Chris@18: <?php
Chris@18: 
Chris@18: namespace Drupal\Core\Database;
Chris@18: 
Chris@18: use Drupal\Component\Datetime\TimeInterface;
Chris@18: use Drupal\Core\Site\Settings;
Chris@18: use Symfony\Component\EventDispatcher\EventSubscriberInterface;
Chris@18: use Symfony\Component\HttpFoundation\Session\SessionInterface;
Chris@18: use Symfony\Component\HttpKernel\Event\GetResponseEvent;
Chris@18: use Symfony\Component\HttpKernel\KernelEvents;
Chris@18: 
Chris@18: /**
Chris@18:  * Provides replica server kill switch to ignore it.
Chris@18:  */
Chris@18: class ReplicaKillSwitch implements EventSubscriberInterface {
Chris@18: 
Chris@18:   /**
Chris@18:    * The settings object.
Chris@18:    *
Chris@18:    * @var \Drupal\Core\Site\Settings
Chris@18:    */
Chris@18:   protected $settings;
Chris@18: 
Chris@18:   /**
Chris@18:    * The time service.
Chris@18:    *
Chris@18:    * @var \Drupal\Component\Datetime\TimeInterface
Chris@18:    */
Chris@18:   protected $time;
Chris@18: 
Chris@18:   /**
Chris@18:    * The session.
Chris@18:    *
Chris@18:    * @var \Symfony\Component\HttpFoundation\Session\SessionInterface
Chris@18:    */
Chris@18:   protected $session;
Chris@18: 
Chris@18:   /**
Chris@18:    * Constructs a ReplicaKillSwitch object.
Chris@18:    *
Chris@18:    * @param \Drupal\Core\Site\Settings $settings
Chris@18:    *   The settings object.
Chris@18:    * @param \Drupal\Component\Datetime\TimeInterface $time
Chris@18:    *   The time service.
Chris@18:    * @param \Symfony\Component\HttpFoundation\Session\SessionInterface $session
Chris@18:    *   The session.
Chris@18:    */
Chris@18:   public function __construct(Settings $settings, TimeInterface $time, SessionInterface $session) {
Chris@18:     $this->settings = $settings;
Chris@18:     $this->time = $time;
Chris@18:     $this->session = $session;
Chris@18:   }
Chris@18: 
Chris@18:   /**
Chris@18:    * Denies access to replica database on the current request.
Chris@18:    *
Chris@18:    * @see https://www.drupal.org/node/2286193
Chris@18:    */
Chris@18:   public function trigger() {
Chris@18:     $connection_info = Database::getConnectionInfo();
Chris@18:     // Only set ignore_replica_server if there are replica servers being used,
Chris@18:     // which is assumed if there are more than one.
Chris@18:     if (count($connection_info) > 1) {
Chris@18:       // Five minutes is long enough to allow the replica to break and resume
Chris@18:       // interrupted replication without causing problems on the Drupal site
Chris@18:       // from the old data.
Chris@18:       $duration = $this->settings->get('maximum_replication_lag', 300);
Chris@18:       // Set session variable with amount of time to delay before using replica.
Chris@18:       $this->session->set('ignore_replica_server', $this->time->getRequestTime() + $duration);
Chris@18:     }
Chris@18:   }
Chris@18: 
Chris@18:   /**
Chris@18:    * Checks and disables the replica database server if appropriate.
Chris@18:    *
Chris@18:    * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
Chris@18:    *   The Event to process.
Chris@18:    */
Chris@18:   public function checkReplicaServer(GetResponseEvent $event) {
Chris@18:     // Ignore replica database servers for this request.
Chris@18:     //
Chris@18:     // In Drupal's distributed database structure, new data is written to the
Chris@18:     // master and then propagated to the replica servers.  This means there is a
Chris@18:     // lag between when data is written to the master and when it is available
Chris@18:     // on the replica. At these times, we will want to avoid using a replica
Chris@18:     // server temporarily. For example, if a user posts a new node then we want
Chris@18:     // to disable the replica server for that user temporarily to allow the
Chris@18:     // replica server to catch up.
Chris@18:     // That way, that user will see their changes immediately while for other
Chris@18:     // users we still get the benefits of having a replica server, just with
Chris@18:     // slightly stale data. Code that wants to disable the replica server should
Chris@18:     // use the 'database.replica_kill_switch' service's trigger() method to set
Chris@18:     // 'ignore_replica_server' session flag to the timestamp after which the
Chris@18:     // replica can be re-enabled.
Chris@18:     if ($this->session->has('ignore_replica_server')) {
Chris@18:       if ($this->session->get('ignore_replica_server') >= $this->time->getRequestTime()) {
Chris@18:         Database::ignoreTarget('default', 'replica');
Chris@18:       }
Chris@18:       else {
Chris@18:         $this->session->remove('ignore_replica_server');
Chris@18:       }
Chris@18:     }
Chris@18:   }
Chris@18: 
Chris@18:   /**
Chris@18:    * {@inheritdoc}
Chris@18:    */
Chris@18:   public static function getSubscribedEvents() {
Chris@18:     $events[KernelEvents::REQUEST][] = ['checkReplicaServer'];
Chris@18:     return $events;
Chris@18:   }
Chris@18: 
Chris@18: }