consumers.rst

Consumers

Implementations of PMG\Queue\Consumer pull message out of a driver backend and handle (process) them in some way. The default consumer accomplishes this a message handler <handlers>.

In all cases $queueName in the consume should correspond to queues into which your producer <producers> put messages.

The script to run your consumer might look something like this. Check out the handlers <handlers> documentation for more information about what $handler is below.

<?php

use PMG\Queue\DefaultConsumer;
use PMG\Queue\Driver\MemoryDriver;

$driver = new MemoryDriver();

/** @var PMG\Queue\MessageHandler $handler */
$consumer = new DefaultConsumer($driver, $handler);

exit($consumer->run(isset($argv[1]) ? $argv[1] : 'defaultQueue'));

Retrying Messages

When a message fails -- by throwing an exception or returns false from a MessageHandler -- the consumer puts it back in the queue to retry up to 5 times by default. This behavior can be adjusted by providing a RetrySpec as the third argument to DefaultConsumers constructor. pmg/queue provides a few by default.

Retry specs look at PMG\Queue\Envelope instances, not raw messages. See the internals documentation <envelopes> for more info about them.

Limited Retries

Use PMG\\Queue\\Retry\\LimitedSpec.

<?php

use PMG\Queue\DefaultConsumer;
use PMG\Queue\Retry\LimitedSpec;

// five retries by default. This is what the consumer does automatically
$retry = new LimitedSpec();

// Or limit to a specific number of retries
$retry = new LimitedSpec(2);

// $driver and $handler as above
$consumer = new DefaultConsumer($driver, $handler, $retry);

Never Retry a Message

Sometimes you don't want to retry a message, for those cases use PMG\\Queue\\Retry\\NeverSpec.

<?php

use PMG\Queue\DefaultConsumer;
use PMG\Queue\Retry\NeverSpec;

$retry = new NeverSpec();

// $driver and $handler as above
$consumer = new DefaultConsumer($driver, $handler, $retry);

Logging

When something goes wrong DefaultConsumer logs it with a PSR-3 Logger implementation. The default is to use a NullLogger, but you can provide your own logger as the fourth argument to DefaultConsumer's constructor.

<?php

use PMG\Queue\DefaultConsumer;

$monolog = new Monolog\Logger('yourApp');

// $driver, $handler, $retry as above
$consumer = new DefaultConsumer($driver, $handler, $retry, $monolog);

Build Custom Consumers

Extend PMG\\Queue\\AbstractConsumer to make things easy and only have to implement the once method. Here's an example that decorates another Consumer with events.

<?php

use PMG\Queue\AbstractConsumer;
use PMG\Queue\Consumer;
use PMG\Queue\Message;
use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;

final class EventingConsumer extends AbstractConsumer
{
    /** @var Consumer */
    private $wrapped;

    /** @var EventDispatcherInterface $events */

    // constructor that takes a consumer and dispatcher to set the props ^

    public function once($queueName)
    {
        $this->events->dispatch('queue:before_once', new Event());
        $this->wrapped->once($queueName);
        $this->events->disaptch('queue:after_once', new Event());
    }
}