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Memcached for MediaWiki

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This page is about Memcached for MediaWiki.

This page is not about other Memcached clusters in production, such as those for Thumbor, WMCS, Wikitech wiki, and Swift.


MediaWiki's Memcached cluster runs on mc10XX hosts (in Eqiad) and mc20XX hosts (in Codfw).


WANObjectCache is the abstraction layer in MediaWiki PHP that deals with multi-datacenter concerns and Mcrouter. It builds on top of BagOStuff which is the generic key-value class that abstracts the Memcached protocol itself.

See also:

High level

  • Like a replica. There is generally no proactive setting of values during HTTP write actions. Instead, values are computed based on information from replica DBs, and computed on-demand using the getWithSet(key, ttl, callable) idiom. This means the application generally only expects cache values to be as up to date as a replica DB would be. Historically, it was common for MediaWiki to populate its cache during HTTP write actions instead. This meant that in a single-DC setup it could loosely be expected that the cache was as up-to-date as the master DB. As part of the multi-dc effort, this was changed starting in 2015, and thus its expectations were loosened to that of a replica DB.
  • No synchronisation. MediaWiki's WANObjectCache layer does not require synchronisation of cached values across data centers. Instead, it considers each datacenter's Memcached cluster as independent. Each populating its own values as-needed on dc-local app servers from dc-local replica DBs.
  • Tombstones (broadcasted purge). During HTTP write actions, MediaWiki asks WANObjectCache to purge cache keys of which it has modified the source data. These purges take the form of short-lived Memcached keys known as "tombstones". We do not use the DELETE command because we want each data center to be able to populate its memcached independently, thus requiring no cross-dc master database connection, thus reading from a local replica, thus values ingested in the cache may be as stale as a replica can be. Implementing a Memcached purge as DELETE would mean both in the same DC and other DCs, the same key could be re-populated immediately with the same stale value we just deleted. Instead, WANObjectCache formulates its purge as a SET operation that stores a placeholder value known as a "tombstone" (lasts for approx. 10 seconds for local and remote replica DBs to catch up).
  • Interim values. Upon seeing such tombstone, WANObjectCache acts much like a cache miss, except that the newly computed value is not written back over the tombstone (as the computed value may be stale). Instead, to avoid a recompute stampede these maybe-stale values are stored as an "interim value" in a sister key which is only kept for a few seconds.

Memcached commands


  • Read traffic from the getWithSet idiom results in a GETS command (getMulti) that fetches the main key, plus any sister keys that might exist.
  • Write trafffic from the getWithSet idiom results in either ADD if the key was known to be absent, or Memcached->mergeViaCas if a value existed but either required (or was elected for) regeneration.


  • Purge traffic uses the /*/mw-wan/ prefix to tell Mcouter to broadcast this to other pools and clusters as well. The actual command is generally SET as it needs to induce a "hold-off" period using the tombstone (per the above). In rare cases where a hold-off is not needed (e.g. if the purge is not related to a DB write), then the broadcasted event will use DELETE

Getting revision/page from WANObjectCache key

If you're trying to track down the specific revision text given an SqlBlobStore key, the somewhat convoluted procedure is documented at mw:Manual:Caching#Revision_text.

Magic numbers

Last updated: May 2020.

  • Tombstone (aka "hold-off TTL"): 11 seconds.
  • Interim value: 1 second.


Main article: Memcached for MediaWiki/mcrouter

Each MediaWiki api/appserver sees memcached through a local proxy called Mcrouter[1]. This daemon is configured with all the mc[1,2]0XX hosts as shards, and applies consistent hashing on the key name to know where to store it. The following example should help clarifying how things flow at this layer.

The Memcached key WANCache:v:metawiki:translate-groups (belongs to the Translate extension) is formatted by the WANObjectCache library. When Translate wants to get the value of this key, WANObjectCache will send a GET command from MediaWiki to localhost:11213, where Mcrouter is listening. The command is then further routed to mc1022 (based on key hashing). MediaWiki it totally ignorant about the mc[1,2]0XX host, it only knows about sending commands to a localhost port. A Mcrouter admin command helps figure out where keys are hashed/routed to:

elukey@mw1345:~$ echo "get __mcrouter__.route(get,WANCache:v:metawiki:translate-groups)" | nc localhost 11213 -q 2
VALUE __mcrouter__.route(get,WANCache:v:metawiki:translate-groups) 0 16

elukey@mw1345:~$ dig -x +short

Some things to notice:

  • The special prefix __mcrouter__.route is intercepted by Mcrouter. These are admin commands, for which proxy returns directly without contacting the Memcached hosts. This function returns the target of the consistent hashing of the key name.
  • Mcrouter listens on port 11213 on all MediaWiki app servers, meanwhile on every mc10XX host memcached listens on port 11211.

To get a key and dump it to a file it is sufficient to:

elukey@mw1345:~$ echo "get WANCache:v:metawiki:translate-groups" | nc localhost 11213 -q 2 > dump.txt
elukey@mw1345:~$ du -hs dump.txt
380K	dump.txt

In this case the key's value is pretty big, and it needs PHP to be interpreted correctly (to unserialize it), but nonetheless we got some useful information (like the size of the key). This could be useful when it is necessary to quickly get how big a key is, rather than knowing its content.

See also