    public function archive(Article $article)
    {
        $context = new Context();
        $context->set('action', 'archive');
        $context->set('publishedAt', $article->getPublishedAt());
        $context->set('now', new \DateTime());

        $this->advance($article, $context);
    }

    private function advance($article, ContextInterface $context)
    {
        try {
            $this->workflow->initialize($article->getToken())->next($context);
        } catch (\LogicException $e) {
            // the workflow reports a problem
        }
    }
}

Finally, you have to listen on events dispatched by the workflow to attach the business behavior:

namespace BlogEngine\Domain\Event;

use Alterway\Component\Workflow\Event;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class ArticleSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return array(
            'article.draft' => array('onDraft', 0),
            'article.published' => array('onPublished', 0),
            'article.deleted' => array('onDeleted', 0),
            'article.archived' => array('onArchived', 0),
        );
    }

    public function onDraft(Event $event) { /* ... */ }

    public function onPublished(Event $event) { /* ... */ }

    public function onDeleted(Event $event) { /* ... */ }

    public function onArchived(Event $event) { /* ... */ }
}

Contributing

Pretty please, with sugar on top, phpspec specifications are provided and should be green when contributing code.

References

Theory

• Petri net
• Workflow patterns
• Graph theory
• Specification pattern

PHP

• An activity based workflow engine
• eZ Workflow component
• Yii simpleWorkflow extension
• Galaxia workflow engine
• State pattern by Sebastian Bergmann
• Petrinet Framework

Licencing

See the bundled LICENSE file for details.

Sponsors

• Alter Way
• La Ruche Qui Dit Oui !