Chris@0: <?php
Chris@0: 
Chris@0: namespace Drupal\Core\Cache;
Chris@0: 
Chris@0: /**
Chris@0:  * Defines a backend with a fast and a consistent backend chain.
Chris@0:  *
Chris@0:  * In order to mitigate a network roundtrip for each cache get operation, this
Chris@0:  * cache allows a fast backend to be put in front of a slow(er) backend.
Chris@0:  * Typically the fast backend will be something like APCu, and be bound to a
Chris@0:  * single web node, and will not require a network round trip to fetch a cache
Chris@0:  * item. The fast backend will also typically be inconsistent (will only see
Chris@0:  * changes from one web node). The slower backend will be something like Mysql,
Chris@0:  * Memcached or Redis, and will be used by all web nodes, thus making it
Chris@0:  * consistent, but also require a network round trip for each cache get.
Chris@0:  *
Chris@0:  * In addition to being useful for sites running on multiple web nodes, this
Chris@0:  * backend can also be useful for sites running on a single web node where the
Chris@0:  * fast backend (e.g., APCu) isn't shareable between the web and CLI processes.
Chris@0:  * Single-node configurations that don't have that limitation can just use the
Chris@0:  * fast cache backend directly.
Chris@0:  *
Chris@0:  * We always use the fast backend when reading (get()) entries from cache, but
Chris@0:  * check whether they were created before the last write (set()) to this
Chris@0:  * (chained) cache backend. Those cache entries that were created before the
Chris@0:  * last write are discarded, but we use their cache IDs to then read them from
Chris@0:  * the consistent (slower) cache backend instead; at the same time we update
Chris@0:  * the fast cache backend so that the next read will hit the faster backend
Chris@0:  * again. Hence we can guarantee that the cache entries we return are all
Chris@0:  * up-to-date, and maximally exploit the faster cache backend. This cache
Chris@0:  * backend uses and maintains a "last write timestamp" to determine which cache
Chris@0:  * entries should be discarded.
Chris@0:  *
Chris@0:  * Because this backend will mark all the cache entries in a bin as out-dated
Chris@0:  * for each write to a bin, it is best suited to bins with fewer changes.
Chris@0:  *
Chris@0:  * Note that this is designed specifically for combining a fast inconsistent
Chris@0:  * cache backend with a slower consistent cache back-end. To still function
Chris@0:  * correctly, it needs to do a consistency check (see the "last write timestamp"
Chris@0:  * logic). This contrasts with \Drupal\Core\Cache\BackendChain, which assumes
Chris@0:  * both chained cache backends are consistent, thus a consistency check being
Chris@0:  * pointless.
Chris@0:  *
Chris@0:  * @see \Drupal\Core\Cache\BackendChain
Chris@0:  *
Chris@0:  * @ingroup cache
Chris@0:  */
Chris@0: class ChainedFastBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {
Chris@0: 
Chris@0:   /**
Chris@0:    * Cache key prefix for the bin-specific entry to track the last write.
Chris@0:    */
Chris@0:   const LAST_WRITE_TIMESTAMP_PREFIX = 'last_write_timestamp_';
Chris@0: 
Chris@0:   /**
Chris@0:    * @var string
Chris@0:    */
Chris@0:   protected $bin;
Chris@0: 
Chris@0:   /**
Chris@0:    * The consistent cache backend.
Chris@0:    *
Chris@0:    * @var \Drupal\Core\Cache\CacheBackendInterface
Chris@0:    */
Chris@0:   protected $consistentBackend;
Chris@0: 
Chris@0:   /**
Chris@0:    * The fast cache backend.
Chris@0:    *
Chris@0:    * @var \Drupal\Core\Cache\CacheBackendInterface
Chris@0:    */
Chris@0:   protected $fastBackend;
Chris@0: 
Chris@0:   /**
Chris@0:    * The time at which the last write to this cache bin happened.
Chris@0:    *
Chris@0:    * @var float
Chris@0:    */
Chris@0:   protected $lastWriteTimestamp;
Chris@0: 
Chris@0:   /**
Chris@0:    * Constructs a ChainedFastBackend object.
Chris@0:    *
Chris@0:    * @param \Drupal\Core\Cache\CacheBackendInterface $consistent_backend
Chris@0:    *   The consistent cache backend.
Chris@0:    * @param \Drupal\Core\Cache\CacheBackendInterface $fast_backend
Chris@0:    *   The fast cache backend.
Chris@0:    * @param string $bin
Chris@0:    *   The cache bin for which the object is created.
Chris@0:    */
Chris@0:   public function __construct(CacheBackendInterface $consistent_backend, CacheBackendInterface $fast_backend, $bin) {
Chris@0:     $this->consistentBackend = $consistent_backend;
Chris@0:     $this->fastBackend = $fast_backend;
Chris@0:     $this->bin = 'cache_' . $bin;
Chris@0:     $this->lastWriteTimestamp = NULL;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function get($cid, $allow_invalid = FALSE) {
Chris@0:     $cids = [$cid];
Chris@0:     $cache = $this->getMultiple($cids, $allow_invalid);
Chris@0:     return reset($cache);
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function getMultiple(&$cids, $allow_invalid = FALSE) {
Chris@0:     $cids_copy = $cids;
Chris@0:     $cache = [];
Chris@0: 
Chris@0:     // If we can determine the time at which the last write to the consistent
Chris@0:     // backend occurred (we might not be able to if it has been recently
Chris@0:     // flushed/restarted), then we can use that to validate items from the fast
Chris@0:     // backend, so try to get those first. Otherwise, we can't assume that
Chris@0:     // anything in the fast backend is valid, so don't even bother fetching
Chris@0:     // from there.
Chris@0:     $last_write_timestamp = $this->getLastWriteTimestamp();
Chris@0:     if ($last_write_timestamp) {
Chris@0:       // Items in the fast backend might be invalid based on their timestamp,
Chris@0:       // but we can't check the timestamp prior to getting the item, which
Chris@0:       // includes unserializing it. However, unserializing an invalid item can
Chris@0:       // throw an exception. For example, a __wakeup() implementation that
Chris@0:       // receives object properties containing references to code or data that
Chris@0:       // no longer exists in the application's current state.
Chris@0:       //
Chris@0:       // Unserializing invalid data, whether it throws an exception or not, is
Chris@0:       // a waste of time, but we only incur it while a cache invalidation has
Chris@0:       // not yet finished propagating to all the fast backend instances.
Chris@0:       //
Chris@0:       // Most cache backend implementations should not wrap their internal
Chris@0:       // get() implementations with a try/catch, because they have no reason to
Chris@0:       // assume that their data is invalid, and doing so would mask
Chris@0:       // unserialization errors of valid data. We do so here, only because the
Chris@0:       // fast backend is non-authoritative, and after discarding its
Chris@0:       // exceptions, we proceed to check the consistent (authoritative) backend
Chris@0:       // and allow exceptions from that to bubble up.
Chris@0:       try {
Chris@0:         $items = $this->fastBackend->getMultiple($cids, $allow_invalid);
Chris@0:       }
Chris@0:       catch (\Exception $e) {
Chris@0:         $cids = $cids_copy;
Chris@0:         $items = [];
Chris@0:       }
Chris@0: 
Chris@0:       // Even if items were successfully fetched from the fast backend, they
Chris@0:       // are potentially invalid if older than the last time the bin was
Chris@0:       // written to in the consistent backend, so only keep ones that aren't.
Chris@0:       foreach ($items as $item) {
Chris@0:         if ($item->created < $last_write_timestamp) {
Chris@0:           $cids[array_search($item->cid, $cids_copy)] = $item->cid;
Chris@0:         }
Chris@0:         else {
Chris@0:           $cache[$item->cid] = $item;
Chris@0:         }
Chris@0:       }
Chris@0:     }
Chris@0: 
Chris@0:     // If there were any cache entries that were not available in the fast
Chris@0:     // backend, retrieve them from the consistent backend and store them in the
Chris@0:     // fast one.
Chris@0:     if ($cids) {
Chris@0:       foreach ($this->consistentBackend->getMultiple($cids, $allow_invalid) as $item) {
Chris@0:         $cache[$item->cid] = $item;
Chris@0:         // Don't write the cache tags to the fast backend as any cache tag
Chris@0:         // invalidation results in an invalidation of the whole fast backend.
Chris@0:         $this->fastBackend->set($item->cid, $item->data, $item->expire);
Chris@0:       }
Chris@0:     }
Chris@0: 
Chris@0:     return $cache;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = []) {
Chris@0:     $this->consistentBackend->set($cid, $data, $expire, $tags);
Chris@0:     $this->markAsOutdated();
Chris@0:     // Don't write the cache tags to the fast backend as any cache tag
Chris@0:     // invalidation results in an invalidation of the whole fast backend.
Chris@0:     $this->fastBackend->set($cid, $data, $expire);
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function setMultiple(array $items) {
Chris@0:     $this->consistentBackend->setMultiple($items);
Chris@0:     $this->markAsOutdated();
Chris@0:     // Don't write the cache tags to the fast backend as any cache tag
Chris@0:     // invalidation results in an invalidation of the whole fast backend.
Chris@0:     foreach ($items as &$item) {
Chris@0:       unset($item['tags']);
Chris@0:     }
Chris@0:     $this->fastBackend->setMultiple($items);
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function delete($cid) {
Chris@0:     $this->consistentBackend->deleteMultiple([$cid]);
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function deleteMultiple(array $cids) {
Chris@0:     $this->consistentBackend->deleteMultiple($cids);
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function deleteAll() {
Chris@0:     $this->consistentBackend->deleteAll();
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function invalidate($cid) {
Chris@0:     $this->invalidateMultiple([$cid]);
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function invalidateMultiple(array $cids) {
Chris@0:     $this->consistentBackend->invalidateMultiple($cids);
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function invalidateTags(array $tags) {
Chris@0:     if ($this->consistentBackend instanceof CacheTagsInvalidatorInterface) {
Chris@0:       $this->consistentBackend->invalidateTags($tags);
Chris@0:     }
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function invalidateAll() {
Chris@0:     $this->consistentBackend->invalidateAll();
Chris@0:     $this->markAsOutdated();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function garbageCollection() {
Chris@0:     $this->consistentBackend->garbageCollection();
Chris@0:     $this->fastBackend->garbageCollection();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function removeBin() {
Chris@0:     $this->consistentBackend->removeBin();
Chris@0:     $this->fastBackend->removeBin();
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * @todo Document in https://www.drupal.org/node/2311945.
Chris@0:    */
Chris@0:   public function reset() {
Chris@0:     $this->lastWriteTimestamp = NULL;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Gets the last write timestamp.
Chris@0:    */
Chris@0:   protected function getLastWriteTimestamp() {
Chris@0:     if ($this->lastWriteTimestamp === NULL) {
Chris@0:       $cache = $this->consistentBackend->get(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin);
Chris@0:       $this->lastWriteTimestamp = $cache ? $cache->data : 0;
Chris@0:     }
Chris@0:     return $this->lastWriteTimestamp;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Marks the fast cache bin as outdated because of a write.
Chris@0:    */
Chris@0:   protected function markAsOutdated() {
Chris@0:     // Clocks on a single server can drift. Multiple servers may have slightly
Chris@0:     // differing opinions about the current time. Given that, do not assume
Chris@0:     // 'now' on this server is always later than our stored timestamp.
Chris@0:     // Also add 1 millisecond, to ensure that caches written earlier in the same
Chris@0:     // millisecond are invalidated. It is possible that caches will be later in
Chris@0:     // the same millisecond and are then incorrectly invalidated, but that only
Chris@0:     // costs one additional roundtrip to the persistent cache.
Chris@0:     $now = round(microtime(TRUE) + .001, 3);
Chris@0:     if ($now > $this->getLastWriteTimestamp()) {
Chris@0:       $this->lastWriteTimestamp = $now;
Chris@0:       $this->consistentBackend->set(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin, $this->lastWriteTimestamp);
Chris@0:     }
Chris@0:   }
Chris@0: 
Chris@0: }