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Cache

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Table of Contents

Introduction
Class overview
Usage
More Information

Introduction

The Cache package provides a collection of lightweight classes to cache different kinds of data. It provides a manager class, which takes care of instantiating and reusing caches.

Class overview

This section gives you an overview of the most important classes.

ezcCacheManager: This is the optional manager, which is recommended if your application needs to cache different data for different purposes. It allows you to configure all caches in a central place and to retrieve them through ezcCacheManager. The cache manager will store only the configurations by default, only instantiating the cache object when requested.
ezcCacheStorage: This is the base class for all cache storage (the cache classes themselves). All cache classes inherit from this base class.
ezcCacheStack: Using this class, multiple cache storages can be combined using a technique called "hierarchical caching". This works similar to how CPU and file system caches work. For more information please refer to the Hierarchical caching section.
ezcCacheStorageFilePlain: Cache objects of this class are capable of storing plain text data on the file system. It utilizes the file_get_contents() and file_put_contents() PHP functions.
ezcCacheStorageFileArray: In contrast to ezcCacheStorageFilePlain, objects of this class can store array structures and will keep PHP data types intact. The ezcCacheStorageFileArray class generates PHP code, which will be stored on the file system. Restoring data from the cache uses the require() construct.
ezcCacheStorageFileEvalArray: Objects of this storage class follow a similar approach to ezcCacheStorageFileArray; they are also capable of storing array structures. The major difference between both classes is that ezcCacheStorageFileEvalArray will use PHP's eval() method instead of require() to restore the cached data. As a result, the stored data will not be cached again in PHP accelerators like APC. This might be desirable if you store large amounts of data at once.
ezcCacheStorageMemcachePlain: Uses the PHP Memcache extension to store cache objects in memory between requests, thus improving speed. The zlib extension is also required by Memcache for on-the-fly compression.
ezcCacheStorageApcPlain: Uses the PHP APC extension to store cache objects in memory between requests, thus improving speed.
ezcCacheStorageFileApcArray: Uses the PHP APC extension to store array structures in memory between requests, thus improving speed. It is basically a replacement for ezcCacheStorageFileArray, if the APC extension is installed.

Usage

Terminology

A cache is identified by a cache identifier and contains a number of cache item s.

Cache: A location that stores cache item s. A cache is created with ezcCacheManager::createCache(), preferably at the beginning of your script. A cache with its cache identifier - the first parameter to ezcCacheManager::createCache() - can only be created once. After it has been created, you can reference it by calling ezcCacheManager::getCache(), using the corresponding $id as the first parameter.
Cache identifier: An identifier that uniquely identifies a cache.
Cache item: One single entry stored in a cache. Cache items can be stored and retrieved by using the ezcCacheStorage object that is returned by calling ezcCacheManager::getCache(). Cache items use a cache key to identify specific entries.
Cache key: A string representing a single cache item in a cache.
Attribute: Additional information to complement the cache key in cache item s.

A simple example

This example shows how to create and use a simple cache with ezcCacheManager:

<?php
require_once 'tutorial_autoload.php';
$options = array(
'ttl' => 30,
);
ezcCacheManager::createCache( 'simple', '/tmp/cache/plain', 'ezcCacheStorageFilePlain', $options );
$myId = 'unique_id_1';
$mySecondId = 'id_2';
$cache = ezcCacheManager::getCache( 'simple' );
if ( ( $dataOfFirstItem = $cache->restore( $myId ) ) === false )
{
$dataOfFirstItem = "Plain cache stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $myId, $dataOfFirstItem );
}
if ( ( $dataOfSecondItem = $cache->restore( $mySecondId ) ) === false )
{
$dataOfSecondItem = "Plain cache 2 stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $mySecondId, $dataOfSecondItem );
}
var_dump( $dataOfFirstItem, $dataOfSecondItem );
?>

Time-to-live is defined as 30 seconds in this case; if left out, the cache will have a lifespan of 24 hours. On line 9, the cache configuration is stored in the cache manager. The created cache uses the cache identifier "simple" and will reside in the directory /tmp/cache/plain. (Note: This directory must exist and must be writable!) To store a cache item, the storage class ezcCacheStorageFilePlain will be used.

Line 11 defines a cache key for a cache item. The next line defines a second unique cache key for a second cache item. After that (line 14), the newly-created cache object is retrieved from ezcCacheManager. Lines 16 and 22 show how to check for cached data: ezcCacheStorage::restore() will return bool false if no valid cache data is found for the given ID.

If no valid data is found, the data will be generated and stored in the cache later (lines 17-18 and 24-25). The last line outputs the data, so you can follow how it's cached for 30 seconds by running the example multiple times in a short time frame. After 30 seconds, the cache data will become invalid and will be regenerated.

Delayed initialization

Instead of calling the ezcCacheManager::getCache() method yourself it is also possible to use delayed initialization. When using this it is not required to configure all the caches first by calling the ezcCacheManager::createCache() method. Instead it would allow you to configure/create caches on demand, this is called lazy, or delayed initialization. You can find a description how you can use it for your own components and how it works in the ezcBase tutorial. The keyword for the cache component is ezcInitConfigurationManager.

<?php
require_once 'tutorial_autoload.php';
class customLazyCacheConfiguration implements ezcBaseConfigurationInitializer
{
public static function configureObject( $id )
{
$options = array( 'ttl' => 30 );
switch ( $id )
{
case 'simple':
ezcCacheManager::createCache( 'simple', '/tmp/cache/plain', 'ezcCacheStorageFilePlain', $options );
break;
}
}
}
ezcBaseInit::setCallback(
'ezcInitCacheManager',
'customLazyCacheConfiguration'
);
$myId = 'unique_id_1';
$mySecondId = 'id_2';
$cache = ezcCacheManager::getCache( 'simple' );
if ( ( $dataOfFirstItem = $cache->restore( $myId ) ) === false )
{
$dataOfFirstItem = "Plain cache stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $myId, $dataOfFirstItem );
}
if ( ( $dataOfSecondItem = $cache->restore( $mySecondId ) ) === false )
{
$dataOfSecondItem = "Plain cache 2 stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $mySecondId, $dataOfSecondItem );
}
var_dump( $dataOfFirstItem, $dataOfSecondItem );
?>

Differences with the previous example can be seen in lines 5 to 23. In lines 20 to 23 you tell the delayed initialization mechanism to use the customLazyCacheConfiguration as lazy initialization provider for the ezcInitConfigurationManager context. The customLazyCacheConfiguration class implements the configureObject() method that will automatically be called when ezcCacheManager::getCache() is called with a cache identifier that has not been created yet through ezcCacheManager::createCache(), as you can see in line 14.

Using multiple caches

The following example shows how the cache manager deals with multiple caches:

<?php
require_once 'tutorial_autoload.php';
$optionsPlain = array(
'ttl' => 30,
);
$optionsArray = array(
'ttl' => 45,
);
ezcCacheManager::createCache( 'plain', '/tmp/cache/plain', 'ezcCacheStorageFilePlain', $optionsPlain );
ezcCacheManager::createCache( 'array', '/tmp/cache/array', 'ezcCacheStorageFileArray', $optionsArray );
$myId = 'unique_id_2';
$cache = ezcCacheManager::getCache( 'plain' );
if ( ( $plainData = $cache->restore( $myId ) ) === false )
{
$plainData = "Plain cache stored on " . date( 'Y-m-d, H:m:s' );
$cache->store( $myId, $plainData );
sleep( 2 );
}
echo "Plain cache data:\n";
var_dump( $plainData );
$cache = ezcCacheManager::getCache( 'array' );
if ( ( $arrayData = $cache->restore( $myId ) ) === false )
{
$arrayData = array(
$plainData,
"Array cache stored on " . date( 'Y-m-d, H:m:s'),
true,
23
);
$cache->store( $myId, $arrayData );
}
echo "Array cache data:\n";
var_dump( $arrayData );
?>

In lines 12 and 13, two caches are created. Each cache must reside in its own location and must have a different cache identifier. We use two different options for the lifetime of the caches to show how they act independently.

Since the first cache reuses the location already used in example 1, we use a different cache key here. Lines 15 to 25 are almost identical to the code from example 1, except that the program will pause for two seconds when generating the plain cache, in order to show different generation times for the two caches.

On line 30, the second cache object is retrieved, which is capable of storing arrays. Therefore, we store the data from the plain cache here and generate some additional data to be stored in an array. Running this example multiple times will give you different results since the second cache has a longer lifetime and will therefore hold its data longer than the first one.

Complex caching

As the next example shows, the ezcCacheStorage class is capable of more advanced features. This example uses extra attributes in addition to the cache key:

<?php
require_once 'tutorial_autoload.php';
$options = array(
'ttl' => 30,
);
ezcCacheManager::createCache( 'array', '/tmp/cache/array', 'ezcCacheStorageFileArray', $options );
$exampleData = array(
'unique_id_3_a' => array( 'language' => 'en', 'section' => 'articles' ),
'unique_id_3_b' => array( 'language' => 'de', 'section' => 'articles' ),
'unique_id_3_c' => array( 'language' => 'no', 'section' => 'articles' ),
'unique_id_3_d' => array( 'language' => 'de', 'section' => 'tutorials' ),
);
$cache = ezcCacheManager::getCache( 'array' );
foreach ( $exampleData as $myId => $exampleDataArr )
{
if ( ( $data = $cache->restore( $myId, $exampleDataArr ) ) === false )
{
$cache->store( $myId, $exampleDataArr, $exampleDataArr );
}
}
echo "Data items with attribute <section> set to <articles>: " .
$cache->countDataItems( null, array( 'section' => 'articles' ) ) .
"\n";
echo "Data items with attribute <language> set to <de>: " .
$cache->countDataItems( null, array( 'language' => 'de' ) ) .
"\n\n";
$cache->delete( null, array( 'language' => 'de' ) );
echo "Data items with attribute <section> set to <articles>: " .
$cache->countDataItems( null, array( 'section' => 'articles' ) ) .
"\n";
echo "Data items with attribute <language> set to <de>: " .
$cache->countDataItems( null, array( 'language' => 'de' ) ) .
"\n\n";
?>

After the creation of an array cache, some sample data is created (lines 11-16). Data is identified by cache key s, which are associated with arrays. Each array will be used to store the content and the attributes together. Attribute s describe a cache item s in further detail.

In line 20, a foreach loop starts, which stores all example data in the cache. After that, the method ezcCacheStorageFile::countDataItems() is used to count cache items that meet certain criteria. The first parameter here would be a cache key. When this is set, the method should always return 1 or 0, because only one cache item per cache key may exist. In this example, the cache items with the specified attribute are counted. The attributes to match are supplied as the second parameter. The first method call will return 3 (line 28), since we have three cache items that have the attribute "section" set to "articles". The second call (line 32) should return 2, because two data items have the attribute "language" set to the value "de".

On line 36 the storage object is told to delete all cache items that have the attribute "language" set to "de". Therefore, the next calls to ezcCacheStorageFile::countDataItems() will return 2 and 0.

Memory caching

If either of Memcache or APC PHP extensions is installed, then the caching performance can be improved considerably by storing the cache data in memory between requests.

APC

To use the APC cache storage, the APC PHP extension must be installed. The corresponding storage class is ezcCacheStorageApcPlain. Note, that the other existing APC storage class, ezcCacheStorageApcArray has been deprecated in favor of hierarchical caching provided by ezcCacheStack. It is not recommended to use this class further on, since it will be removed in the next major version of the Cache component.

The following example shows how to create and use a plain APC storage. It is the same as the simple example above, but using the ezcCacheStorageApcPlain class instead of ezcCacheStorageFilePlain. The second parameter of the createCache() method is arbitrary, because an existing path is not needed. However, the $location ensures the uniqueness of a cache storage in the manager. Therefore this parameter should be used accordingly.

<?php
require_once 'tutorial_autoload.php';
$options = array(
'ttl' => 30,
);
ezcCacheManager::createCache( 'apc', 'apc', 'ezcCacheStorageApcPlain', $options );
$myId = 'unique_id_1';
$mySecondId = 'id_2';
$cache = ezcCacheManager::getCache( 'apc' );
if ( ( $dataOfFirstItem = $cache->restore( $myId ) ) === false )
{
$dataOfFirstItem = "Plain cache stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $myId, $dataOfFirstItem );
}
if ( ( $dataOfSecondItem = $cache->restore( $mySecondId ) ) === false )
{
$dataOfSecondItem = "Plain cache 2 stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $mySecondId, $dataOfSecondItem );
}
var_dump( $dataOfFirstItem, $dataOfSecondItem );
?>

Memcache

Certain options must be set before creating a Memcache cache storage. Changing these options after the storage has been created does not affect the object in any way, since the connection to the Memcache is established in the constructor.

host: The name of the host running Memcache. The default is 'localhost'.
port: The port on which to connect to the Memcache host. The default is 11211.
persistent: Determines if the connection to the Memcache server should be persistent. A persistent connection does not close when the script ends. By default this is false.
compressed: If the data is to be compressed in the cache. The zlib extension is required, if this option is set to true. False is the default.

The following example shows how to create and use a plain Memcache storage. It is the same as the simple example above, but using the ezcCacheStorageMemcachePlain class instead of ezcCacheStorageFilePlain. The second parameter of the createCache() method is chosen arbitrary, because an existing path is not needed. However, the $location parameter takes care for the uniqueness of the storage in the manager. The $host an $port options just reflect the defaul here, they could also have been left out.

<?php
require_once 'tutorial_autoload.php';
$options = array(
'ttl' => 30,
'host' => 'localhost',
'port' => 11211
);
ezcCacheManager::createCache( 'memcache', 'memcache', 'ezcCacheStorageMemcachePlain', $options );
$myId = 'unique_id_1';
$mySecondId = 'id_2';
$cache = ezcCacheManager::getCache( 'memcache' );
if ( ( $dataOfFirstItem = $cache->restore( $myId ) ) === false )
{
$dataOfFirstItem = "Plain cache stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $myId, $dataOfFirstItem );
}
if ( ( $dataOfSecondItem = $cache->restore( $mySecondId ) ) === false )
{
$dataOfSecondItem = "Plain cache 2 stored on " . date( 'Y-m-d, H:i:s' );
$cache->store( $mySecondId, $dataOfSecondItem );
}
var_dump( $dataOfFirstItem, $dataOfSecondItem );
?>

Hierarchical caching

The ezcCacheStack class allows you to build a stack of arbitrary cache storages that implement ezcCacheStackable storage. This stack allows you to combine very fast caches (like APC and Memcache), which are mostly small, with slower ones, that are usually quite large. Similar techniques are used in CPU caches and file system caches.

Whenever data is stored in a cache stack, it is stored in all of the stacked storages. For each of the storages, an $itemLimit is configured that determines how many items may reside in a cache. If this limit is reached for a storage during the store operation of a new item, a certain amount of items is removed from that storage to free up space. The fraction of $itemLimit that is freed when reaching the limit is the $freeRate of the storage.

A simple stack

The following example shows how 2 caches are combined to a stack. Assume that $storageApc contains an instance of ezcCacheStorageApcPlain and $storageFile contains an instance of ezcCacheStorageFileArray.

<?php
require_once 'tutorial_autoload.php';
$stack = new ezcCacheStack( 'stack' );
$stack->pushStorage(
new ezcCacheStackStorageConfiguration(
'file',
$fileStorage,
1000000,
.5
)
);
$stack->pushStorage(
new ezcCacheStackStorageConfiguration(
'apc',
$apcStorage,
1000,
.3
)
);
$stack->options->replacementStrategy = 'ezcCacheStackLfuReplacementStrategy';
// ... somewhere else...
$stack->store(
'id_1', 'id_1_data'
);
$stack->store(
'id_2', 'id_2_data', array( 'attribute' => 'value' )
);
$id1data = $stack->restore( 'id_1' );
?>

The ezcCacheStack class (which is itself a descendant of ezcCacheStorage) is initialized with an arbitrary location string. This string is actually not used, since a stack does not need a dedicated location. However, the ezcCacheManager needs this identifier for sanity checks, so it should be a sensible key.

The method ezcCacheStack->push() adds a new ezcCacheStackableStorage to the stack. Therefore, the $fileStorage is resides on the very bottom of the stack and the APC storage is at the top. All implementations of ezcCacheStorage inside the Cache component also implement ezcCacheStackableStoragel, so you can stack all of them. To ease the internal handling of the storage configurations, the ezcCacheStackStorageConfiguration class is used. In addition, objects of this type perform sanity checks on their parameters.

The first parameter to the constructor of a storage configuration is an identifier which identifies the storage uniquely inside a stack. If you make consequent use of ezcCacheManager and the $location parameter of each storage, you should use the ezcCacheStorage->$location here, since it is already unique over all storages. Remember, that you never change the identifier of the storage in the stack between requests, without calling ezcCacheStack->reset(). This will considerably harm your stored data and produce unpredictable behavior of the stack.

The third parameter to ezcCacheStackStorageConfiguration->__construct() is the item limit for that storage. In this sense, $fileStorage may contain up to a million items, while $apcStorage may only contain 1,000. The last parameter is the $freeRate assigned to the storage. If the $fileStorage reaches 1,000,000 items, 500,000 of them will be freed. For the APC storage, only 300 items will be removed when the item limit is reached.

For the freeing of items, the cache stack first purges all outdated items from the affected storage using ezcCacheStackableStorage->purge(). If this does not remove the desired number of items, a special replacement strategy is utilized to free up more items. In the example above, the replacement strategy ezcCacheStackLfuReplacementStrategy (LRU = Least Frequently Used) is chosen. This strategy will record every access (store/restore) to a cache item and will purge those first, which have been least frequently accessed. The default replacement strategy is ezcCacheStackLruReplacementStrategy (LRU = Least Recently Used), which removes such items first, that have been accessed least recently.

Usually, the faster caches will run full first in will therefore be purged first. The result is, that these caches contain the "most important" (depending on the replacement strategy) items. Items which are not used that much will be purged from fast caches soon and will only be stored in the slower caches. The stack will always restore an item from the storage in which it is found first.

As you have seen in the above example, storing and restoring with caches works exactly like with other cache storages.

Stacks and ezcCacheManager

The ezcCacheManager is used to lazy initialize cache objects when they are first needed in your application and to make caches globally available. On a first glance this does not work with ezcCacheStack, since ready to use ezcCacheStackableStorages are needed for its configuration. To allow lazy initialization, a class that implements ezcCacheStackConfigurator can be used.

<?php
require_once 'tutorial_autoload.php';
class myCustomConfigurator extends ezcCacheStackConfigurator
{
public static function configure( ezcCacheStack $stack )
{
// ... create your storages here or fetch from manager...
$stack->pushStorage(
new ezcCacheStackStorageConfiguration(
'file',
$fileStorage,
1000000,
.5
)
);
$stack->pushStorage(
new ezcCacheStackStorageConfiguration(
'apc',
$apcStorage,
1000,
.3
)
);
}
}
$stackOptions = array(
'bubbleUpOnRestore' => true,
'configurator' => 'myCustomConfigurator',
);
$stack = new ezcCacheStack( 'stack' );
ezcCacheManager::createCache(
'stack',
'stack',
'ezcCacheStack',
$stackOptions
);
// ... somewhere else...
$stack = ezcCacheManager::getCache( 'stack' );
?>

This example configures a similar stack than the previous one. Instead of doing so directly, a new class myCustomConfigurator is created which extends ezcCacheStackConfigurator. The static configure() method is defined by this interface and creates the storages (or receives them from the manager) to add them to the given stack.

The class is configured to be used as the $configurator through the options array that is given to ezcCacheManager->createCache(). The speciality about this option is, that it is only used once: During the actual construction of the stack, when it is first requested from the manager. In this case, ezcCacheStack::__construct() calls myCustomConfigurator::configure() and submits the fresh created stack instance to it.

Another option is shown in this example: $bubbleUpOnRestore. If this option is set, every item that is restored from a lower level storage will be stored in the caches above it again. While this sounds sensible at a first glance, it has some fundamental draw backs:

The "bubble up" process might again result in other items being purged from upper storages. This can make restoring much slower.
The restored item is only stored with the attributes assigned which have been used for restoring it. If more attributes were assigned to that item, these are lost in the upper storages.
The time to live (TTL) of the item will be re-newed for the upper storages.

Stack meta data

An ezcCacheStack instance needs to store certain meta data, which is actually related to the used replacement strategy. The meta data is quite sensitive and depends heavily on the layout of your stack. Therefore you must always perform a ezcCacheStack->reset() in case you change the layout of your stack. This will clean up all data from the storages in the stack and also clean up the meta data. Remember that you also reset a storage that has been removed from a stack, before using it with another one, since it might have meta data stored.

The meta data used by a stack is stored in one of the storages inside it. If not configured otherwise, the top most storage is chosen for this, since it is usually the fastest one. However cases exist where the top most storage is not suitable as the meta data storage. These are:

If you implemented your own storage which does not implement ezcCacheMetaDataStorage.
If the top most storage is only local to the current request or server, but the environment is distributed on several servers.

In both cases you can configure the meta data storage manually, using the $metaStorage property of ezcCacheStackOptions, which must contain an object implementing the ezcCacheMetaDataStorage interface.

More Information

For more information, see the ezcCacheManager, ezcCacheStorageFile, ezcCacheStorageMemory and ezcCacheStack API documentation.

Last updated: Mon, 05 Jan 2009

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