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Improving Performance with Cache

Phalcon provides the Phalcon\Cache class allowing faster access to frequently used or already processed data. Phalcon\Cache is written in C, achieving higher performance and reducing the overhead when getting items from the backends. This class uses an internal structure of frontend and backend components. Front-end components act as input sources or interfaces, while backend components offer storage options to the class.

When to implement cache?

Although this component is very fast, implementing it in cases that are not needed could lead to a loss of performance rather than gain. We recommend you check this cases before using a cache:

  • You are making complex calculations that every time return the same result (changing infrequently)
  • You are using a lot of helpers and the output generated is almost always the same
  • You are accessing database data constantly and these data rarely change
NOTE Even after implementing the cache, you should check the hit ratio of your cache over a period of time. This can easily be done, especially in the case of Memcache or Apc, with the relevant tools that backends provide.

Caching Behavior

The caching process is divided into 2 parts:

  • Frontend: This part is responsible for checking if a key has expired and perform additional transformations to the data before storing and after retrieving them from the backend-
  • Backend: This part is responsible for communicating, writing/reading the data required by the frontend.

Caching Output Fragments

An output fragment is a piece of HTML or text that is cached as is and returned as is. The output is automatically captured from the ob_* functions or the PHP output so that it can be saved in the cache. The following example demonstrates such usage. It receives the output generated by PHP and stores it into a file. The contents of the file are refreshed every 172800 seconds (2 days).

The implementation of this caching mechanism allows us to gain performance by not executing the helper Phalcon\Tag::linkTo call whenever this piece of code is called.

<?php

//Create an Output frontend. Cache the files for 2 days
$frontCache = new Phalcon\Cache\Frontend\Output(array(
    "lifetime" => 172800
));

// Create the component that will cache from the "Output" to a "File" backend
// Set the cache file directory - it's important to keep the "/" at the end of
// the value for the folder
$cache = new Phalcon\Cache\Backend\File($frontCache, array(
    "cacheDir" => "../app/cache/"
));

// Get/Set the cache file to ../app/cache/my-cache.html
$content = $cache->start("my-cache.html");

// If $content is null then the content will be generated for the cache
if ($content === null) {

    //Print date and time
    echo date("r");

    //Generate a link to the sign-up action
    echo Phalcon\Tag::linkTo(
        array(
            "user/signup",
            "Sign Up",
            "class" => "signup-button"
        )
    );

    // Store the output into the cache file
    $cache->save();

} else {

    // Echo the cached output
    echo $content;
}

NOTE In the example above, our code remains the same, echoing output to the user as it has been doing before. Our cache component transparently captures that output and stores it in the cache file (when the cache is generated) or it sends it back to the user pre-compiled from a previous call, thus avoiding expensive operations.

Caching Arbitrary Data

Caching just data is equally important for your application. Caching can reduce database load by reusing commonly used (but not updated) data, thus speeding up your application.

File Backend Example

One of the caching adapters is ‘File’. The only key area for this adapter is the location of where the cache files will be stored. This is controlled by the cacheDir option which must have a backslash at the end of it.

<?php

// Cache the files for 2 days using a Data frontend
$frontCache = new Phalcon\Cache\Frontend\Data(array(
    "lifetime" => 172800
));

// Create the component that will cache "Data" to a "File" backend
// Set the cache file directory - important to keep the "/" at the end of
// of the value for the folder
$cache = new Phalcon\Cache\Backend\File($frontCache, array(
    "cacheDir" => "../app/cache/"
));

// Try to get cached records
$cacheKey = 'robots_order_id.cache';
$robots    = $cache->get($cacheKey);
if ($robots === null) {

    // $robots is null because of cache expiration or data does not exist
    // Make the database call and populate the variable
    $robots = Robots::find(array("order" => "id"));

    // Store it in the cache
    $cache->save($cacheKey, $robots);
}

// Use $robots :)
foreach ($robots as $robot) {
   echo $robot->name, "\n";
}

Memcached Backend Example

The above example changes slightly (especially in terms of configuration) when we are using a Memcached backend.

<?php

//Cache data for one hour
$frontCache = new Phalcon\Cache\Frontend\Data(array(
    "lifetime" => 3600
));

// Create the component that will cache "Data" to a "Memcached" backend
// Memcached connection settings
$cache = new Phalcon\Cache\Backend\Libmemcached($frontCache, array(
    "host" => "localhost",
    "port" => "11211"
));

// Try to get cached records
$cacheKey = 'robots_order_id.cache';
$robots    = $cache->get($cacheKey);
if ($robots === null) {

    // $robots is null because of cache expiration or data does not exist
    // Make the database call and populate the variable
    $robots = Robots::find(array("order" => "id"));

    // Store it in the cache
    $cache->save($cacheKey, $robots);
}

// Use $robots :)
foreach ($robots as $robot) {
   echo $robot->name, "\n";
}

Querying the cache

The elements added to the cache are uniquely identified by a key. In the case of the File backend, the key is the actual filename. To retrieve data from the cache, we just have to call it using the unique key. If the key does not exist, the get method will return null.

<?php

// Retrieve products by key "myProducts"
$products = $cache->get("myProducts");

If you want to know which keys are stored in the cache you could call the queryKeys method:

<?php

// Query all keys used in the cache
$keys = $cache->queryKeys();
foreach ($keys as $key) {
    $data = $cache->get($key);
    echo "Key=", $key, " Data=", $data;
}

//Query keys in the cache that begins with "my-prefix"
$keys = $cache->queryKeys("my-prefix");

Deleting data from the cache

There are times where you will need to forcibly invalidate a cache entry (due to an update in the cached data). The only requirement is to know the key that the data have been stored with.

<?php

// Delete an item with a specific key
$cache->delete("someKey");

// Delete all items from the cache
$keys = $cache->queryKeys();
foreach ($keys as $key) {
    $cache->delete($key);
}

Checking cache existence

It is possible to check if a cache already exists with a given key:

<?php

if ($cache->exists("someKey")) {
    echo $cache->get("someKey");
} else {
    echo "Cache does not exists!";
}

Lifetime

A “lifetime” is a time in seconds that a cache could live without expire. By default, all the created caches use the lifetime set in the frontend creation. You can set a specific lifetime in the creation or retrieving of the data from the cache:

Setting the lifetime when retrieving:

<?php

$cacheKey = 'my.cache';

//Setting the cache when getting a result
$robots = $cache->get($cacheKey, 3600);
if ($robots === null) {

    $robots = "some robots";

    // Store it in the cache
    $cache->save($cacheKey, $robots);
}

Setting the lifetime when saving:

<?php

$cacheKey = 'my.cache';

$robots = $cache->get($cacheKey);
if ($robots === null) {

    $robots = "some robots";

    //Setting the cache when saving data
    $cache->save($cacheKey, $robots, 3600);
}

Multi-Level Cache

This feature ​of the cache component, ​allows ​the developer to implement a multi-level cache​. This new feature is very ​useful because you can save the same data in several cache​ locations​ with different lifetimes, reading ​first from the one with the faster adapter and ending with the slowest one until the data expire​s​:

<?php

use Phalcon\Cache\Frontend\Data as DataFrontend,
    Phalcon\Cache\Multiple,
    Phalcon\Cache\Backend\Apc as ApcCache,
    Phalcon\Cache\Backend\Memcache as MemcacheCache,
    Phalcon\Cache\Backend\File as FileCache;

$ultraFastFrontend = new DataFrontend(array(
    "lifetime" => 3600
));

$fastFrontend = new DataFrontend(array(
    "lifetime" => 86400
));

$slowFrontend = new DataFrontend(array(
    "lifetime" => 604800
));

//Backends are registered from the fastest to the slower
$cache = new Multiple(array(
    new ApcCache($ultraFastFrontend, array(
        "prefix" => 'cache',
    )),
    new MemcacheCache($fastFrontend, array(
        "prefix" => 'cache',
        "host" => "localhost",
        "port" => "11211"
    )),
    new FileCache($slowFrontend, array(
        "prefix" => 'cache',
        "cacheDir" => "../app/cache/"
    ))
));

//Save, saves in every backend
$cache->save('my-key', $data);

Frontend Adapters

The available frontend adapters that are used as interfaces or input sources to the cache are:

Adapter Description Example
Output Read input data from standard PHP output Phalcon\Cache\Frontend\Output
Data It’s used to cache any kind of PHP data (big arrays, objects, text, etc). Data is serialized before stored in the backend. Phalcon\Cache\Frontend\Data
Base64 It’s used to cache binary data. The data is serialized using base64_encode before be stored in the backend. Phalcon\Cache\Frontend\Base64
Json Data is encoded in JSON before be stored in the backend. Decoded after be retrieved. This frontend is useful to share data with other languages or frameworks. Phalcon\Cache\Frontend\Json
IgBinary It’s used to cache any kind of PHP data (big arrays, objects, text, etc). Data is serialized using IgBinary before be stored in the backend. Phalcon\Cache\Frontend\Igbinary
None It’s used to cache any kind of PHP data without serializing them. Phalcon\Cache\Frontend\None

Implementing your own Frontend adapters

The Phalcon\Cache\FrontendInterface interface must be implemented in order to create your own frontend adapters or extend the existing ones.

Backend Adapters

The backend adapters available to store cache data are:

Adapter Description Info Required Extensions Example
File Stores data to local plain files     Phalcon\Cache\Backend\File
Memcached Stores data to a memcached server Memcached memcache Phalcon\Cache\Backend\Memcache
APC Stores data to the Alternative PHP Cache (APC) APC APC extension Phalcon\Cache\Backend\Apc
Mongo Stores data to Mongo Database MongoDb Mongo Phalcon\Cache\Backend\Mongo
XCache Stores data in XCache XCache xcache extension Phalcon\Cache\Backend\Xcache

Implementing your own Backend adapters

The Phalcon\Cache\BackendInterface interface must be implemented in order to create your own backend adapters or extend the existing ones.

File Backend Options

This backend will store cached content into files in the local server. The available options for this backend are:

Option Description
prefix A prefix that is automatically prepended to the cache keys
cacheDir A writable directory on which cached files will be placed

Memcached Backend Options

This backend will store cached content on a memcached server. The available options for this backend are:

Option Description
prefix A prefix that is automatically prepended to the cache keys
host memcached host
port memcached port
persistent create a persistent connection to memcached?

APC Backend Options

This backend will store cached content on Alternative PHP Cache (APC). The available options for this backend are:

Option Description
prefix A prefix that is automatically prepended to the cache keys

Mongo Backend Options

This backend will store cached content on a MongoDB server. The available options for this backend are:

Option Description
prefix A prefix that is automatically prepended to the cache keys
server A MongoDB connection string
db Mongo database name
collection Mongo collection in the database

XCache Backend Options

This backend will store cached content on XCache (XCache). The available options for this backend are:

Option Description
prefix A prefix that is automatically prepended to the cache keys

There are more adapters available for this components in the Phalcon Incubator