inanzzz | Implementing tagged Strategy Pattern services without Symfony Compiler Pass feature

In this example we are going to avoid using Compiler Pass by injecting services tagged with a specific tag into another service. Our example implements Strategy Pattern.

Design

We have two consumer services and one console command. Console command watches messages in two different queues (image and upload) and passes them to main consumer (Consumer). The main consumer then passes messages to the relevant consumers (ImageConsumer and UploadConsumer) where they will be processed. The main aim here is not to create more than one command.

Files

service.yml

service:
    # CONSUMER
    App\Consumer\Consumer:
        arguments: [!tagged consumer]

    App\Consumer\ImageConsumer:
        tags:
            - { name: consumer }

    App\Consumer\UploadConsumer:
        tags:
            - { name: consumer }

QueueWorkerCommand

declare(strict_types=1);

namespace App\Command;

use App\Consumer\Consumer;
use Symfony\Component\Console\Command\Command;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Input\InputOption;
use Symfony\Component\Console\Output\OutputInterface;

class QueueWorkerCommand extends Command
{
    private $consumer;

    public function __construct(Consumer $consumer)
    {
        parent::__construct();

        $this->consumer = $consumer;
    }

    protected function configure()
    {
        $this
            ->setName('app:queue-worker')
            ->setDescription('Helps consumer consume messages in queues.')
            ->setHelp('Watches messages in queues and passes them to the consumer.')
            ->addOption(
                'queue',
                null,
                InputOption::VALUE_REQUIRED,
                'What is the name of the queue to watch?'
            );
    }

    protected function execute(InputInterface $input, OutputInterface $output): void
    {
        $queue = trim($input->getOption('queue'));
        if (!$queue) {
            return;
        }

        $output->writeln(sprintf('Watching the messages in "%s" queue...', $queue));

        $this->watch($queue);
    }

    private function watch(string $queue): void
    {
        // Grab the message from the queue. e.g. RabbitMQ, Beanstalk etc.
        $message = 'I am the message';

        $result = $this->consumer->consume($queue, $message);

        // Do something next
    }
}

Consumer

declare(strict_types=1);

namespace App\Consumer;

use Traversable;

class Consumer
{
    private $consumers;

    public function __construct(Traversable $consumers)
    {
        $this->consumers = $consumers;
    }

    public function consume(string $queue, $message): bool
    {
        /** @var ConsumerInterface $consumer */
        foreach ($this->consumers as $consumer) {
            if ($consumer->canConsume($queue)) {
                return $consumer->consume($message);
            }
        }
    }
}

ConsumerInterface

declare(strict_types=1);

namespace App\Consumer;

interface ConsumerInterface
{
    public function canConsume(string $queue): bool;

    public function consume($message): bool;
}

ImageConsumer

declare(strict_types=1);

namespace App\Consumer;

class ImageConsumer implements ConsumerInterface
{
    public function canConsume(string $queue): bool
    {
        return $queue === 'image';
    }

    public function consume($message): bool
    {
        echo sprintf('Consumed message "%s" in queue "image".'.PHP_EOL, $message);

        return true;
    }
}

UploadConsumer

declare(strict_types=1);

namespace App\Consumer;

class UploadConsumer implements ConsumerInterface
{
    public function canConsume(string $queue): bool
    {
        return $queue === 'upload';
    }

    public function consume($message): bool
    {
        echo sprintf('Consumed message "%s" in queue "upload".'.PHP_EOL, $message);

        return true;
    }
}

Test

$ bin/console app:queue-worker --queue=image
Watching the messages in "image" queue...
Consumed message "I am the message" in queue "image".

$ bin/console app:queue-worker --queue=upload
Watching the messages in "upload" queue...
Consumed message "I am the message" in queue "upload".