class ChainedFastBackend in Supercache 2.0.x

Defines a backend with a fast and a consistent backend chain.

In order to mitigate a network roundtrip for each cache get operation, this cache allows a fast backend to be put in front of a slow(er) backend. Typically the fast backend will be something like APCu, and be bound to a single web node, and will not require a network round trip to fetch a cache item. The fast backend will also typically be inconsistent (will only see changes from one web node). The slower backend will be something like Mysql, Memcached or Redis, and will be used by all web nodes, thus making it consistent, but also require a network round trip for each cache get.

In addition to being useful for sites running on multiple web nodes, this backend can also be useful for sites running on a single web node where the fast backend (For example, APCu) isn't shareable between the web and CLI processes. Single-node configurations that don't have that limitation can just use the fast cache backend directly.

We always use the fast backend when reading (get()) entries from cache, but check whether they were created before the last write (set()) to this (chained) cache backend. Those cache entries that were created before the last write are discarded, but we use their cache IDs to then read them from the consistent (slower) cache backend instead; at the same time we update the fast cache backend so that the next read will hit the faster backend again. Hence we can guarantee that the cache entries we return are all up-to-date, and maximally exploit the faster cache backend. This cache backend uses and maintains a "last write timestamp" to determine which cache entries should be discarded.

Because this backend will mark all the cache entries in a bin as out-dated for each write to a bin, it is best suited to bins with fewer changes.

Note that this is designed specifically for combining a fast inconsistent cache backend with a slower consistent cache back-end. To still function correctly, it needs to do a consistency check (see the "last write timestamp" logic). This contrasts with \Drupal\Core\Cache\BackendChain, which assumes both chained cache backends are consistent, thus a consistency check being pointless.

Hierarchy

• class \Drupal\supercache\Cache\ChainedFastBackend implements CacheBackendInterface uses CoordinatedWriteCounterTrait

See also

\Drupal\Core\Cache\BackendChain

File

src/Cache/ChainedFastBackend.php, line 57

Contains \Drupal\supercache\Cache\ChainedFastBackend.

Namespace

Drupal\supercache\Cache

class ChainedFastBackend implements CacheBackendInterface {
  use CoordinatedWriteCounterTrait;

  /**
   * @var string
   */
  protected $bin;

  /**
   * The consistent cache backend.
   *
   * @var \Drupal\Core\Cache\CacheBackendInterface
   */
  protected $consistentBackend;

  /**
   * The fast cache backend.
   *
   * @var \Drupal\Core\Cache\CacheBackendInterface
   */
  protected $fastBackend;

  /**
   * Constructs a ChainedFastBackend object.
   *
   * @param \Drupal\Core\Cache\CacheBackendInterface $consistent_backend
   *   The consistent cache backend.
   * @param \Drupal\Core\Cache\CacheBackendInterface $fast_backend
   *   The fast cache backend.
   * @param string $bin
   *   The cache bin for which the object is created.
   * @throws \Exception
   *   When the consistent cache backend and the fast cache backend are the same
   *   service.
   */
  public function __construct(CacheBackendInterface $consistent_backend, CacheBackendInterface $fast_backend, $bin, $mark_as_outdated_explicit = FALSE) {
    if ($consistent_backend == $fast_backend) {

      // @todo: should throw a proper exception. See https://www.drupal.org/node/2751847.
      trigger_error('Consistent cache backend and fast cache backend cannot use the same service.', E_USER_ERROR);
    }
    $this->consistentBackend = $consistent_backend;
    $this->fastBackend = $fast_backend;
    $this->bin = 'cache_' . $bin;

    // We explicitly execute the outdated call on shutdown.
    $this->doMarkAsOutdatedExplicit = $mark_as_outdated_explicit;
  }

  /**
   * {@inheritdoc}
   */
  public function get($cid, $allow_invalid = FALSE) {
    $cids = array(
      $cid,
    );
    $cache = $this
      ->getMultiple($cids, $allow_invalid);
    return reset($cache);
  }

  /**
   * {@inheritdoc}
   */
  public function getMultiple(&$cids, $allow_invalid = FALSE) {
    $cids_copy = $cids;
    $cache = array();

    // If we can determine the time at which the last write to the consistent
    // backend occurred (we might not be able to if it has been recently
    // flushed/restarted), then we can use that to validate items from the fast
    // backend, so try to get those first. Otherwise, we can't assume that
    // anything in the fast backend is valid, so don't even bother fetching
    // from there.
    $last_write_timestamp = $this
      ->getLastWrite();
    if ($last_write_timestamp) {

      // Items in the fast backend might be invalid based on their timestamp,
      // but we can't check the timestamp prior to getting the item, which
      // includes unserializing it. However, unserializing an invalid item can
      // throw an exception. For example, a __wakeup() implementation that
      // receives object properties containing references to code or data that
      // no longer exists in the application's current state.
      //
      // Unserializing invalid data, whether it throws an exception or not, is
      // a waste of time, but we only incur it while a cache invalidation has
      // not yet finished propagating to all the fast backend instances.
      //
      // Most cache backend implementations should not wrap their internal
      // get() implementations with a try/catch, because they have no reason to
      // assume that their data is invalid, and doing so would mask
      // unserialization errors of valid data. We do so here, only because the
      // fast backend is non-authoritative, and after discarding its
      // exceptions, we proceed to check the consistent (authoritative) backend
      // and allow exceptions from that to bubble up.
      try {
        $items = $this->fastBackend
          ->getMultiple($cids, $allow_invalid);
      } catch (\Exception $e) {
        $cids = $cids_copy;
        $items = array();
      }

      // Even if items were successfully fetched from the fast backend, they
      // are potentially invalid if older than the last time the bin was
      // written to in the consistent backend, so only keep ones that aren't.
      foreach ($items as $item) {
        if ($item->created < $last_write_timestamp) {
          $cids[array_search($item->cid, $cids_copy)] = $item->cid;
        }
        else {
          $cache[$item->cid] = $item;
        }
      }
    }

    // If there were any cache entries that were not available in the fast
    // backend, retrieve them from the consistent backend and store them in the
    // fast one.
    if ($cids) {
      $missing = array();
      foreach ($this->consistentBackend
        ->getMultiple($cids, $allow_invalid) as $item) {
        $cache[$item->cid] = $item;
        $missing[$item->cid] = (array) $item;
      }
      if (!empty($missing)) {

        // TODO: Expiration data from the consistent
        // backend is lost here. We are setting items
        // in the fast backend with permanent status...
        // But time based expirations are becoming less
        // relevant, and considering that the items
        // in the consistent backend will actually expire
        // properly, this might not be that of an issue.
        $this->fastBackend
          ->setMultiple($missing);
      }

      // This means that the persistent backend has no lastWriteTimestamp
      // but we were able to retrieve data from it....
      if (!$last_write_timestamp && !empty($missing)) {
        $this
          ->markAsOutdated();
      }
    }
    return $cache;
  }

  /**
   * {@inheritdoc}
   */
  public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
    $this->consistentBackend
      ->set($cid, $data, $expire, $tags);
    $this->fastBackend
      ->set($cid, $data, $expire, $tags);
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function setMultiple(array $items) {
    if (empty($items)) {
      return;
    }
    $this->consistentBackend
      ->setMultiple($items);
    $this->fastBackend
      ->setMultiple($items);
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function delete($cid) {
    $this->consistentBackend
      ->deleteMultiple(array(
      $cid,
    ));
    $this->fastBackend
      ->deleteMultiple(array(
      $cid,
    ));
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function deleteMultiple(array $cids) {
    $this->consistentBackend
      ->deleteMultiple($cids);
    $this->fastBackend
      ->deleteMultiple($cids);
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function deleteAll() {
    $this->consistentBackend
      ->deleteAll();
    $this->fastBackend
      ->deleteAll();
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function garbageCollection() {
    $this->consistentBackend
      ->garbageCollection();
    $this->fastBackend
      ->garbageCollection();
  }

  /**
   * {@inheritdoc}
   */
  public function removeBin() {
    $this->consistentBackend
      ->removeBin();
    $this->fastBackend
      ->removeBin();
  }

  /**
   * @todo Document in https://www.drupal.org/node/2311945.
   */
  public function reset() {
    $this->lastWrite = NULL;
  }

  /**
   * {@inheritdoc}
   */
  public function invalidate($cid) {
    $this
      ->invalidateMultiple(array(
      $cid,
    ));
  }

  /**
   * {@inheritdoc}
   */
  public function invalidateMultiple(array $cids) {
    $this->consistentBackend
      ->invalidateMultiple($cids);
    $this->fastBackend
      ->invalidateMultiple($cids);
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function invalidateAll() {
    $this->consistentBackend
      ->invalidateAll();
    $this->fastBackend
      ->invalidateAll();
    $this
      ->markAsOutdated();
  }

  /**
   * Shutdown functions.
   *
   * Using __destruct() proved to be problematic
   * with some some cache backends such as couchbase
   * with custom transcoders or the Drupal.org
   * test bot.
   *
   * But because binaries are not services... we rely
   * on the ChainedFastBackend factory to subscribe to
   * the onKernelTerminate event and call us.
   *
   */
  public function onKernelTerminate() {
    $this
      ->doMarkAsOutdatedExplicitCall();

    // Once this is done here, any further invalidations
    // must be done as they come.
    $this->doMarkAsOutdatedExplicit = FALSE;
  }

  #region Helper implementations for CoordinatedWriteCounterTrait

  /**
   * {@inheritdoc}
   */
  public function getFastStorage($cid) {
    return $this->fastBackend
      ->get($cid);
  }

  /**
   * {@inheritdoc}
   */
  public function getPersistentStorage($cid) {
    return $this->consistentBackend
      ->get($cid);
  }

  /**
   * {@inheritdoc}
   */
  public function setFastStorage($cid, $value) {
    $this->fastBackend
      ->set($cid, $value);
  }

  /**
   * {@inheritdoc}
   */
  public function setPersistentStorage($cid, $value) {
    $this->consistentBackend
      ->set($cid, $value);
  }

  /**
   * {@inheritdoc}
   */
  public function clearFastStorage($cid, $value) {
    $this->fastBackend
      ->deleteAll();
  }

}

Members

Name	Modifiers	Type	Description	Overrides
CacheBackendInterface::CACHE_PERMANENT		constant	Indicates that the item should never be removed unless explicitly deleted.
ChainedFastBackend::$bin	protected	property
ChainedFastBackend::$consistentBackend	protected	property	The consistent cache backend.
ChainedFastBackend::$fastBackend	protected	property	The fast cache backend.
ChainedFastBackend::clearFastStorage	public	function	Overrides CoordinatedWriteCounterTrait::clearFastStorage
ChainedFastBackend::delete	public	function	Deletes an item from the cache. Overrides CacheBackendInterface::delete
ChainedFastBackend::deleteAll	public	function	Deletes all cache items in a bin. Overrides CacheBackendInterface::deleteAll
ChainedFastBackend::deleteMultiple	public	function	Deletes multiple items from the cache. Overrides CacheBackendInterface::deleteMultiple
ChainedFastBackend::garbageCollection	public	function	Performs garbage collection on a cache bin. Overrides CacheBackendInterface::garbageCollection
ChainedFastBackend::get	public	function	Returns data from the persistent cache. Overrides CacheBackendInterface::get
ChainedFastBackend::getFastStorage	public	function	Overrides CoordinatedWriteCounterTrait::getFastStorage
ChainedFastBackend::getMultiple	public	function	Returns data from the persistent cache when given an array of cache IDs. Overrides CacheBackendInterface::getMultiple
ChainedFastBackend::getPersistentStorage	public	function	Overrides CoordinatedWriteCounterTrait::getPersistentStorage
ChainedFastBackend::invalidate	public	function	Marks a cache item as invalid. Overrides CacheBackendInterface::invalidate
ChainedFastBackend::invalidateAll	public	function	Marks all cache items as invalid. Overrides CacheBackendInterface::invalidateAll
ChainedFastBackend::invalidateMultiple	public	function	Marks cache items as invalid. Overrides CacheBackendInterface::invalidateMultiple
ChainedFastBackend::onKernelTerminate	public	function	Shutdown functions.
ChainedFastBackend::removeBin	public	function	Remove a cache bin. Overrides CacheBackendInterface::removeBin
ChainedFastBackend::reset	public	function	@todo Document in https://www.drupal.org/node/2311945.
ChainedFastBackend::set	public	function	Stores data in the persistent cache. Overrides CacheBackendInterface::set
ChainedFastBackend::setFastStorage	public	function	Overrides CoordinatedWriteCounterTrait::setFastStorage
ChainedFastBackend::setMultiple	public	function	Store multiple items in the persistent cache. Overrides CacheBackendInterface::setMultiple
ChainedFastBackend::setPersistentStorage	public	function	Overrides CoordinatedWriteCounterTrait::setPersistentStorage
ChainedFastBackend::__construct	public	function	Constructs a ChainedFastBackend object.
CoordinatedWriteCounterTrait::$doMarkAsOutdatedExplicit	protected	property	For the persistent backend to be marked as outdated the implementing class must explictly call
CoordinatedWriteCounterTrait::$fastStorageInvalid	protected	property	Summary of $fastStorageInvalid
CoordinatedWriteCounterTrait::$headId	protected	property	Identifier for the instance of the volatile storage.
CoordinatedWriteCounterTrait::$headKey	protected	property	Key used to store the coordinated write counters.
CoordinatedWriteCounterTrait::$lastWrite	protected	property	The time at which the last write to this cache bin happened.
CoordinatedWriteCounterTrait::$last_invalidation	protected	property	Last timestamp that a local invalidation took place.
CoordinatedWriteCounterTrait::$writeKey	protected	property	Key used to store the coordinated write counters.
CoordinatedWriteCounterTrait::clearFastStorageIfInvalid	protected	function	Some storage backends cannot rely on the information provided by $this->getLastWrite() to tell what items they should invalidate. Calling this method will clear all of the fast backend if it is considered not to be consistent with the contents of…
CoordinatedWriteCounterTrait::doMarkAsOutdatedExplicitCall	protected	function	To be called by the implementing class whenever it wants to persistent the last invalidation.
CoordinatedWriteCounterTrait::getHeadId	protected	function	Retrieve the key that identifies the volatile storage head.
CoordinatedWriteCounterTrait::getLastWrite	protected	function	Items retrieve from the persistent backend that have been modified prior to this timestamp are to be considered outdated.
CoordinatedWriteCounterTrait::markAsOutdated	protected	function	Notify that a write has happened, it does not inmediately invalidate the persistent storage.
CoordinatedWriteCounterTrait::_doMarkAsOutdated	protected	function	Mark as outdated.