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docs/guide/caching-data.md
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:
[[yii\caching\Cache::set()|set()]] and [[yii\caching\Cache::get()|get()]]. Note, only serializable variables and objects could be cached successfully.
Data caching is about storing some PHP variable in cache and retrieving it later from cache.
It is also the foundation for more advanced caching features, such as [query caching](#query-caching)
and [content caching](caching-content.md).
To store a variable `$value` in cache, we choose a unique `$key` and call [[yii\caching\Cache::set()|set()]] to store it:
The following code is a typical usage pattern of data caching, where `$cache` refers to
a [cache component](#cache-components):
```php
Yii::$app->cache->set($key, $value);
```
// try retrieving $data from cache
$data = $cache->get($key);
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 [[yii\caching\Cache::set()|set()]] so that the data will be removed from the cache after
a certain period of time:
```php
// keep the value in cache for at most 45 seconds
Yii::$app->cache->set($key, $value, 45);
```
if ($data === false) {
Later when we need to access this variable (in either the same or a different web request), we call [[yii\caching\Cache::get()|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:
// $data is not found in cache, calculate it from scratch
```php
public function getCachedData()
{
$key = /* generate unique key here */;
$value = Yii::$app->cache->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($key, $value);
}
return $value;
// store $data in cache so that it can be retrieved next time
$cache->set($key, $data);
}
// $data is available here
```
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.
## Cache Components <a name="cache-components"></a>
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 [[yii\caching\Cache::mget()|mget()]] is provided
to exploit this feature. In case the underlying cache storage does not support this feature,
[[yii\caching\Cache::mget()|mget()]] will still simulate it.
Data caching relies on the so-called *cache components* which represent various cache storage,
such as memory, files, databases.
To remove a cached value from cache, call [[yii\caching\Cache::delete()|delete()]]; and to remove everything from cache, call
[[yii\caching\Cache::flush()|flush()]].
Be very careful when calling [[yii\caching\Cache::flush()|flush()]] because it also removes cached data that are from
other applications if the cache is shared among different applications.
Note, because [[yii\caching\Cache]] implements `ArrayAccess`, a cache component can be used liked an array. The followings
are some examples:
Cache components are usually registered as application components so that they can be globally configurable
and accessible. The following code shows how to configure the `cache` application component to use
[memcached](http://memcached.org/) with two cache servers:
```php
$cache = Yii::$app->cache;
$cache['var1'] = $value1; // equivalent to: $cache->set('var1', $value1);
$value2 = $cache['var2']; // equivalent to: $value2 = $cache->get('var2');
'components' => [
'cache' => [
'class' => 'yii\caching\MemCache',
'servers' => [
[
'host' => 'server1',
'port' => 11211,
'weight' => 100,
],
[
'host' => 'server2',
'port' => 11211,
'weight' => 50,
],
],
],
],
```
### Cache Dependency
You can then access the above cache component using the expression `Yii::$app->cache`.
Besides expiration setting, cached data may also be invalidated according to some dependency changes. For example, if we
are caching the content of some file and the file is changed, we should invalidate the cached copy and read the latest
content from the file instead of the cache.
Because all cache components support the same set of APIs, you can swap the underlying cache component
with a different one by reconfiguring it in the application configuration without modifying the code that uses the cache.
For example, you can modify the above configuration to use [[yii\caching\ApcCache|APC cache]]:
We represent a dependency as an instance of [[yii\caching\Dependency]] or its child class. We pass the dependency
instance along with the data to be cached when calling [[yii\caching\Cache::set()|set()]].
```php
use yii\caching\FileDependency;
// the value will expire in 30 seconds
// it may also be invalidated earlier if the dependent file is changed
Yii::$app->cache->set($id, $value, 30, new FileDependency(['fileName' => 'example.txt']));
'components' => [
'cache' => [
'class' => 'yii\caching\ApcCache',
],
],
```
Now if we retrieve $value from cache by calling `get()`, the dependency will be evaluated and if it is changed, we will
get a false value, indicating the data needs to be regenerated.
Below is a summary of the available cache dependencies:
- [[yii\caching\FileDependency]]: the dependency is changed if the file's last modification time is changed.
- [[yii\caching\GroupDependency]]: marks a cached data item with a group name. You may invalidate the cached data items
with the same group name all at once by calling [[yii\caching\GroupDependency::invalidate()]].
- [[yii\caching\DbDependency]]: the dependency is changed if the query result of the specified SQL statement is changed.
- [[yii\caching\ChainedDependency]]: the dependency is changed if any of the dependencies on the chain is changed.
- [[yii\caching\ExpressionDependency]]: the dependency is changed if the result of the specified PHP expression is
changed.
### Query Caching
For caching the result of database queries you can wrap them in calls to [[yii\db\Connection::beginCache()]]
and [[yii\db\Connection::endCache()]]:
> Tip: You can register multiple cache application components. The component named `cache` is used
by default by many cache-dependent classes (e.g. [[yii\web\UrlManager]]).
### Supported Cache Storage <a name="supported-cache-storage"></a>
Yii supports a wide range of cache storage. The following is a summary:
* [[yii\caching\ApcCache]]: uses PHP [APC](http://php.net/manual/en/book.apc.php) 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. To use this cache, you must
create a table as specified in [[yii\caching\DbCache::cacheTable]].
* [[yii\caching\DummyCache]]: serves as a cache placeholder which does no real caching.
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, you may configure
a cache component to use this cache. When an actual cache support is enabled, you can switch to use
the corresponding cache component. In both cases, you may use the same code
`Yii::$app->cache->get($key)` to attempt retrieving data from the cache without worrying that
`Yii::$app->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 page content.
* [[yii\caching\MemCache]]: uses PHP [memcache](http://php.net/manual/en/book.memcache.php)
and [memcached](http://php.net/manual/en/book.memcached.php) 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\redis\Cache]]: implements a cache component based on [Redis](http://redis.io/) key-value store
(redis version 2.6.12 or higher is required).
* [[yii\caching\WinCache]]: uses PHP [WinCache](http://iis.net/downloads/microsoft/wincache-extension)
([see also](http://php.net/manual/en/book.wincache.php)) extension.
* [[yii\caching\XCache]]: uses PHP [XCache](http://xcache.lighttpd.net/) extension.
* [[yii\caching\ZendDataCache]]: uses
[Zend Data Cache](http://files.zend.com/help/Zend-Server-6/zend-server.htm#data_cache_component.htm)
as the underlying caching medium.
> Tip: You may use different cache storage in the same application. A common strategy is to use memory-based
cache storage to store data that is small but constantly used (e.g. statistical data), and use file-based
or database-based cache storage to store data that is big and less frequently used (e.g. page content).
## Cache APIs <a name="cache-apis"></a>
All cache components have the same base class [[yii\caching\Cache]] and thus support the following APIs:
* [[yii\caching\Cache::get()|get()]]: retrieves a data item from cache with a specified key. A false
value will be returned if the data item is not found in the cache or is expired/invalidated.
* [[yii\caching\Cache::set()|set()]]: stores a data item identified by a key in cache.
* [[yii\caching\Cache::add()|add()]]: stores a data item identified by a key in cache if the key is not found in the cache.
* [[yii\caching\Cache::mget()|mget()]]: retrieves multiple data items from cache with the specified keys.
* [[yii\caching\Cache::mset()|mset()]]: stores multiple data items in cache. Each item is identified by a key.
* [[yii\caching\Cache::madd()|madd()]]: stores multiple data items in cache. Each item is identified by a key.
If a key already exists in the cache, the data item will be skipped.
* [[yii\caching\Cache::exists()|exists()]]: returns a value indicating whether the specified key is found in the cache.
* [[yii\caching\Cache::delete()|delete()]]: removes a data item identified by a key from the cache.
* [[yii\caching\Cache::flush()|flush()]]: removes all data items from the cache.
Some cache storage, such as MemCache, APC, support retrieving multiple cached values in a batch mode,
which may reduce the overhead involved in retrieving cached data. The APIs [[yii\caching\Cache::mget()|mget()]]
and [[yii\caching\Cache::madd()|madd()]] are provided to exploit this feature. In case the underlying cache storage
does not support this feature, it will be simulated.
Because [[yii\caching\Cache]] implements `ArrayAccess`, a cache component can be used liked an array. The followings
are some examples:
```php
$connection->beginCache(60); // cache all query results for 60 seconds.
// your db query code here...
$connection->endCache();
$cache['var1'] = $value1; // equivalent to: $cache->set('var1', $value1);
$value2 = $cache['var2']; // equivalent to: $value2 = $cache->get('var2');
```
Data Caching
============
### Cache Keys <a name="cache-keys"></a>
Data caching is about storing some PHP variable in cache and retrieving it
later from cache. For this purpose, the cache component base class [CCache]
provides two methods that are used most of the time: [set()|CCache::set]
and [get()|CCache::get].
To store a variable `$value` in cache, we choose a unique ID and call
[set()|CCache::set] to store it:
~~~
[php]
Yii::app()->cache->set($id, $value);
~~~
The cached data will remain in the cache forever unless it is removed
because of some caching policy (e.g. caching space is full and the oldest
data are removed). To change this behavior, we can also supply an
expiration parameter when calling [set()|CCache::set] so that the data will
be removed from the cache after, at most, that period of time:
~~~
[php]
// keep the value in cache for at most 30 seconds
Yii::app()->cache->set($id, $value, 30);
~~~
Later when we need to access this variable (in either the same or a
different Web request), we call [get()|CCache::get] with the ID to retrieve
it from cache. If the returned value is false, it means the value is not
available in cache and we have to regenerate it.
~~~
[php]
$value=Yii::app()->cache->get($id);
if($value===false)
{
// regenerate $value because it is not found in cache
// and save it in cache for later use:
// Yii::app()->cache->set($id,$value);
}
~~~
When choosing the ID for a variable to be cached, make sure the ID is
unique among all other variables that may be cached in the application. It
is NOT required that the ID is unique across applications because the cache
component is intelligent enough to differentiate IDs 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()|CCache::mget] is provided to achieve this feature. In case the underlying
cache storage does not support this feature, [mget()|CCache::mget] will still
simulate it.
To remove a cached value from cache, call [delete()|CCache::delete]; and
to remove everything from cache, call [flush()|CCache::flush]. Be very
careful when calling [flush()|CCache::flush] because it also removes cached
data that are from other applications.
> Tip: Because [CCache] implements `ArrayAccess`, a cache component can be
> used liked an array. The followings are some examples:
> ~~~
> [php]
> $cache=Yii::app()->cache;
> $cache['var1']=$value1; // equivalent to: $cache->set('var1',$value1);
> $value2=$cache['var2']; // equivalent to: $value2=$cache->get('var2');
> ~~~
Cache Dependency
----------------
Besides expiration setting, cached data may also be invalidated according
to some dependency changes. For example, if we are caching the content of
some file and the file is changed, we should invalidate the cached copy and
read the latest content from the file instead of the cache.
We represent a dependency as an instance of [CCacheDependency] or its
child class. We pass the dependency instance along with the data to be
cached when calling [set()|CCache::set].
~~~
[php]
// the value will expire in 30 seconds
// it may also be invalidated earlier if the dependent file is changed
Yii::app()->cache->set($id, $value, 30, new CFileCacheDependency('FileName'));
~~~
Now if we retrieve `$value` from cache by calling [get()|CCache::get], the
dependency will be evaluated and if it is changed, we will get a false
value, indicating the data needs to be regenerated.
Below is a summary of the available cache dependencies:
- [CFileCacheDependency]: the dependency is changed if the file's last
modification time is changed.
Each data item stored in cache is uniquely identified by a key. When you store a data item in cache,
you have to specify a key for it. Later when you retrieve the data item from cache, you should provide
the corresponding key.
- [CDirectoryCacheDependency]: the dependency is changed if any of the
files under the directory and its subdirectories is changed.
You may use a string or an arbitrary value as a cache key. When a key is not a string, it will be automatically
serialized into a string.
- [CDbCacheDependency]: the dependency is changed if the query result
of the specified SQL statement is changed.
A common strategy of defining a cache key is to include all determining factors in terms of an array.
For example, [[yii\db\Schema]] uses the following key to cache schema information about a database table:
- [CGlobalStateCacheDependency]: the dependency is changed if the value
of the specified global state is changed. A global state is a variable that
is persistent across multiple requests and multiple sessions in an
application. It is defined via [CApplication::setGlobalState()].
- [CChainedCacheDependency]: the dependency is changed if any of the
dependencies on the chain is changed.
```php
[
__CLASS__, // schema class name
$this->db->dsn, // DB connection data source name
$this->db->username, // DB connection login user
$name, // table name
];
```
- [CExpressionDependency]: the dependency is changed if the result of
the specified PHP expression is changed.
As you can see, the key includes all necessary information needed to uniquely specify a database table.
When the same cache storage is used by different applications, you should specify a unique cache key prefix
for each application to avoid conflicts of cache keys. This can be done by configuring the [[yii\caching\Cache::keyPrefix]]
property. For example, in the application configuration you can write the following code:
Query Caching
-------------
```php
'components' => [
'cache' => [
'class' => 'yii\caching\ApcCache',
'keyPrefix' => 'myapp', // a unique cache key prefix
],
],
```
Since version 1.1.7, Yii has added support for query caching.
Built on top of data caching, query caching stores the result of a DB query
in cache and may thus save the DB query execution time if the same query is requested
in future, as the result can be directly served from the cache.
To ensure interoperability, only alphanumeric characters should be used.
> Info: Some DBMS (e.g. [MySQL](http://dev.mysql.com/doc/refman/5.1/en/query-cache.html))
> also support query caching on the DB server side. Compared with the server-side
> query caching, the same feature we support here offers more flexibility and
> may be potentially more efficient.
### Cache Expiration <a name="cache-expiration"></a>
### Enabling Query Caching
A data item stored in a cache will remain there forever unless it is removed because of some caching policy
enforcement (e.g. caching space is full and the oldest data are removed). To change this behavior, you can provide
an expiration parameter when calling [[yii\caching\Cache::set()|set()]] to store a data item. The parameter
indicates for how many seconds the data item can remain valid in the cache. When you call
[[yii\caching\Cache::get()|get()]] to retrieve the data item, if it has passed the expiration time, the method
will return false, indicating the data item is not found in the cache. For example,
To enable query caching, make sure [CDbConnection::queryCacheID] refers to the ID of a valid
cache application component (it defaults to `cache`).
```php
// keep the data in cache for at most 45 seconds
$cache->set($key, $data, 45);
sleep(50);
### Using Query Caching with DAO
$data = $cache->get($key);
if ($data === false) {
// $data is expired or is not found in the cache
}
```
To use query caching, we call the [CDbConnection::cache()] method when we perform DB queries.
The following is an example:
~~~
[php]
$sql = 'SELECT * FROM tbl_post LIMIT 20';
$dependency = new CDbCacheDependency('SELECT MAX(update_time) FROM tbl_post');
$rows = Yii::app()->db->cache(1000, $dependency)->createCommand($sql)->queryAll();
~~~
### Cache Dependencies <a name="cache-dependencies"></a>
When running the above statements, Yii will first check if the cache contains a valid
result for the SQL statement to be executed. This is done by checking the following three conditions:
Besides expiration setting, cached data item may also be invalidated by changes of the so-called *cache dependencies*.
For example, [[yii\caching\FileDependency]] represents the dependency of a file's modification time.
When this dependency changes, it means the corresponding file is modified. As a result, any outdated
file content found in the cache should be invalidated and the [[yii\caching\Cache::get()|get()]] call
should return false.
- if the cache contains an entry indexed by the SQL statement.
- if the entry is not expired (less than 1000 seconds since it was first saved in the cache).
- if the dependency has not changed (the maximum `update_time` value is the same as when
the query result was saved in the cache).
Cache dependencies are represented as objects of [[yii\caching\Dependency]] descendant classes. When you call
[[yii\caching\Cache::set()|set()]] to store a data item in the cache, you can pass along an associated cache
dependency object. For example,
If all of the above conditions are satisfied, the cached result will be returned directly from the cache.
Otherwise, the SQL statement will be sent to the DB server for execution, and the corresponding
result will be saved in the cache and returned.
```php
// Create a dependency on the modification time of file example.txt.
$dependency = new \yii\caching\FileDependency(['fileName' => 'example.txt']);
// The data will expire in 30 seconds.
// It may also be invalidated earlier if example.txt is modified.
$cache->set($key, $data, 30, $dependency);
### Using Query Caching with ActiveRecord
// The cache will check if the data has expired.
// It will also check if the associated dependency was changed.
// It will return false if any of these conditions is met.
$data = $cache->get($key);
```
Query caching can also be used with [Active Record](/doc/guide/database.ar).
To do so, we call a similar [CActiveRecord::cache()] method like the following:
Below is a summary of the available cache dependencies:
~~~
[php]
$dependency = new CDbCacheDependency('SELECT MAX(update_time) FROM tbl_post');
$posts = Post::model()->cache(1000, $dependency)->findAll();
// relational AR query
$posts = Post::model()->cache(1000, $dependency)->with('author')->findAll();
~~~
- [[yii\caching\ChainedDependency]]: the dependency is changed if any of the dependencies on the chain is changed.
- [[yii\caching\DbDependency]]: the dependency is changed if the query result of the specified SQL statement is changed.
- [[yii\caching\ExpressionDependency]]: the dependency is changed if the result of the specified PHP expression is changed.
- [[yii\caching\FileDependency]]: the dependency is changed if the file's last modification time is changed.
- [[yii\caching\GroupDependency]]: marks a cached data item with a group name. You may invalidate the cached data items
with the same group name all at once by calling [[yii\caching\GroupDependency::invalidate()]].
The `cache()` method here is essentially a shortcut to [CDbConnection::cache()].
Internally, when executing the SQL statement generated by ActiveRecord, Yii will
attempt to use query caching as we described in the last subsection.
## Query Caching <a name="query-caching"></a>
### Caching Multiple Queries
Query caching is a special caching feature built on top of data caching. It is provided to cache the result
of database queries.
By default, each time we call the `cache()` method (of either [CDbConnection] or [CActiveRecord]),
it will mark the next SQL query to be cached. Any other SQL queries will NOT be cached
unless we call `cache()` again. For example,
Query caching requires a [[yii\db\Connection|DB connection]] and a valid `cache` application component.
The basic usage of query caching is as follows, assuming `$db` is a [[yii\db\Connection]] instance:
~~~
[php]
$sql = 'SELECT * FROM tbl_post LIMIT 20';
$dependency = new CDbCacheDependency('SELECT MAX(update_time) FROM tbl_post');
```php
$duration = 60; // cache query results for 60 seconds.
$dependency = ...; // optional dependency
$rows = Yii::app()->db->cache(1000, $dependency)->createCommand($sql)->queryAll();
// query caching will NOT be used
$rows = Yii::app()->db->createCommand($sql)->queryAll();
~~~
$db->beginCache($duration, $dependency);
By supplying an extra `$queryCount` parameter to the `cache()` method, we can enforce
multiple queries to use query caching. In the following example, when we call `cache()`,
we specify that query caching should be used for the next 2 queries:
// ...performs DB queries here...
~~~
[php]
// ...
$rows = Yii::app()->db->cache(1000, $dependency, 2)->createCommand($sql)->queryAll();
// query caching WILL be used
$rows = Yii::app()->db->createCommand($sql)->queryAll();
~~~
$db->endCache();
```
As we know, when performing a relational AR query, it is possible several SQL queries will
be executed (by checking the [log messages](/doc/guide/topics.logging)).
For example, if the relationship between `Post` and `Comment` is `HAS_MANY`,
then the following code will actually execute two DB queries:
As you can see, any SQL queries in between the `beginCache()` and `endCache()` calls will be cached.
If the result of the same query is found valid in the cache, the query will be skipped and the result
will be served from the cache instead.
- it first selects the posts limited by 20;
- it then selects the comments for the previously selected posts.
Query caching can be used for [DAO](db-dao.md) as well as [ActiveRecord](db-active-record.md).
~~~
[php]
$posts = Post::model()->with('comments')->findAll(array(
'limit'=>20,
));
~~~
> Info: Some DBMS (e.g. [MySQL](http://dev.mysql.com/doc/refman/5.1/en/query-cache.html))
also support query caching on the DB server side. You may choose to use either query caching mechanism.
The query caching described above has the advantage that you may specify flexible cache dependencies
and are potentially more efficient.
If we use query caching as follows, only the first DB query will be cached:
~~~
[php]
$posts = Post::model()->cache(1000, $dependency)->with('comments')->findAll(array(
'limit'=>20,
));
~~~
### Configurations <a name="query-caching-configs"></a>
In order to cache both DB queries, we need supply the extra parameter indicating how
many DB queries we want to cache next:
Query caching has two two configurable options through [[yii\db\Connection]]:
~~~
[php]
$posts = Post::model()->cache(1000, $dependency, 2)->with('comments')->findAll(array(
'limit'=>20,
));
~~~
* [[yii\db\Connection::queryCacheDuration|queryCacheDuration]]: this represents the number of seconds
that a query result can remain valid in the cache. The duration will be overwritten if you call
[[yii\db\Connection::beginCache()]] with an explicit duration parameter.
* [[yii\db\Connection::queryCache|queryCache]]: this represents the ID of the cache application component.
It defaults to `'cache'`. Query caching is enabled only when there is a valid cache application component.
### Limitations
### Limitations <a name="query-caching-limitations"></a>
Query caching does not work with query results that contain resource handles. For example,
when using the `BLOB` column type in some DBMS, the query result will return a resource
......@@ -358,7 +288,3 @@ Some caching storage has size limitation. For example, memcache limits the maxim
of each entry to be 1MB. Therefore, if the size of a query result exceeds this limit,
the caching will fail.
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).
docs/guide/caching-overview.md
......@@ -3,7 +3,7 @@ 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, the application saves the time that would be
required to generate the data from scratch.
required to generate the data from scratch every time.
Caching can occur at different levels and places in a Web application. On the server side, at the lower level,
cache may be used to store basic data, such as a list of most recent article information fetched from database;
......@@ -11,86 +11,8 @@ and at the higher level, cache may be used to store the page content, such as th
recent articles. On the client side, HTTP caching may be used to keep most recently visited page content in
the browser cache.
Yii supports all these caching mechanisms which are described in the following sections:
Yii supports all these caching mechanisms:
* [Data caching](caching-data.md)
* [Content caching](caching-content.md)
* [HTTP caching](caching-http.md)
## Cache Components
Server-side caching (data caching and content caching) relies on the so-called *cache components*.
Each cache component represents a caching storage and provides a common set of APIs
that may be called to store data in the cache and retrieve it later.
Cache components are usually registered as application components so that they can be globally
configurable and accessible. You may register multiple cache components in a single application.
In most cases you would configure at least the [[yii\base\Application::getCache()|cache]] component
because it is the default cache component being used by most cache-dependent classes, such as [[yii\web\UrlManager]].
The following code shows how to configure the `cache` application component to
use [memcached](http://memcached.org/) with two cache servers:
```php
'components' => [
'cache' => [
'class' => 'yii\caching\MemCache',
'servers' => [
[
'host' => 'server1',
'port' => 11211,
'weight' => 100,
],
[
'host' => 'server2',
'port' => 11211,
'weight' => 50,
],
],
],
],
```
You may access the `cache` component by using the expression `Yii::$app->cache`.
The following is a summary of the built-in cache components supported by Yii:
* [[yii\caching\ApcCache]]: uses PHP [APC](http://php.net/manual/en/book.apc.php) 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. To use this cache, you must
create a table as specified in [[yii\caching\DbCache::cacheTable]].
* [[yii\caching\DummyCache]]: serves as a cache placeholder which does no real caching.
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, you may configure
a cache component to use this cache. When an actual cache support is enabled, you can switch to use
the corresponding cache component. In both cases, you may use the same code
`Yii::$app->cache->get($key)` to attempt retrieving data from the cache without worrying that
`Yii::$app->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 page content.
* [[yii\caching\MemCache]]: uses PHP [memcache](http://php.net/manual/en/book.memcache.php)
and [memcached](http://php.net/manual/en/book.memcached.php) 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\redis\Cache]]: implements a cache component based on [Redis](http://redis.io/) key-value store
(redis version 2.6.12 or higher is required).
* [[yii\caching\WinCache]]: uses PHP [WinCache](http://iis.net/downloads/microsoft/wincache-extension)
([see also](http://php.net/manual/en/book.wincache.php)) extension.
* [[yii\caching\XCache]]: uses PHP [XCache](http://xcache.lighttpd.net/) extension.
* [[yii\caching\ZendDataCache]]: uses
[Zend Data Cache](http://files.zend.com/help/Zend-Server-6/zend-server.htm#data_cache_component.htm)
as the underlying caching medium.
> Because all cache components extend from the same base class [[yii\caching\Cache]], you can switch to use
a different type of cache without modifying the code that uses a cache.
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