Drupal and the memcache module

About Memcached

Drupal core provides a Cache API that provides a structure for cached data objects (both key and value). This API allows for access to multiple storage implementations through a consistent set of functions — one of which is the Memcache API & Integration Module.

Memcache API and Integration module

The Memcache API and Integration module acts as an interface between Drupal core and the PECL memcache PHP library. The following files play a large role:

  • settings.php - Provides the following storage implementation (in this case, Memcached) configuration and connection information:
    • cache_backends - use memcached
    • cache_class_ - select these particular cache bins
    • Acquia Hosting default memcached port - 11211
    • Memcache_servers
  • memcache.inc - Part of the Memcache API and Integration module that plugs in to Drupal, pulling all configuration and connection data into objects that can be used by the module. This is the heart of the storage implementation
  • dmemcache.inc - Plugs in to the PECL memcache extension, and performs the following tasks, in order:
    1. Creates the key (memcache key prefix, cache bin, and the CID​).
    2. Trims the key using the SHA1 hashing algorithm if it exceeds 250 characters.
    3. Dmemcache_get​ takes the full key name, and then generates a new dmemcache_object.
    4. Determines where to write the object on our hash ring.

Memory management over 1MB

On Acquia's hosting platform, Memcached is configured to save items of up to 1MB in size. Starting with Memcache API and Integration version 7.x-1.5, the module includes the ability to takes objects larger than 1MB and then break them into smaller, equal objects (called multi-part data). One method to determine that this has occurred is when you see the string _multi in your Memcached item's key. Here is a key from a random website's slab 42 (which contains items ranging in size between 754KB and 1MB):

ITEM randomcustomer_-cache_views_data-randomcustomer_-cache-_multi5-randomcustomer_user_c ontent%253Apanel_pane_1%253A7ece55917020be962b0b81f576f55022%253Aresults%253A924f8f375f1b9cd3431700faf3acd15d [776277 b; 1478647027 s]

The end result is an object (a panel pane) being saved to Memcached that is approximately 5MB.

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