class ChainedFastBackendFactory

class ChainedFastBackendFactory implements CacheFactoryInterface {
    use ContainerAwareTrait;
    
    /**
     * The service name of the consistent backend factory.
     *
     * @var string
     */
    protected $consistentServiceName;
    
    /**
     * The service name of the fast backend factory.
     *
     * @var string
     */
    protected $fastServiceName;
    
    /**
     * Constructs ChainedFastBackendFactory object.
     *
     * @param \Drupal\Core\Site\Settings|null $settings
     *   (optional) The settings object.
     * @param string|null $consistent_service_name
     *   (optional) The service name of the consistent backend factory. Defaults
     *   to:
     *   - $settings->get('cache')['default'] (if specified)
     *   - 'cache.backend.database' (if the above isn't specified)
     * @param string|null $fast_service_name
     *   (optional) The service name of the fast backend factory. Defaults to:
     *   - 'cache.backend.apcu' (if the PHP process has APCu enabled)
     *   - NULL (if the PHP process doesn't have APCu enabled)
     */
    public function __construct(Settings $settings = NULL, $consistent_service_name = NULL, $fast_service_name = NULL) {
        // Default the consistent backend to the site's default backend.
        if (!isset($consistent_service_name)) {
            $cache_settings = isset($settings) ? $settings->get('cache') : [];
            $consistent_service_name = isset($cache_settings['default']) ? $cache_settings['default'] : 'cache.backend.database';
        }
        // Default the fast backend to APCu if it's available.
        if (!isset($fast_service_name) && function_exists('apcu_fetch')) {
            $fast_service_name = 'cache.backend.apcu';
        }
        $this->consistentServiceName = $consistent_service_name;
        // Do not use the fast chained backend during installation. In those cases,
        // we expect many cache invalidations and writes, the fast chained cache
        // backend performs badly in such a scenario.
        if (!InstallerKernel::installationAttempted()) {
            $this->fastServiceName = $fast_service_name;
        }
    }
    
    /**
     * Instantiates a chained, fast cache backend class for a given cache bin.
     *
     * @param string $bin
     *   The cache bin for which a cache backend object should be returned.
     *
     * @return \Drupal\Core\Cache\CacheBackendInterface
     *   The cache backend object associated with the specified bin.
     */
    public function get($bin) {
        // Use the chained backend only if there is a fast backend available;
        // otherwise, just return the consistent backend directly.
        if (isset($this->fastServiceName)) {
            return new ChainedFastBackend($this->container
                ->get($this->consistentServiceName)
                ->get($bin), $this->container
                ->get($this->fastServiceName)
                ->get($bin), $bin);
        }
        else {
            return $this->container
                ->get($this->consistentServiceName)
                ->get($bin);
        }
    }

}