Caching
=======

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 memcache with two cache servers.

~~~
[php]
array(
	......
	'components'=>array(
		......
		'cache'=>array(
			'class'=>'system.caching.CMemCache',
			'servers'=>array(
				array('host'=>'server1', 'port'=>11211, 'weight'=>60),
				array('host'=>'server2', 'port'=>11211, 'weight'=>40),
			),
		),
	),
);
~~~

When the application is running, the cache component can be accessed via
`Yii::app()->cache`.

Yii provides various cache components that can store cached data in
different media. For example, the [CMemCache] component encapsulates the
PHP memcache extension and uses memory as the medium of cache storage; the
[CApcCache] component encapsulates the PHP APC extension; and the
[CDbCache] component stores cached data in database. The following is a
summary of the available cache components:

   - [CMemCache]: uses PHP [memcache extension](http://www.php.net/manual/en/book.memcache.php).

   - [CApcCache]: uses PHP [APC extension](http://www.php.net/manual/en/book.apc.php).

   - [CXCache]: uses PHP [XCache extension](http://xcache.lighttpd.net/).

   - [CEAcceleratorCache]: uses PHP [EAccelerator extension](http://eaccelerator.net/).

   - [CRedisCache]: uses [redis](http://redis.io/) key value-store as storage backend.

   - [CDbCache]: 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
[connectionID|CDbCache::connectionID] property.

   - [CZendDataCache]: uses [Zend Data Cache](http://files.zend.com/help/Zend-Server-Community-Edition/data_cache_component.htm)
   as the underlying caching medium.

   - [CFileCache]: uses files to store cached data. This is particular suitable to
cache large chunk of data (such as pages).

   - [CDummyCache]: 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::app()->cache` might be `null`.

> Tip: Because all these cache components extend from the same base class
[CCache], 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).

