Hello,

The WMF’s technology department has for this quarter the goal of testing and temporarily switching the main operational data centre from Eqiad (located in Chicago) to Codfw (located in Dallas)~[1,2]. This includes both back-end-processing as well as serving live traffic from it.

As a part of this effort, we are scheduling a switch-over for RESTBase and its back-end services, including: Parsoid, the Mobile Content Service, CXServer, Mathoid, Citoid, Apertium and Zotero~[3]. Technically, it will not be a real switch-over per se, because we will keep all of those services active in both DCs. However, external traffic will be directed to the Dallas DC only.

=== When is it and what does it mean for me? === The switch-over test is planned for this Thursday, 2016-03-17. We have allotted a three-hour window for this~[4]. There is nothing users should do before or after the switch; it will be transparent for them. There are two things users should note, though:

1) At the time of the switch-over, users might receive error responses for a while (both 4xx and 5xx status codes). While we will test most of the things ahead of time, we cannot test the actual traffic shifting, so small bumps might be noticed. 2) After the switch to the Dallas DC, users will likely see their response latencies slightly elevated. During the test, some requests might experience a slightly larger latency. This will occur because all of the services that will be responding to live requests still need to contact the main MediaWiki cluster, which will remain in Eqiad (the other DC) until a complete switch-over of the infrastructure is performed. However, given the multiple levels of caching, the 40 ms of penalty to go cross-DC for an uncached API request does not seem too taxing.

=== Wait, what about my service X running in WMF production? === If you are a service owner of one the aforementioned services, there are no explicit actions you should take prior to, during or after the switch-over test. This test could, however, affect your service depending on whether it usually serves live traffic or is mostly operational during various internal updates. MediaWiki and JobQueue processing will still be performed in Eqiad, so in the latter case your service should not see a change in the usage pattern. If, however, your service is mostly in charge of responding to live requests coming through RESTBase, those will be handled by instances in Codfw. However, as these services are full replicas of their Eqiad counterparts and are stateless, no major breakage will happen.

Should you have any questions or concerns, don’t hesitate to contact us here or on IRC (#wikimedia-services @ freenode).

Best, Marko Obrovac, PhD Senior Services Engineer Wikimedia Foundation

[1] https://www.mediawiki.org/wiki/Wikimedia_Engineering/2015-16_Q3_Goals#Techno... [2] https://phabricator.wikimedia.org/project/profile/1723/ [3] https://phabricator.wikimedia.org/T127974 [4] https://wikitech.wikimedia.org/wiki/Deployments#Thursday.2C.C2.A0March.C2.A0...