README

Yii2 Refresh-Ahead Cache can decorate Yii2 cache components or other components to implement a refresh-ahead cache strategy.

The Refresh-Ahead cache strategy (also called Read-Ahead) is used to refresh cached data before it expires. By refreshing cached data before it expires (and doing it asynchronously), end-users never have to suffer the delay of the refresh. Furthermore, it can also help avoid a Cache Stampede.

For license information check the LICENSE-file.

Installation

The preferred way to install this extension is through composer.

php composer.phar require --prefer-dist thamtech/yii2-read-ahead-cache

or add

"thamtech/yii2-read-ahead-cache": "*"

to the require section of your composer.json file.

Usage

Decorate Cache Component

You can add Refresh-Ahead capability to your application's cache component by attaching the RefreshAheadCacheBehavior. For example, in your application configuration:

<?php
[
    'components' => [
        'cache' => [
            'class' => 'yii\redis\Cache',
            'as refreshAhead' => 'thamtech\caching\refreshahead\RefreshAheadCacheBehavior',
        ],
    ],
];

There are a number of parameters you can configure if you declare the behavior as a configuration array:

<?php
[
    'components' => [
        'redisCache' => [
            'class' => 'yii\redis\Cache',
        ],
        'appMutex' => [
            'class' => 'yii\redis\Mutex',
        ],
        'cache' => [
            'class' => 'yii\caching\FileCache',
            'as refreshAhead' => [
                'class' => 'thamtech\caching\refreshahead\RefreshAheadCacheBehavior',
                'refreshTimeoutCache' => 'redisCache',
                'refreshAheadFactor' => 0.5,
                'refreshTimeoutKeySuffix' => 'refresh-ahead-timeout',
                'mutex' => 'appMutex',
            ],
        ],
    ],
];

Decorate any Component

By default, RefreshAheadCacheBehavior assumes that its owner (the component it is attached to as a behavior) is a cache component that will be used for both storage of the cached data as well as the storage of the refresh timeout key. However, you can specify which cache components to use in these cases, so you do not have to attach RefreshAheadCacheBehavior to a Cache component. You can attach it to any Yii Component provided that you specify the cache component(s) to use in the behavior's configuration.

<?php
$dataManager = Yii::createObject([
    'class' => DataManager::class,
    'as refreshAhead' => [
        'class' => 'thamtech\caching\refreshahead\RefreshAheadCacheBehavior',
        
        // use application 'cache' component for data storage
        'dataCache' => 'cache',
        
        // use application 'cache' component for refresh timeout key storage
        'refreshTimeoutCache' => 'cache',
    ],
]);

If both data values and refresh timeout keys will be stored in the same cache component, you can set the single cache property as a shortcut for setting both dataCache and refreshTimeoutCache. The following configuration is equivalent to the one above:

<?php
$dataManager = Yii::createObject([
    'class' => DataManager::class,
    'as refreshAhead' => [
        'class' => 'thamtech\caching\refreshahead\RefreshAheadCacheBehavior',
        
        // use application 'cache' component for both data storage and refresh
        // timeout key storage
        'cache' => 'cache',
    ],
]);

Drop-In Replacement for getOrSet

RefreshAheadCacheBehavior adds a getRefreshOrSet() method to the cache or any other component it decorates. This method has the same signature as getOrSet(), so you can perform a drop-in replacement where you currently use getOrSet(). For example,

<?php
$data = $cache->getOrSet($key, function ($cache) {
    return $this->calculateSomething();
});

// drop-in replacement:
$data = $cache->getRefreshOrSet($key, function ($cache) {
    return $this->calculateSomething();
});


// with specified $duration and $dependency
$data = $cache->getOrSet($key, function ($cache) {
    return $this->calculateSomething();
}, $duration, $dependency);

// drop-in replacement
$data = $cache->getRefreshOrSet($key, function ($cache) {
    return $this->calculateSomething();
}, $duration, $dependency);

The Refresh Ahead strategy attempts to add() a refresh timeout key in the refresh timeout cache component with a duration shorter than the requested $duration (half of $duration by default). If the add is successful, it means any previous refresh timeout key had expired and the cached data is due for a refresh.

When getRefreshOrSet() is called with a single callable parameter like the examples above, the Refresh Ahead strategy calls the callable, stores the returned value into the data cache component using the specified $duration, and returns that value (into the $data variable in the examples above).

On the other hand, if the attempt to add the refresh timeout key was not successful, it means the key already exists and is not expired, and therefore, no refresh is currently called for. The Refresh Ahead strategy uses $key to look up the cached value in the data cache component and returns it if it finds it. If it doesn't find it in the data cache (perhaps the cache was flushed or the key was evicted), then the callable is invoked to calculate the new value. The new value is set in the data cache component using the specified $duration and the value is returned.

Typical Usage

The usage can be further improved if you can support asynchronous refreshing. In order to do this, we must provide the Refresh Ahead strategy with two callables: one to trigger a refresh asynchronously and one that will refresh the data synchronously and return the result.

The usage is similar to the examples above, except that the second parameter to getRefreshOrSet() will be a GeneratorInterface object or configuration array instead of a single callable. For example,

<?php
$data = $cache->getRefreshOrSet($key, [
    // called by Refresh Ahead strategy if the data is still cached, but it is
    // time to refresh it
    'refresh' => function ($cache) {
        // queue the refresh task to be run at a later time
        // return `true` when task is queued, `false` if the task was not queued
        // (in which case the `refresh` callable will be called again in a
        // subsequent request)
        return $this->taskQueue->append('calculateSomething');
    },
    
    // called by Refresh Ahead strategy if the data is not in cache (it may
    // have expired before it could be refreshed, or it could have been
    // flushed or evicted, etc.)
    'generate' => function ($cache) {
        return $this->calculateSomething();
    }
], $duration, $dependency);

If you've configured a mutex component in the RefreshAheadCacheBehavior, you can specify a timeout for acquiring a lock using the mutexLockTimeout property:

<?php
$generator = [
    'refresh' => function ($cache) {
        return $this->taskQueue->append('calculateSomething');
    },
    'generate' => function ($cache) {
        return $this->calculateSomething();
    },
    // Attempt to acquire a mutex lock for 12 seconds before invoking
    // the 'generate' callback. If the lock is acquired, Refresh Ahead will
    // check to see if the value is in the data cache once more before invoking
    // 'generate', in case another process was generating and caching the value
    // already.
    'mutexLockTimeout' => 12,
];

$data = $cache->getRefreshOrSet($key, $generator, $duration, $dependency);

By configuring a mutex component on the behavior and setting the mutexLockTimeout as a property on the generator, the Refresh Ahead strategy will attempt to acquire a lock to invoke the generate callable. This way, if multiple requests come in around the same time when the value has expired (a Cache Stampede), the process that first acquires the lock will compute the value and store it in cache. The other processes wait for the lock to be released. Once the first process releases the lock, the value has been computed and is in cache, so the other processes will check for it in cache, find it, and return it without having to invoke the generate callable.

If your task queue can run asynchronously, such as in a cron task, you can use the same $generator in a call to generateAndSet() to complete the refresh process and update the cache value in the background. For example,

<?php
// using the same parameters defined in the previous example:
$data = $cache->generateAndSet($key, $generator, $duration, $dependency);

This will invoke the generate callable (if the item hasn't already been cached by another invocation of generate at the same time), and sets the result in the cache before returning it.

See Also

• Cache Stampede

• Cache Concurrency Control - Case Study

• All things caching