Cache Pools and Supported Adapters

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Cache Pools and Supported Adapters

Cache Pools are the logical repositories of cache items. They perform all the common operations on items, such as saving them or looking for them. Cache pools are independent of the actual cache implementation. Therefore, applications can keep using the same cache pool even if the underlying cache mechanism changes from a file system based cache to a Redis or database based cache.

Creating Cache Pools

Cache Pools are created through the cache adapters, which are classes that implement both CacheInterface and Psr\Cache\CacheItemPoolInterface. This component provides several adapters ready to use in your applications.

Using the Cache Contracts

The CacheInterface allows fetching, storing and deleting cache items using only two methods and a callback:

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        use Symfony\Component\Cache\Adapter\FilesystemAdapter;
use Symfony\Contracts\Cache\ItemInterface;

$cache = new FilesystemAdapter();

// The callable will only be executed on a cache miss.
$value = $cache->get('my_cache_key', function (ItemInterface $item): string {
    $item->expiresAfter(3600);

    // ... do some HTTP request or heavy computations
    $computedValue = 'foobar';

    return $computedValue;
});

echo $value; // 'foobar'

// ... and to remove the cache key
$cache->delete('my_cache_key');
    

Out of the box, using this interface provides stampede protection via locking and early expiration. Early expiration can be controlled via the third "beta" argument of the get() method. See the The Cache Component article for more information.

Early expiration can be detected inside the callback by calling the isHit() method: if this returns true, it means we are currently recomputing a value ahead of its expiration date.

For advanced use cases, the callback can accept a second bool &$save argument passed by reference. By setting $save to false inside the callback, you can instruct the cache pool that the returned value should not be stored in the backend.

Using PSR-6

Looking for Cache Items

Cache Pools define three methods to look for cache items. The most common method is getItem($key), which returns the cache item identified by the given key:

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        use Symfony\Component\Cache\Adapter\FilesystemAdapter;

$cache = new FilesystemAdapter('app.cache');
$latestNews = $cache->getItem('latest_news');
    

If no item is defined for the given key, the method doesn't return a null value but an empty object which implements the CacheItem class.

If you need to fetch several cache items simultaneously, use instead the getItems([$key1, $key2, ...]) method:

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        // ...
$stocks = $cache->getItems(['AAPL', 'FB', 'GOOGL', 'MSFT']);
    

Again, if any of the keys doesn't represent a valid cache item, you won't get a null value but an empty CacheItem object.

The last method related to fetching cache items is hasItem($key), which returns true if there is a cache item identified by the given key:

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        // ...
$hasBadges = $cache->hasItem('user_'.$userId.'_badges');
    

Saving Cache Items

The most common method to save cache items is Psr\Cache\CacheItemPoolInterface::save, which stores the item in the cache immediately (it returns true if the item was saved or false if some error occurred):

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        // ...
$userFriends = $cache->getItem('user_'.$userId.'_friends');
$userFriends->set($user->getFriends());
$isSaved = $cache->save($userFriends);
    

Sometimes you may prefer to not save the objects immediately in order to increase the application performance. In those cases, use the Psr\Cache\CacheItemPoolInterface::saveDeferred method to mark cache items as "ready to be persisted" and then call to Psr\Cache\CacheItemPoolInterface::commit method when you are ready to persist them all:

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        // ...
$isQueued = $cache->saveDeferred($userFriends);
// ...
$isQueued = $cache->saveDeferred($userPreferences);
// ...
$isQueued = $cache->saveDeferred($userRecentProducts);
// ...
$isSaved = $cache->commit();
    

The saveDeferred() method returns true when the cache item has been successfully added to the "persist queue" and false otherwise. The commit() method returns true when all the pending items are successfully saved or false otherwise.

Removing Cache Items

Cache Pools include methods to delete a cache item, some of them or all of them. The most common is Psr\Cache\CacheItemPoolInterface::deleteItem, which deletes the cache item identified by the given key (it returns true when the item is successfully deleted or doesn't exist and false otherwise):

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        // ...
$isDeleted = $cache->deleteItem('user_'.$userId);
    

Use the Psr\Cache\CacheItemPoolInterface::deleteItems method to delete several cache items simultaneously (it returns true only if all the items have been deleted, even when any or some of them don't exist):

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        // ...
$areDeleted = $cache->deleteItems(['category1', 'category2']);
    

Finally, to remove all the cache items stored in the pool, use the Psr\Cache\CacheItemPoolInterface::clear method (which returns true when all items are successfully deleted):

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        // ...
$cacheIsEmpty = $cache->clear();
    

Tip

If the cache component is used inside a Symfony application, you can remove items from cache pools using the following commands (which reside within the framework bundle):

To remove one specific item from the given pool:

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        $ php bin/console cache:pool:delete <cache-pool-name> <cache-key-name>

# deletes the "cache_key" item from the "cache.app" pool
$ php bin/console cache:pool:delete cache.app cache_key
    

You can also remove all items from the given pool(s):

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        $ php bin/console cache:pool:clear <cache-pool-name>

# clears the "cache.app" pool
$ php bin/console cache:pool:clear cache.app

# clears the "cache.validation" and "cache.app" pool
$ php bin/console cache:pool:clear cache.validation cache.app
    

Pruning Cache Items

Some cache pools do not include an automated mechanism for pruning expired cache items. For example, the FilesystemAdapter cache does not remove expired cache items until an item is explicitly requested and determined to be expired, for example, via a call to Psr\Cache\CacheItemPoolInterface::getItem. Under certain workloads, this can cause stale cache entries to persist well past their expiration, resulting in a sizable consumption of wasted disk or memory space from excess, expired cache items.

This shortcoming has been solved through the introduction of PruneableInterface, which defines the abstract method prune(). The ChainAdapter, FilesystemAdapter, PdoAdapter, and PhpFilesAdapter all implement this new interface, allowing manual removal of stale cache items:

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        use Symfony\Component\Cache\Adapter\FilesystemAdapter;

$cache = new FilesystemAdapter('app.cache');
// ... do some set and get operations
$cache->prune();
    

The ChainAdapter implementation does not directly contain any pruning logic itself. Instead, when calling the chain adapter's prune() method, the call is delegated to all its compatible cache adapters (and those that do not implement PruneableInterface are silently ignored):

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        use Symfony\Component\Cache\Adapter\ApcuAdapter;
use Symfony\Component\Cache\Adapter\ChainAdapter;
use Symfony\Component\Cache\Adapter\FilesystemAdapter;
use Symfony\Component\Cache\Adapter\PdoAdapter;
use Symfony\Component\Cache\Adapter\PhpFilesAdapter;

$cache = new ChainAdapter([
    new ApcuAdapter(),       // does NOT implement PruneableInterface
    new FilesystemAdapter(), // DOES implement PruneableInterface
    new PdoAdapter(),        // DOES implement PruneableInterface
    new PhpFilesAdapter(),   // DOES implement PruneableInterface
    // ...
]);

// prune will proxy the call to PdoAdapter, FilesystemAdapter and PhpFilesAdapter,
// while silently skipping ApcuAdapter
$cache->prune();
    

Tip

If the cache component is used inside a Symfony application, you can prune all items from all pools using the following command (which resides within the framework bundle):

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        $ php bin/console cache:pool:prune
    

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.

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