Hierarchical PSR-6 cache pool

Note: You will find the best performance when using a driver that automatically purges stale records. e.g. Memcache(d) or Redis

The trait for hierarchy pools creates a form of tag for each level in the hierarchy. That means that for every input key there are multiple path keys. The path keys are used to fetch a path value in the storage which is used to create a storage key for the CacheItem. The idea is complex, but easy to use. Consider this example:

// Input key is form the user
$inputKey = '|foo|bar';
$pathKey1 = 'path!foo';
$pathKey2 = 'path!foo![foo_idx]!bar';
$storageKey = 'foo![foo_idx]!bar![bar_idx]';

$this->storage->set($storageKey, $item);

To clear the input key |foo you need to update foo_idx simply by increasing the value by one. The implementation becomes a bit more complex when you add support for tags but luckily there is a trait to help you with this.

To implement hierarchy cache, there are four things you need to do:

  • Implement HierarchicalPoolInterface and use HierarchicalCachePoolTrait
  • Use HierarchicalCachePoolTrait::getHierarchyKey($key)
  • Delete items better
  • Implement HierarchicalCachePoolTrait::getValueFormStore($key)

Implement the interface and use the trait

The trait has two protected functions getHierarchyKey($inputKey) and clearHierarchyKeyCache() and one abstract function getValueFormStore($key). We will talk about those later.

use Cache\Hierarchy\HierarchicalCachePoolTrait;
use Cache\Hierarchy\HierarchicalPoolInterface;

class MyCachePool extends AbstractCachePool implements HierarchicalPoolInterface
  use HierarchicalCachePoolTrait;

  // ..

Use HierarchicalCachePoolTrait::getHierarchyKey

To convert an input key to a storage key you need to run HierarchicalCachePoolTrait::getHierarchyKey($key). It will create the storage key for you. You could give in a tagged key like foo:tagHash.

public function getItem($key)
  $storageKey = $this->getHierarchyKey($key);

  // ...

You should do this for the following functions:

  • getItem
  • getItems
  • hasItem
  • clear
  • deleteItem
  • deleteItems

Delete items better

When you are clearing the cache there are quite a few things to think about. You need to update the stored value for the path key and also clear the local hierarchy cache key storage. To get a path key you can use the second argument of HierarchicalCachePoolTrait::getHierarchyKey($inputKey, $pathKey) that is passed by reference.

public function deleteItem($key)
  // Get storage key and path key
  $pathKey = null;
  $storageKey = $this->getHierarchyKey($key, $pathKey);

  // Update index for the path key
  $pathIndex = $this->storage->get($pathKey);
  $this->storage->set($pathKey, $pathIndex + 1);

  // Clear the local key cache

  // ..


Implement getValueFormStore($key)

The trait has one abstract function HierarchicalCachePoolTrait::getValueFormStore($key) which is used to get the value for a path key. This function should just fetch data from the storage without modifications.

 protected function getValueFormStore($key)
   return $this->storage->get($key);