Parser cache

Our implementation of the MediaWiki parser cache primarily uses MariaDB, with hot keys also residing in Memcached.

The configuration is in CommonSettings.php (operations/mediawiki-config ) as part of a MultiWrite wgObjectCaches key. Although the mapping between parser cache sections and DB hosts used to be configured in mediawiki-config as well, this is now managed with dbctl.

ParserCache in Memcached (tier 1 and tier 2).
ParserCache DB (tier 3). MediaWiki uses consistent hashing to distribute entries over the pc1, pc2, and pc3 sections.

Current configuration

Cache retention period is set to (as of 2021-06-07 - see T282761):
- 21 days for wiki pages
- 10 days for talk pages
Semi-random tasks from 2021 containing various current knowledge about parser cache: T282761.
Grafana: Parser Cache
Each data center has its own parser cache; since multi-DC MediaWiki, it is possible for eqiad and codfw to consistently respond with different parser cache contents (can be tested with WikimediaDebug). A purge appears to correctly purge the parser cache in both data centers. Lucas Werkmeister (WMDE) (talk) 14:11, 13 October 2023 (UTC)[reply]

History

Prior to 2011, the parser cache was solely memcached-based.

In 2011, we introduced the MySQL-based parser cache. See Parser cache expansion and Disk-backed object cache for details of the 2011 project.

In 2020, an additional local on-host Memcached tier was enabled for parser cache (T264604).