class BackendChain

class BackendChain implements CacheBackendInterface, CacheTagsInvalidatorInterface {
    
    /**
     * Ordered list of CacheBackendInterface instances.
     *
     * @var array
     */
    protected $backends = [];
    
    /**
     * Appends a cache backend to the cache chain.
     *
     * @param CacheBackendInterface $backend
     *   The cache backend to be appended to the cache chain.
     *
     * @return $this
     *   The called object.
     */
    public function appendBackend(CacheBackendInterface $backend) {
        $this->backends[] = $backend;
        return $this;
    }
    
    /**
     * Prepends a cache backend to the cache chain.
     *
     * @param CacheBackendInterface $backend
     *   The backend to be prepended to the cache chain.
     *
     * @return $this
     *   The called object.
     */
    public function prependBackend(CacheBackendInterface $backend) {
        array_unshift($this->backends, $backend);
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function get($cid, $allow_invalid = FALSE) {
        foreach ($this->backends as $index => $backend) {
            if (($return = $backend->get($cid, $allow_invalid)) !== FALSE) {
                // We found a result, propagate it to all missed backends.
                if ($index > 0) {
                    for ($i = $index - 1; 0 <= $i; --$i) {
                        $this->backends[$i]
                            ->set($cid, $return->data, $return->expire, $return->tags);
                    }
                }
                return $return;
            }
        }
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getMultiple(&$cids, $allow_invalid = FALSE) {
        $return = [];
        foreach ($this->backends as $index => $backend) {
            $items = $backend->getMultiple($cids, $allow_invalid);
            // Propagate the values that could be retrieved from the current cache
            // backend to all missed backends.
            if ($index > 0 && !empty($items)) {
                for ($i = $index - 1; 0 <= $i; --$i) {
                    foreach ($items as $cached) {
                        $this->backends[$i]
                            ->set($cached->cid, $cached->data, $cached->expire, $cached->tags);
                    }
                }
            }
            // Append the values to the previously retrieved ones.
            $return += $items;
            if (empty($cids)) {
                // No need to go further if we don't have any cid to fetch left.
                break;
            }
        }
        return $return;
    }
    
    /**
     * {@inheritdoc}
     */
    public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = []) {
        foreach ($this->backends as $backend) {
            $backend->set($cid, $data, $expire, $tags);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function setMultiple(array $items) {
        foreach ($this->backends as $backend) {
            $backend->setMultiple($items);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete($cid) {
        foreach ($this->backends as $backend) {
            $backend->delete($cid);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteMultiple(array $cids) {
        foreach ($this->backends as $backend) {
            $backend->deleteMultiple($cids);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteAll() {
        foreach ($this->backends as $backend) {
            $backend->deleteAll();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidate($cid) {
        foreach ($this->backends as $backend) {
            $backend->invalidate($cid);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateMultiple(array $cids) {
        foreach ($this->backends as $backend) {
            $backend->invalidateMultiple($cids);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateTags(array $tags) {
        foreach ($this->backends as $backend) {
            if ($backend instanceof CacheTagsInvalidatorInterface) {
                $backend->invalidateTags($tags);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateAll() {
        foreach ($this->backends as $backend) {
            $backend->invalidateAll();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function garbageCollection() {
        foreach ($this->backends as $backend) {
            $backend->garbageCollection();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function removeBin() {
        foreach ($this->backends as $backend) {
            $backend->removeBin();
        }
    }

}