class ChainedFastBackend in Drupal 10

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 (e.g., 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\Core\Cache\ChainedFastBackend implements \Drupal\Core\Cache\CacheBackendInterface, \Drupal\Core\Cache\CacheTagsInvalidatorInterface

See also

\Drupal\Core\Cache\BackendChain

Related topics

Cache API

Information about the Drupal Cache API

2 files declare their use of ChainedFastBackend

ChainedFastBackendTest.php in core/tests/Drupal/Tests/Core/Cache/ChainedFastBackendTest.php

ChainedFastBackendTest.php in core/tests/Drupal/KernelTests/Core/Cache/ChainedFastBackendTest.php

File

core/lib/Drupal/Core/Cache/ChainedFastBackend.php, line 48

Namespace

Drupal\Core\Cache

class ChainedFastBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {

  /**
   * Cache key prefix for the bin-specific entry to track the last write.
   */
  const LAST_WRITE_TIMESTAMP_PREFIX = 'last_write_timestamp_';

  /**
   * @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;

  /**
   * The time at which the last write to this cache bin happened.
   *
   * @var float
   */
  protected $lastWriteTimestamp;

  /**
   * 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.
   */
  public function __construct(CacheBackendInterface $consistent_backend, CacheBackendInterface $fast_backend, $bin) {
    $this->consistentBackend = $consistent_backend;
    $this->fastBackend = $fast_backend;
    $this->bin = 'cache_' . $bin;
    $this->lastWriteTimestamp = NULL;
  }

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

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

    // 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 = $this
      ->getLastWriteTimestamp();
    if ($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 = [];
      }

      // 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 ($cids) {
      foreach ($this->consistentBackend
        ->getMultiple($cids, $allow_invalid) as $item) {
        $cache[$item->cid] = $item;

        // Don't write the cache tags to the fast backend as any cache tag
        // invalidation results in an invalidation of the whole fast backend.
        $this->fastBackend
          ->set($item->cid, $item->data, $item->expire);
      }
    }
    return $cache;
  }

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

    // Don't write the cache tags to the fast backend as any cache tag
    // invalidation results in an invalidation of the whole fast backend.
    $this->fastBackend
      ->set($cid, $data, $expire);
  }

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

    // Don't write the cache tags to the fast backend as any cache tag
    // invalidation results in an invalidation of the whole fast backend.
    foreach ($items as &$item) {
      unset($item['tags']);
    }
    $this->fastBackend
      ->setMultiple($items);
  }

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

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

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

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

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

  /**
   * {@inheritdoc}
   */
  public function invalidateTags(array $tags) {
    if ($this->consistentBackend instanceof CacheTagsInvalidatorInterface) {
      $this->consistentBackend
        ->invalidateTags($tags);
    }
    $this
      ->markAsOutdated();
  }

  /**
   * {@inheritdoc}
   */
  public function invalidateAll() {
    $this->consistentBackend
      ->invalidateAll();
    $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->lastWriteTimestamp = NULL;
  }

  /**
   * Gets the last write timestamp.
   */
  protected function getLastWriteTimestamp() {
    if ($this->lastWriteTimestamp === NULL) {
      $cache = $this->consistentBackend
        ->get(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin);
      $this->lastWriteTimestamp = $cache ? $cache->data : 0;
    }
    return $this->lastWriteTimestamp;
  }

  /**
   * Marks the fast cache bin as outdated because of a write.
   */
  protected function markAsOutdated() {

    // Clocks on a single server can drift. Multiple servers may have slightly
    // differing opinions about the current time. Given that, do not assume
    // 'now' on this server is always later than our stored timestamp.
    // Also add 1 millisecond, to ensure that caches written earlier in the same
    // millisecond are invalidated. It is possible that caches will be later in
    // the same millisecond and are then incorrectly invalidated, but that only
    // costs one additional roundtrip to the persistent cache.
    $now = round(microtime(TRUE) + 0.001, 3);
    if ($now > $this
      ->getLastWriteTimestamp()) {
      $this->lastWriteTimestamp = $now;
      $this->consistentBackend
        ->set(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin, $this->lastWriteTimestamp);
    }
  }

}