IdentityCache

IdentityCache is a drop-in array replacement object cache designed to work with ORM and Identity Map patterns, providing Weakref-based unique identity-to-object resolution and smart caching of unused objects based on objects popularity

Abstract

The Identity Map pattern is very common in ORM implementations. It consists of an intermediate cache between the persistence and application layer, which is able to retrieve object instances by an unique object id. It must satisfy two requirements:

• For as long as an Identity is in use (the application layer holds any references to it), the Identity Map will retrieve the same instance when its id is queried. Otherwise, data consistency will be compromised when the application layer mutates different instances of the same identity.
• For Identities not in use, the Cache provides performance benefits by keeping instances for eventual reuse. It should however have a mechanism to discard cached instances so as not to consume excessive resources (i.e. memory).

Many implementations use a PHP native array to store an identity => instance map; however this has the disadvantage of storing instances indefinitely, ultimately filling up memory even though the references are not still in use.

Implementation

The IdentityCache implements Weakref-based caching to satisfy the requirements and a popularity-based garbage collection algorithm to keep the size of the cached unused instances into configurable limits.

The IdentityCache and IdentityMap are designed as drop-in replacements for a native PHP array. The IdentityMap does not provide caching functionality; as soon as an object is not used anymore (its refcount drops to 0), the object is freed.

Requirements

• For PHP > 7.4, use Exteon\IdentityCache\Weakreference\IdentityMap, Exteon\IdentityCache\Weakreference\IdentityMap which use Weakreference
• For PHP < 7.4, the Weakref extension is needed. Then use Exteon\IdentityCache\Weakref\IdentityMap, Exteon\IdentityCache\Weakref\IdentityMap
• PHP < 7.2 not supported

Usage

Installing with composer

composer config repositories.exteon-identity-map vcs https://github.com/exteon/identity-cache
composer require exteon/identity-cache

IdentityMap

$map = new \Exteon\IdentityCache\WeakReference\IdentityMap();

$instance = new stdClass();
$map[1] = $instance;

// While object is in use via $instance, the map will provide a reference to it
// via its id.

assert($map[1] === $instance);

// Put object out of use by releasing its reference

unset($instance);

// Object has been freed and its reference unset, because all references to it
// were released

assert(isset($map[1]) === false);

IdentityCache

$cache = new \Exteon\IdentityCache\WeakReference\IdentityCache([
    'trigger' => 'maxRetainedObjects',
    'maxRetainedObjects' => 1,
    'purgeStrategy' => 'popularity',
    'purgePressure' => 50
]);

// Create 2 object instances

$instance1 = new stdClass();
$instance2 = new stdClass();

$cache[1] = $instance1;
$cache[2] = $instance2;

// While objects are in use via $instance1, $instance2, the map holds references
// to them via their ids

assert($cache[1] === $instance1);
assert($cache[1] === $instance1);

// Increase instance 1's popularity by accessing it repeatedly

$cache[1];
$cache[1];
$cache[1];

// Put objects out of use by releasing their references

unset($instance1);
unset($instance2);

// maxRetainedObjects was configured to 1; instances will be purged

$cache->gc();

// instance 1's popularity was greater so it was preserved in the cache

assert(is_object($cache[1]));

// instance 2 was of a lesser popularity so it was purged

assert(isset($cache[2]) === false);

Configuration

An associative array can be passed to the constructor of \Exteon\IdentityCache\WeakReference\IdentityCache. Example usage, default and possible values are shown below. If a configuration option is not passed to the constructor, its default value will be used:

use \Exteon\IdentityCache\WeakReference\IdentityCache;

$cache = new IdentityCache([
    //  Specifies the condition that triggers the cache to purge unused 
    //  references.
    //  Possible values:
    //      'maxRetainedObjects'    :   purging will be initiated when we are
    //                                  caching a number of references greater
    //                                  than the maxRetainedObjects parameter
    //      'maxScriptMemory'       :   purging will be initiated when the total
    //                                  memory consumed by the running script is
    //                                  greater than the value specified in the
    //                                  maxScriptMemory parameter.
    //      'none'                  :   purging will not be done. This can be
    //                                  changed later by using the setConfig()
    //                                  method.
    'trigger' => 'maxRetainedObjects',

    //  Number of objects that can be retained before purging if trigger is
    //  set to 'maxRetainedObjects'
    'maxRetainedObjects' => 1000,

    //  Maximum total memory consumed by the running script before purging if
    //  trigger is 'maxScriptMemory'. Format is the same as php.ini's sizes// 
    //  format.
    'maxScriptMemory' => '64M',

    //  When purging, which percent of the cached objects to purge?
    'purgePressure' => 10,

    //  When purging, how to select which instances to keep?
    //  Possible values:
    //      'popularity'    :   Keep the most popular objects. See the following 
    //                          section for more details on how popularity-based
    //                          caching works.
    //      'random'        :   Randomly purge the number of objects dictated
    //                          by purgePressure.  
    'purgeStrategy' => 'popularity',

    //  See the Popularity-based caching section for an explanation of this// 
    //  parameter
    'popularityDecay' => IdentityCache::getPopularityDecay(1000, 10000, 2)
]);

Popularity-based caching

When the purgeStrategy config option is popularity, a popularity index will be kept for the cached objects. An object's popularity increases by 1 whenever it is accessed from the cache.

In order to not keep forever objects that were once very popular but have not been used in a long time, an object's popularity also decays exponentially every time another object is accessed from the cache.

The value of the decay parameter can be specified by the popularityDecay config option. This is a very sensitive float value that can be computed using the getPopularityDecay() method, as such:

$popularityDecay = \Exteon\IdentityCache\WeakReference\IdentityCache::getPopularityDecay(
    1000,   //  initialPopularity
    10000,  //  rounds
    2       //  targetPopularity
);

The meaning of the parameters above, shown with default values, is: If an object has at the present moment a popularity of initialPopularity (1000), then other objects are accessed from the cache a number of rounds (10000) times, the popularity of the initial object will drop to targetPopularity (2). By tweaking the value of popularityDecay, you can balance between keeping in the cache more of formerly popular objects or more of recent objects.

Acquiring objects

Mapped objects can be explicitly 'acquired', meaning the map/cache will hold them indefinitely even if they are no longer in use (they won't be purged). This is needed for 'dirty' objects that have been mutated and will need to be persisted later in a write-behind strategy scenario.

Example:

$cache = new \Exteon\IdentityCache\WeakReference\IdentityCache([
    'trigger' => 'maxRetainedObjects',
    'maxRetainedObjects' => 0
]);

$instance = new stdClass();
$cache[1] = $instance;

$instance->foo = 'bar';
$cache->acquire(1);

//  The acquired identity with id 1 will never be purged, even if the cache's
//  purge strategy is triggered by number of objects or memory trigger 

unset($instance);
$cache->gc();
assert(
    is_object($cache[1]) &&
    $cache[1]->foo === 'bar'
);

//  Acquired identities can be released and then they can be purged if no longer 
//  in use:

$cache->release(1);
$cache->gc();
assert(isset($cache[1]) === false);

Invoking the garbage collector explicitly

The garbage collector is normally invoked every time an identity is added to the cache; it will then check if the trigger condition is satisfied and if so it will proceed to purge excess objects from the cache. The gc can be also invoked manually by calling the gc() method on IdentityCache.

Native PHP array replacements

If the Weakref extension is not available, or for testing purposes, or for an emergency in case the Weakref / WeakReference implementation is failing, we provide regular array-based replacements for the Weakref IdentityMap and IdentityCache:

\Exteon\IdentityCache\NativeArray\IdentityMap
\Exteon\IdentityCache\NativeArray\IdentityCache

These implement the same interfaces as their WeakRef / WeakReference counterparts, so they can be dropped in their place, but they will not implement any reference releasing functionality: objects added will remain in the IdentityMap or IdentityCache until explicitly unset().