Caching

Overview and Base Concepts

Caching is a cheap and effective way to improve the performance of a web application. By storing relatively static data in cache and serving it from cache when requested, we save the time needed to generate the data.

Using cache in Yii mainly involves configuring and accessing a cache application component. The following application configuration specifies a cache component that uses memcached with two cache servers.

'components' => array(
	'cache' => array(
		'class' => '\yii\caching\MemCache',
		'servers' => array(
			array(
				'host' => 'server1',
				'port' => 11211,
				'weight' => 100,
			),
			array(
				'host' => 'server2',
				'port' => 11211,
				'weight' => 50,
			),
		),
	),
),

When the application is running, the cache component can be accessed through Yii::$app->cache call.

Yii provides various cache components that can store cached data in different media. For example, the \yii\caching\MemCache component encapsulates the PHP memcache and memcached extensions and uses memory as the medium of cache storage; the \yii\caching\ApcCache component encapsulates the PHP APC extension; and the \yii\caching\DbCache component stores cached data in database table.

The following is a summary of the available cache components:

\yii\caching\ApcCache: uses PHP APC extension. This option can be considered as the fastest one when dealing with cache for a centralized thick application (e.g. one server, no dedicated load balancers, etc.).
\yii\caching\DbCache: uses a database table to store cached data. By default, it will create and use a SQLite3 database under the runtime directory. You can explicitly specify a database for it to use by setting its db property.
\yii\caching\DummyCache: presents dummy cache that does no caching at all. The purpose of this component is to simplify the code that needs to check the availability of cache. For example, during development or if the server doesn't have actual cache support, we can use this cache component. When an actual cache support is enabled, we can switch to use the corresponding cache component. In both cases, we can use the same code Yii::$app->cache->get($key) to attempt retrieving a piece of data without worrying that Yii::$all->cache might be null.
\yii\caching\FileCache: uses standard files to store cached data. This is particular suitable to cache large chunk of data (such as pages).
\yii\caching\MemCache: uses PHP memcache and memcached extensions. This option can be considered as the fastest one when dealing with cache in a distributed applications (e.g. with several servers, load balancers, etc.)
\yii\caching\RedisCache: implements a cache component based on Redis NoSQL database.
\yii\caching\WinCache: uses PHP WinCache (see also) extension.
\yii\caching\XCache: uses PHP XCache extension.
\yii\caching\ZendDataCache: uses Zend Data Cache as the underlying caching medium.

Tip: because all these cache components extend from the same base class Cache, one can switch to use a different type of cache without modifying the code that uses cache.

Caching can be used at different levels. At the lowest level, we use cache to store a single piece of data, such as a variable, and we call this data caching. At the next level, we store in cache a page fragment which is generated by a portion of a view script. And at the highest level, we store a whole page in cache and serve it from cache as needed.

In the next few subsections, we elaborate how to use cache at these levels.

Note, by definition, cache is a volatile storage medium. It does not ensure the existence of the cached data even if it does not expire. Therefore, do not use cache as a persistent storage (e.g. do not use cache to store session data or other valuable information).

Data Caching

Data caching is about storing some PHP variable in cache and retrieving it later from cache. For this purpose, the cache component base class \yii\caching\Cache provides two methods that are used most of the time: set() and get(). Note, only serializable variables and objects could be cached successfully.

To store a variable $value in cache, we choose a unique $key and call set() to store it:

Yii::$app->cache->set($key, $value);

The cached data will remain in the cache forever unless it is removed because of some caching policy (e.g. caching space is full and the oldest data are removed). To change this behavior, we can also supply an expiration parameter when calling set() so that the data will be removed from the cache after a certain period of time:

// keep the value in cache for at most 45 seconds
Yii::$app->cache->set($key, $value, 45);

Later when we need to access this variable (in either the same or a different web request), we call get() with the key to retrieve it from cache. If the value returned is false, it means the value is not available in cache and we should regenerate it:

public function getCachedData()
{
	$key = /* generate unique key here */;
	$value = Yii::$app->getCache()->get($key);
	if ($value === false) {
		$value = /* regenerate value because it is not found in cache and then save it in cache for later use */;
		Yii::$app->cache->set($id, $value);
	}
	return $value;
}

This is the common pattern of arbitrary data caching for general use.

When choosing the key for a variable to be cached, make sure the key is unique among all other variables that may be cached in the application. It is NOT required that the key is unique across applications because the cache component is intelligent enough to differentiate keys for different applications.

Some cache storages, such as MemCache, APC, support retrieving multiple cached values in a batch mode, which may reduce the overhead involved in retrieving cached data. A method named mget() is provided to exploit this feature. In case the underlying cache storage does not support this feature, mget() will still simulate it.

To remove a cached value from cache, call delete(); and to remove everything from cache, call flush(). Be very careful when calling flush() because it also removes cached data that are from other applications.

Note, because CCache implements ArrayAccess, a cache component can be used liked an array. The followings are some examples:

$cache = Yii::$app->getComponent('cache');
$cache['var1'] = $value1;  // equivalent to: $cache->set('var1', $value1);
$value2 = $cache['var2'];  // equivalent to: $value2 = $cache->get('var2');