regarding https://www.mediawiki.org/wiki/Requests_for_comment/Master_%26_slave_datacenter_strategy_for_MediaWiki, we need to help look into Wikibase use of memcached to see what needs broadcasted purges.
per aaron, "hopefully wancache will be enough, but if something is needed let me know" and we can ask him more about this.
@aaron do you think there is still anything for the Wikibase team to investigate here?
If not I might just close this.
I see EntityRevisionCache and CacheAwarePropertyInfoStore seems to use set() on invalidation. Also, EntityRevisionCache
and CachingEntityRevisionLookup, and PopulateInterwiki seems to call delete() on a non-WAN cache instance.
In general when WANCache/getWithSet and purge is used, it works correctly both for single-DC and multi-DC. Using raw sets or deletes to local memcached has been discouraged for a while now, and we've have many years of 2 switchovers a year without memcached being wiped as precaution.
Presumed solved as of T270223: FY2021-2022: Enable basic Multi-DC operations for read traffic (tracking). If bugs are found, new tasks can be filed for specific issues.