2022-03-25 - Cache stampede protection

INFO

This document represents an architecture decision record (ADR) and has been mirrored from the ADR section in our Shopware 6 repository. You can find the original version here

The Cache Stampede protection is a mechanism to prevent several users try to update this cache entry at the same time if a cache entry is no longer hot. This mechanism is very useful if there is a lot of load on the store and a cache entry is expired or invalidated. If there is no cache stampede protection on the system, and several users call a category listing at the same time, which is no longer in the cache, then all users would be let through to the database and the server could collapse under the load.

We have now integrated such a protection into all our services using the \Symfony\Contracts\Cache\CacheInterface of symfony. This mechanic is mainly used in our cached store api routes. Another positive side effect is that the code has become much more concise, since much is done within symfony:

CachedRuleLoader before

<?php declare(strict_types=1);

namespace Shopware\Core\Checkout\Cart;

use Psr\Log\LoggerInterface;
use Shopware\Core\Content\Rule\RuleCollection;
use Shopware\Core\Framework\Context;
use Symfony\Component\Cache\Adapter\TagAwareAdapterInterface;

class CachedRuleLoader extends AbstractRuleLoader
{
    public const CACHE_KEY = 'cart_rules';

    private AbstractRuleLoader $decorated;

    private TagAwareAdapterInterface $cache;

    private LoggerInterface $logger;

    public function __construct(AbstractRuleLoader $decorated, TagAwareAdapterInterface $cache, LoggerInterface $logger)
    {
        $this->decorated = $decorated;
        $this->cache = $cache;
        $this->logger = $logger;
    }

    public function getDecorated(): AbstractRuleLoader
    {
        return $this->decorated;
    }

    public function load(Context $context): RuleCollection
    {
        $item = $this->cache->getItem(self::CACHE_KEY);

        try {
            if ($item->isHit() && $item->get()) {
                $this->logger->info('cache-hit: ' . self::CACHE_KEY);

                return $item->get();
            }
        } catch (\Throwable $e) {
            $this->logger->error($e->getMessage());
        }

        $this->logger->info('cache-miss: ' . self::CACHE_KEY);

        $rules = $this->getDecorated()->load($context);

        $item->set($rules);
        $this->cache->save($item);

        return $rules;
    }
}

CachedRuleLoader after

<?php declare(strict_types=1);

namespace Shopware\Core\Checkout\Cart;

use Shopware\Core\Content\Rule\RuleCollection;
use Shopware\Core\Framework\Context;
use Symfony\Contracts\Cache\CacheInterface;

class CachedRuleLoader extends AbstractRuleLoader
{
    public const CACHE_KEY = 'cart_rules';

    private AbstractRuleLoader $decorated;

    private CacheInterface $cache;

    public function __construct(AbstractRuleLoader $decorated, CacheInterface $cache)
    {
        $this->decorated = $decorated;
        $this->cache = $cache;
    }

    public function getDecorated(): AbstractRuleLoader
    {
        return $this->decorated;
    }

    public function load(Context $context): RuleCollection
    {
        return $this->cache->get(self::CACHE_KEY, function () use ($context): RuleCollection {
            return $this->decorated->load($context);
        });
    }
}

However, since the service itself does not recognize whether it is a cache hit or miss, we have removed the corresponding logging.