Rate Limit Subscriber for Guzzle 4.x + Laravel

RateLimitSubscriber.php

<?php

/**
 * Todo: Decouple from Laravel's Caching system
 **/

use GuzzleHttp\Collection;
use GuzzleHttp\Event\BeforeEvent;
use GuzzleHttp\Event\CompleteEvent;
use GuzzleHttp\Event\EmitterInterface;
use GuzzleHttp\Event\SubscriberInterface;

class RateLimitSubscriber implements SubscriberInterface
{

    /** @var Collection Configuration settings */
    private $config;

    public function __construct(array $config = [])
    {
        $this->config = Collection::fromConfig($config, [
            'msBetweenRequests' => '1000',
        ]);
    }

    public function getEvents()
    {
        return [
            'before'   => ['onBefore'],
            'complete' => ['onComplete'],
        ];
    }

    public function onBefore(BeforeEvent $event)
    {
        $ms = $this->config['msBetweenRequests'] * 1000;

        if ($time = \Cache::get('guzzleRateLimit.'.$this->getKey($event)) ) {
            $elapsed = microtime(true) - $time;

            if ($elapsed < $ms) {
                usleep($ms - $elapsed);
            }
        }
    }

    public function onComplete(CompleteEvent $event)
    {
        \Cache::forever('guzzleRateLimit.'.$this->getKey($event), microtime(true));
    }

    protected function getKey($event)
    {
        return $event->getRequest()->getHost();
    }
    
}

Usage:

$client->getEmitter()->attach(new RateLimitSubscriber([
    'msBetweenRequests' => 2000,
]));