ChainedFastBackend.php

<?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.
        $now = round(microtime(TRUE) + 0.001, 3);
        if ($now > $this->getLastWriteTimestamp()) {
            $this->lastWriteTimestamp = $now;
            $this->consistentBackend
                ->set(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin, $this->lastWriteTimestamp);
        }
    }

}