async_events.md

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{% include support.md %}

Async events

The EnqueueBundle allows you to dispatch events asynchronously. Behind the scene it replaces your listener with one that sends a message to MQ. The message contains the event object. The consumer, once it receives the message, restores the event and dispatches it to only async listeners.

Async listeners benefits:

• Reduces response time. Work is deferred to consumer processes.
• Better fault tolerance. Bugs in async listener do not affect user. Messages will wait till you fix bugs.
• Better scaling. Add more consumers to meet the load.

Note: Prior to Symfony 3.0, events contain eventDispatcher and the default php serializer transformer is unable to serialize the object. A transformer should be registered for every async event. Read the event transformer.

Configuration

Symfony events are currently processed synchronously, enabling the async configuration for EnqueueBundle causes tagged listeners to defer action to a consumer asynchronously. If you already installed the bundle, then enable async_events.

# app/config/config.yml

enqueue:
    default:
        async_events:
            enabled: true
            # if you'd like to send send messages onTerminate use spool_producer (it further reduces response time):
            # spool_producer: true

Usage

To make your listener async you have add async: true and dispatcher: 'enqueue.events.event_dispatcher' attributes to the tag kernel.event_listener, like this:

# app/config/config.yml

services:
    acme.foo_listener:
        class: 'AcmeBundle\Listener\FooListener'
        tags:
            - { name: 'kernel.event_listener', async: true, event: 'foo', method: 'onEvent', dispatcher: 'enqueue.events.event_dispatcher' }

or to kernel.event_subscriber:

# app/config/config.yml

services:
    test_async_subscriber:
        class: 'AcmeBundle\Listener\TestAsyncSubscriber'
        tags:
            - { name: 'kernel.event_subscriber', async: true, dispatcher: 'enqueue.events.event_dispatcher' }

That's basically it. The rest of the doc describes advanced features.

Advanced Usage.

You can also add an async listener directly and register a custom message processor for it:

# app/config/config.yml

services:
    acme.async_foo_listener:
        class: 'Enqueue\AsyncEventDispatcher\AsyncListener'
        public: false
        arguments: ['@enqueue.transport.default.context', '@enqueue.events.registry', 'a_queue_name']
        tags:
          - { name: 'kernel.event_listener', event: 'foo', method: 'onEvent', dispatcher: 'enqueue.events.event_dispatcher' }

Event transformer

The bundle uses php serializer transformer by default to pass events through MQ. You can write a transformer for each event type by implementing the Enqueue\AsyncEventDispatcher\EventTransformer interface. Consider the next example. It shows how to send an event that contains Doctrine entity as a subject

<?php
namespace AcmeBundle\Listener;

// src/AcmeBundle/Listener/FooEventTransformer.php

use Enqueue\Client\Message;
use Enqueue\Consumption\Result;
use Interop\Queue\Message as QueueMessage;
use Enqueue\Util\JSON;
use Symfony\Component\EventDispatcher\Event;
use Enqueue\AsyncEventDispatcher\EventTransformer;
use Doctrine\Bundle\DoctrineBundle\Registry;
use Symfony\Component\EventDispatcher\GenericEvent;

class FooEventTransformer implements EventTransformer
{
    /** @var Registry @doctrine */
    private $doctrine;

    public function __construct(Registry $doctrine)
    {
        $this->doctrine = $doctrine;
    }

    /**
     * {@inheritdoc}
     *
     * @param GenericEvent $event
     */
    public function toMessage($eventName, Event $event = null)
    {
        $entity = $event->getSubject();
        $entityClass = get_class($entity);

        $manager = $this->doctrine->getManagerForClass($entityClass);
        $meta = $manager->getClassMetadata($entityClass);

        $id = $meta->getIdentifierValues($entity);

        $message = new Message();
        $message->setBody([
            'entityClass' => $entityClass,
            'entityId' => $id,
            'arguments' => $event->getArguments()
        ]);

        return $message;
    }

    /**
     * {@inheritdoc}
     */
    public function toEvent($eventName, QueueMessage $message)
    {
        $data = JSON::decode($message->getBody());

        $entityClass = $data['entityClass'];

        $manager = $this->doctrine->getManagerForClass($entityClass);
        if (false == $entity = $manager->find($entityClass, $data['entityId'])) {
            return Result::reject('The entity could not be found.');
        }

        return new GenericEvent($entity, $data['arguments']);
    }
}

and register it:

# app/config/config.yml

services:
    acme.foo_event_transformer:
        class: 'AcmeBundle\Listener\FooEventTransformer'
        arguments: ['@doctrine']
        tags:
            - {name: 'enqueue.event_transformer', eventName: 'foo' }

The eventName attribute accepts a regexp. You can do next eventName: '/foo\..*?/'. It uses this transformer for all event with the name beginning with foo.

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