Cache Items

Cache items are the information units stored in the cache as a key/value pair. In the Cache component they are represented by the CacheItem class. They are used in both the Cache Contracts and the PSR-6 interfaces.

Cache Item Keys and Values

The key of a cache item is a plain string which acts as its identifier, so it must be unique for each cache pool. You can freely choose the keys, but they should only contain letters (A-Z, a-z), numbers (0-9) and the _ and . symbols. Other common symbols (such as {, }, (, ), /, \, @ and :) are reserved by the PSR-6 standard for future uses.

The value of a cache item can be any data represented by a type which is serializable by PHP, such as basic types (string, integer, float, boolean, null), arrays and objects.

Creating Cache Items

The only way to create cache items is via cache pools. When using the Cache Contracts, they are passed as arguments to the recomputation callback:

// $cache pool object was created before
$productsCount = $cache->get('stats.products_count', function (ItemInterface $item) {
    // [...]
});

When using PSR-6, they are created with the getItem($key) method of the cache pool:

// $cache pool object was created before
$productsCount = $cache->getItem('stats.products_count');

Then, use the Psr\Cache\CacheItemInterface::set method to set the data stored in the cache item (this step is done automatically when using the Cache Contracts):

// storing a simple integer
$productsCount->set(4711);
$cache->save($productsCount);

// storing an array
$productsCount->set([
    'category1' => 4711,
    'category2' => 2387,
]);
$cache->save($productsCount);

The key and the value of any given cache item can be obtained with the corresponding getter methods:

$cacheItem = $cache->getItem('exchange_rate');
// ...
$key = $cacheItem->getKey();
$value = $cacheItem->get();

Cache Item Expiration

By default, cache items are stored permanently. In practice, this “permanent storage” can vary greatly depending on the type of cache being used, as explained in the Cache Pools and Supported Adapters article.

However, in some applications it’s common to use cache items with a shorter lifespan. Consider for example an application which caches the latest news just for one minute. In those cases, use the expiresAfter() method to set the number of seconds to cache the item:

$latestNews->expiresAfter(60);  // 60 seconds = 1 minute

// this method also accepts \DateInterval instances
$latestNews->expiresAfter(DateInterval::createFromDateString('1 hour'));

Cache items define another related method called expiresAt() to set the exact date and time when the item will expire:

$mostPopularNews->expiresAt(new \DateTime('tomorrow'));

Cache Item Hits and Misses

Using a cache mechanism is important to improve the application performance, but it should not be required to make the application work. In fact, the PSR-6 document wisely states that caching errors should not result in application failures.

In practice with PSR-6, this means that the getItem() method always returns an object which implements the Psr\Cache\CacheItemInterface interface, even when the cache item doesn’t exist. Therefore, you don’t have to deal with null return values and you can safely store in the cache values such as false and null.

In order to decide if the returned object represents a value coming from the storage or not, caches use the concept of hits and misses:

• Cache Hits occur when the requested item is found in the cache, its value is not corrupted or invalid and it hasn’t expired;
• Cache Misses are the opposite of hits, so they occur when the item is not found in the cache, its value is corrupted or invalid for any reason or the item has expired.

Cache item objects define a boolean isHit() method which returns true for cache hits:

$latestNews = $cache->getItem('latest_news');

if (!$latestNews->isHit()) {
    // do some heavy computation
    $news = ...;
    $cache->save($latestNews->set($news));
} else {
    $news = $latestNews->get();
}