class ChainedFastBackend

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 CacheBackendInterface, CacheTagsInvalidatorInterface

See also

\Drupal\Core\Cache\BackendChain

Related topics

Cache API

Information about the Drupal Cache API

File

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

Namespace

Drupal\Core\Cache

Members

Name	Modifiers	Type	Description
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::$fastBackend	protected	property	The fast cache backend.
ChainedFastBackend::$lastWriteTimestamp	protected	property	The time at which the last write to this cache bin happened.
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::getLastWriteTimestamp	protected	function	Gets the last write timestamp.
ChainedFastBackend::getMultiple	public	function	Returns data from the persistent cache when given an array of cache IDs. Overrides CacheBackendInterface::getMultiple
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::invalidateTags	public	function	Marks cache items with any of the specified tags as invalid. Overrides CacheTagsInvalidatorInterface::invalidateTags
ChainedFastBackend::LAST_WRITE_TIMESTAMP_PREFIX		constant	Cache key prefix for the bin-specific entry to track the last write.
ChainedFastBackend::markAsOutdated	protected	function	Marks the fast cache bin as outdated because of a write.
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::setMultiple	public	function	Store multiple items in the persistent cache. Overrides CacheBackendInterface::setMultiple
ChainedFastBackend::__construct	public	function	Constructs a ChainedFastBackend object.