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Introduction & Installation

An implementation of domain event pattern for Symfony & Doctrine.

What is a Domain Event?​

A domain event is simply a regular event like you would normally use with Symfony's EventDispatcher. The difference is that a domain event represents something that has happened in your domain. It has a name that is meaningful to the underlying business that the domain represents. A domain event is usually dispatched by your entities, as opposed to being dispatched from your controllers or other services.

Why Use Domain Events?​

A domain event represents a business event that has happened. It is a good way to model the business requirements that say "when something happens, do this".

A domain event is raised by the part of your code where the event is actually happening. Different part of your application might call the same method on an entity. In some cases, the method is called indirectly, and the caller has no idea that it is being called. By using domain events, the event will be dispatched in all the cases. No need to make sure to dispatch the event from all the different places where the method is called.

The application layer (controllers, services) can tell an entity to do something, but it cannot reliably know if the action is actually performed, or if an additional action is performed. A controller or a service can ask $bookshelf->removeBook($book), but only the $bookshelf knows if the book was actually removed. And if the event actually happened, the entity can tell the world about it by recording a BookRemoved event.

Some problems might tempt you to inject a service into your entity. With domain events, you can avoid that. Your entity can dispatch an event, and you can set up a listener to react to that event. The relevant services can then correctly act on your entity, instead of the other way around.

Features​

  • Works out of the box. No configuration is required for basic features.
  • Simple, unopinionated architecture. Uses plain event objects, and doesn't require much from your domain entities.
  • Uses standard Symfony's EventDispatcher, with the same dispatching semantics & listener registrations.
  • Transaction support.
  • Works with multiple entity managers.
  • Multiple events considered identical are dispatched only once.
  • Four listening strategies: immediate, pre-flush, post-flush, and event bus.
  • Uses Symfony Messenger as the event bus implementation.
  • Utilizes the transactional outbox pattern when publishing events to the event bus to guarantee consistency and delivery.
  • Utilizes Symfony Scheduler to relay undelivered events to the event bus.
  • Does not require you to change how you work with entities.
  • Should work everywhere without any change: in controllers, message handlers, command line, etc.
  • Separated contracts & framework. Useful for enforcing architectural boundaries. Your domain doesn't have to depend on the framework.
  • Symfony Profiler integration. Debug your events in the profiler's events panel.

To Do​

  • Support for Doctrine MongoDB ODM.
  • Support event inheritance.

Synopsis​

//
// The event
//

final readonly class PostPublished
{
    public function __construct(public string $postId) {}
}

//
// The entity
//

use Rekalogika\Contracts\DomainEvent\DomainEventEmitterInterface;
use Rekalogika\Contracts\DomainEvent\DomainEventEmitterTrait;

class Post implements DomainEventEmitterInterface
{
    use DomainEventEmitterTrait;
    
    // ...

    public function setStatus(string $status): void
    {
        $originalStatus = $this->status;
        $this->status = $status;

        // records the published event if the new status is published and it
        // is different from the original status

        if ($status === 'published' && $originalStatus !== $status) {
            $this->recordEvent(new PostPublished($this->id));
        }
    }

    // ...
}

//
// The listener
//

use Psr\Log\LoggerInterface;
use Rekalogika\Contracts\DomainEvent\Attribute\AsPostFlushDomainEventListener;

class PostEventListener
{
    public function __construct(private LoggerInterface $logger) {}

    // will be called after the post is published and the entity manager is
    // flushed
    
    #[AsPostFlushDomainEventListener]
    public function onPostPublished(PostPublished $event) {
        $postId = $event->postId;

        $this->logger->info("Post $postId has been published.");
    }
}

//
// The caller
//

use Doctrine\ORM\EntityManagerInterface;

/** @var Post $post */
/** @var EntityManagerInterface $entityManager */

$post->setStatus('published');
$entityManager->flush();

// the event will be dispatched after the flush above, afterwards the listener
// above will be called, sending a message to the logger

Installation​

Make sure Composer is installed globally, as explained in the installation chapter of the Composer documentation.

Open a command console, enter your project directory and execute:

composer require rekalogika/domain-event

License​

MIT

Contributing​

This framework consists of multiple repositories split from a monorepo. Be sure to submit issues and pull requests to the rekalogika/domain-event-src monorepo.