diff core/lib/Drupal/Core/Cache/ChainedFastBackend.php @ 0:4c8ae668cc8c

Initial import (non-working)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/core/lib/Drupal/Core/Cache/ChainedFastBackend.php	Wed Nov 29 16:09:58 2017 +0000
@@ -0,0 +1,310 @@
+<?php
+
+namespace Drupal\Core\Cache;
+
+/**
+ * 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.
+ *
+ * @see \Drupal\Core\Cache\BackendChain
+ *
+ * @ingroup 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.
+    $now = round(microtime(TRUE) + .001, 3);
+    if ($now > $this->getLastWriteTimestamp()) {
+      $this->lastWriteTimestamp = $now;
+      $this->consistentBackend->set(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin, $this->lastWriteTimestamp);
+    }
+  }
+
+}