Acquia’s Memcached solution is comprised of the following four major pieces: The Memcached daemon, the PHP Memcached library, the Memcache API and Integration module, and Drupal core.
The Memcached daemon is installed as a service with several components that work together to ensure stability and uptime for a website.
Memcached is a key-value store, which is a data storage paradigm designed for storing, retrieving, and managing associative arrays—commonly known as a hash. This contrasts with relational database object models that enable advanced concepts such as mapping and data types.
The Memcached service includes the following commands for handling data:
get - Retrieve an existing item from storage.
set - Save a new item to storage.
flush_all - Mark an item as
expired so that it cannot be
retrieved. This method pushes the current item to the front of the
line, to be replaced by the next cacheable item. It does not free the
memory used by that item.
delete - Remove an item from storage.
stats - Query the service for aggregated data about the objects
saved to storage.
Environments on Cloud Next technologies have access to alternatives to mcstat with these syntaxes:
When the Memcached service is started, it reserves memory for object storage (by default, 64 MB). The memory allocated is divided into 1MB pages, which by default provides 64 pages. Memcached initializes a memory organization concept called a slab class on startup. A slab defines the number of items (chunks) which can be inserted into a 1MB memory page. By dividing 1MB by that number of chunks, you can determine that slab’s chunk size.
By default, Memcached creates 42 slabs and assigns a single page to each. Each successive slab holds fewer, larger chunks based on the factor (by default, 1.25). In the following sample representation of slabs, notice that the item size (or chunk size) increases by a factor of 1.25 from the second to the third slab:
# Item_Size Max_age Pages Count Full? Evicted Evict_Time OOM 2 120B 1168824s 1 170 no 0 0 0 3 152B 1831103s 1 196 no 0 0 0 4 192B 1847211s 1 988 yes 30 9183 0 ... 36 246.8K 27236s 6 21 yes 7 2676 0 37 308.5K 1224s 2 3 yes 91 29174 0 ... 40 602.5K 1696s 11 4 yes 0 0 0 41 753.1K 64s 4 1 yes 6 35 0 42 1024.0K 44944s 5 1 yes 20 1 20
Memcached saves items by evaluating an item’s size, and then writing it to a slab. In the previous example, note that slab 36 contains items that range in size from 246.8KB to less than 308.5KB.
When a slab cannot fit a new item and there is additional memory available to Memcached, a new page will be assigned to the slab. In the previous example, slab 36 can contain at most four items, but, depending on the items’ size, may contain only three items:
If objects are 248KB, four items equals 992KB, which is less than 1MB.
If objects are 300KB, because four items equals 1.2MB, after three items a new page is created.
set() method is called, Memcached determines the size of
the object, and then locates the slab with the appropriate memory
Each Memcached node is configured to manage its memory with Least Recently Used (LRU) prioritization. The nodes oversee their local slab arrays, producing a list of candidates for removal. When a new item is assigned to a full slab and there are no free pages to assign to that slab, the full slab evicts the LRU item from the same slab. This is both healthy and accepted behavior—assuming that the objects being evicted are outdated and due to be safely removed.
Memcached does not internally provide mechanisms for adding capacity through additional processes. Instead, this functionality is handled entirely by an external client, such as the PECL Memcache extension for PHP.
An item cached by Memcached is stored in a specific location on a single infrastructure, even if your subscription includes multiple Memcached infrastructure.
If a Memcached infrastructure fails, its cached items will no longer be available, and requests for these items will be rerouted to the database layer of your application. Memcached cannot detect failed infrastructure, and will continue to route requests to all infrastructure, resulting in slow or failed requests until the failed infrastructure is repaired or removed from service. Although healthy Memcached infrastructure in your subscription will continue to respond normally, responses may slow or fail if the data source becomes overloaded.
If Memcached must be restarted on a single node, all Memcached
nodes for that cluster must be reset. If this doesn’t occur,
multi_item objects contain incomplete data.
Environments running on Cloud Next technologies have at least two Memcached nodes running at all times for improved resiliency. Acquia leverages mcrouter to handle this transparently without requiring changes to the Drupal application. Memcached nodes might be replaced during resource usage optimization processes, feature releases, or security updates. Feature releases or security updates clear the entire cache. All Memcached nodes operate with the Memcached memory limit configured on a given environment.
Drupal 9 or later provides additional caching flexibility not available in Drupal 7. Instead of caching the entire page, caches in Drupal 9 or later are saved in bins, each of which can be set to cache information in the database, or with Memcached.
Acquia recommends you use Memcached for most cache bins, as it is preferable to local caches, such as PHP’s APCu cache, since it makes cached items available to all layers of PHP.
You can configure Memcached by using Composer. For more information, see Enabling Memcached on Cloud Platform and memcache.settings.php. After you complete the steps documented on these pages, the system configures your website to use Memcached for the following bins:
// Override default fastchained backend for static bins. // @see https://www.drupal.org/node/2754947 $settings['cache']['bins']['bootstrap'] = 'cache.backend.memcache'; $settings['cache']['bins']['discovery'] = 'cache.backend.memcache'; $settings['cache']['bins']['config'] = 'cache.backend.memcache'; // Use Memcache as the default bin. $settings['cache']['default'] = 'cache.backend.memcache';
The bootstrap bin contains information for most Drupal application requests, and is rarely invalidated.
The discovery bin contains discovery data for plugins; Views data; or YAML-discovered data, such as library information.
The config bin contains configuration information provided by modules enabled on your website.
You can configure additional bins to use a Memcached back end, such as:
The data bin contains data that can vary both by path or by context.
The render bin contains cached HTML strings like cached pages and blocks.
For advanced information about cache bins, see the configuration section of the Drupal Cache API documentation on Drupal.org.
Content Hub 2.x requires the Depcalc module which needs to use the database backend.
$settings['cache']['bins']['depcalc'] = 'cache.backend.database';