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Cache Pools

Cache Pools

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 from 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 Symfony\Component\Cache\Adapter\AdapterInterface. This component provides several adapters ready to use in your applications.

Array Cache Adapter

This adapter is only useful for testing purposes because contents are stored in memory and not persisted in any way. Besides, some features explained later are not available, such as the deferred saves:

use Symfony\Component\Cache\Adapter\ArrayAdapter;

$cache = new ArrayAdapter(
    // in seconds; applied to cache items that don't define their own lifetime
    // 0 means to store the cache items indefinitely (i.e. until the current PHP process finishes)
    $defaultLifetime = 0,
    // if ``true``, the values saved in the cache are serialized before storing them
    $storeSerialized = true

Filesystem Cache Adapter

This adapter is useful when you want to improve the application performance but can’t install tools like APC or Redis in the server. This adapter stores the contents as regular files in a set of directories on the local file system:

use Symfony\Component\Cache\Adapter\FilesystemAdapter;

$cache = new FilesystemAdapter(
    // the subdirectory of the main cache directory where cache items are stored
    $namespace = '',
    // in seconds; applied to cache items that don't define their own lifetime
    // 0 means to store the cache items indefinitely (i.e. until the files are deleted)
    $defaultLifetime = 0,
    // the main cache directory (the application needs read-write permissions on it)
    // if none is specified, a directory is created inside the system temporary directory
    $directory = null

APCu Cache Adapter

This adapter can increase the application performance very significantly, because contents are cached in the shared memory of your server, which is much faster than the file system. It requires to have installed and enabled the PHP APCu extension. It’s not recommended to use it when performing lots of write and delete operations because it produces fragmentation in the APCu memory that can degrade performance significantly:

use Symfony\Component\Cache\Adapter\ApcuAdapter;

$cache = new ApcuAdapter(
    // the string prefixed to the keys of the items stored in this cache
    $namespace = '',
    // in seconds; applied to cache items that don't define their own lifetime
    // 0 means to store the cache items indefinitely (i.e. until the APC memory is deleted)
    $defaultLifetime = 0,
    // if present, this string is added to the namespace to simplify the
    // invalidation of the entire cache (e.g. when deploying the application)
    $version = null

Redis Cache Adapter

This adapter stores the contents in the memory of the server. Unlike the APCu adapter, it’s not limited to the shared memory of the current server, so you can store contents in a cluster of servers if needed.

It requires to have installed Redis and have created a connection that implements the \Redis, \RedisArray, \RedisCluster or \Predis classes:

use Symfony\Component\Cache\Adapter\RedisAdapter;

$cache = new RedisAdapter(
    // the object that stores a valid connection to your Redis system
    \Redis $redisConnection,
    // the string prefixed to the keys of the items stored in this cache
    $namespace = '',
    // in seconds; applied to cache items that don't define their own lifetime
    // 0 means to store the cache items indefinitely (i.e. until the Redis memory is deleted)
    $defaultLifetime = 0

Chain Cache Adapter

This adapter allows to combine any number of the previous adapters. Cache items are fetched from the first adapter which contains them. Besides, cache items are saved in all the given adapters, so this is a simple way of creating a cache replication:

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

$apcCache = new ApcuAdapter();
$fileCache = new FilesystemAdapter();

$cache = new ChainAdapter(array($apcCache, $fileCache));

When an item is not found in the first adapters but is found in the next ones, the ChainAdapter ensures that the fetched item is saved in all the adapters where it was missing. Since it’s not possible to know the expiry date and time of a cache item, the second optional argument of ChainAdapter is the default lifetime applied to those cache items (by default it’s 0).

Proxy Cache Adapter

This adapter is useful to integrate in your application cache pools not created with the Symfony Cache component. As long as those cache pools implement the CacheItemPoolInterface interface, this adapter allows you to get items from that external cache and save them in the Symfony cache of your application:

use Symfony\Component\Cache\Adapter\ProxyAdapter;

// ... create $nonSymfonyCache somehow
$cache = new ProxyAdapter($nonSymfonyCache);

The adapter accepts two additional optional arguments: the namespace ('' by default) and the default lifetime (0 by default).

Another use case for this adapter is to get statistics and metrics about the cache hits (getHits()) and misses (getMisses()).

Doctrine Cache Adapter

This adapter wraps any Doctrine Cache provider so you can use them in your application as if they were Symfony Cache adapters:

use Doctrine\Common\Cache\SQLite3Cache;
use Symfony\Component\Cache\Adapter\DoctrineAdapter;

$sqliteDatabase = new \SQLite3(__DIR__.'/cache/data.sqlite');
$doctrineCache = new SQLite3Cache($sqliteDatabase, 'tableName');
$symfonyCache = new DoctrineAdapter($doctrineCache);

This adapter also defines two optional arguments called namespace (default: '') and defaultLifetime (default: 0) and adapts them to make them work in the underlying Doctrine cache.

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:

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 Symfony\Component\Cache\CacheItem class.

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

// ...
$stocks = $cache->getItems(array('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:

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

Saving Cache Items

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

// ...
$userFriends = $cache->get('user_'.$userId.'_friends');
$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 saveDeferred() method to mark cache items as “ready to be persisted” and then call to commit() method when you are ready to persist them all:

// ...
$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 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):

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

Use the 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):

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

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

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

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