It's used for keeping an auto-updated list of relevant articles in particular categories.
|mediawiki/extensions/GettingStarted : master||Add ability to connect to a master or slave Redis instance|
|Open||aaron||T88445 MediaWiki active/active datacenter investigation and work (tracking)|
|Resolved||aaron||T93144 Check multi-datacenter compatibility with GettingStarted's use of Redis|
Looks like RedisCategorySync::getRedis() can just take a master/slave flag. All the writes happen on POST anyway, and those would go to the master server in the main DC. The redis slave would be used for the srandommember call.
Looking at the current production config, I've found the list of session redis servers that GetingStarted picks the first from. That list doesn't indicate if any of these servers are slaves, though. Is there any way for me to know? Or is that information buried in ops' production puppet control panels?
The mc1001-1016 servers have no slaves in eqiad. The only slaves they ever have are in the other DC.
I'd assume for eqiad, the "slave" would just be the master. Not sure if this is in puppet yet.