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Switch Datacenter

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Datacenter switchovers are a standard response to certain types of disasters (web search). Technology companies regularly practice them to make sure that everything will work properly when the process is needed. Switching between datacenters also make it easier to do some maintenance work on non-active servers (e.g. database upgrades/changes), so while we're serving traffic from datacenter A, we can do all that work at datacenter B.

A Wikimedia datacenter switchover (from eqiad to codfw, or vice-versa) comprises switching over multiple different components, some of which can happen independently and many of which need to happen in lockstep. This page documents all the steps needed to switch over from a master datacenter to another one, broken up by component. SRE Service Operations maintains the process and software necessary to run the switchover.

Weeks in advance preparation

  • 10 weeks before: Coordinate dates and communication plan with involved groups: Switch_Datacenter/Coordination
  • 3 weeks before: Run a "live test" of the cookbooks by "switching" from the passive DC to the active DC. The --live-test flag will skip actions that could harm the active DC or do them on the passive DC. This will instrument most of the code paths used in the switchover and help identify issues. This process will !log to SAL so you should coordinate with others, but otherwise should be non-disruptive. Due to changes since the last switchover you can expect code changes to become necessary, so take the time and assistance needed into account.

Live-test limitations


This step of the cookbook will fail if circular replication is not already enabled everywhere. It can be skipped if the live-test is run before circular replication is enabled, but it must be retested during Switch_Datacenter#Days_in_advance_preparation to ensure it is correctly set up.

Overall switchover flow

In a controlled switchover we first deactivate services in the primary datacenter and second deactivate caching in the datacenter. The next step is to switch Mediawiki itself. About a week later we activate caching in the datacenter again, as we believe that testing the situation without caching in the datacenter is sufficient.

Typical scheduling looks like:

  • Monday 14:00 UTC Services
  • Monday 15:00 UTC Caching (traffic)
  • Tuesday 14:00 UTC Mediawiki

The following week reactivate caching. 6+ Weeks later switchback Mediawiki

Per-service switchover instructions


We divide the process in logical phases that should be executed sequentially. Within any phase, top-level tasks can be executed in parallel to each other, while subtasks are to be executed sequentially to each other. The phase number is referred to in the names of the tasks in the operations/cookbooks repository, in the cookbooks/sre/switchdc/mediawiki/ path.

Days in advance preparation

  1. OPTIONAL: SKIP IN AN EMERGENCY: Make sure databases are in a good state. Normally this requires no operation, as the passive datacenter databases are always prepared to receive traffic, so there are no actionables. Some things that #DBAs normally should make sure for the most optimal state possible (sanity checks):
    • There is no ongoing long-running maintenance that affects database availability or lag (schema changes, upgrades, hardware issues, etc.). Depool those servers that are not ready.
    • Replication is flowing from eqiad -> codfw and from codfw -> eqiad (replication is usually stopped in the passive -> active direction to facilitate maintenance)
    • All database servers have its buffer pool filled up. This is taken care automatically with the automatic buffer pool warmup functionality. For sanity checks, some sample load could be sent to the MediaWiki application server to check requests happen as quickly as in the active datacenter.
    • These were the things we prepared/checked for the 2018 switch
  2. Make absolutely sure that parsercache replication is working from the active to the passive DC. Verify that the parsercache servers are set to read-write in the passive DC. This is important
  3. Check appserver weights on servers in the passive DC, make sure that newer hardware is weighted higher (usually 30) and older hardware is less (usually 25)
  4. Run sre.switchdc.mediawiki.03-set-db-readonly and sre.switchdc.mediawiki.06-set-db-readwrite in live-test mode back to back once circular replication is enabled. This is important
Start the following steps about half an hour to an hour before the scheduled switchover time

Phase 0 - preparation

  1. Add a scheduled maintenance on StatusPage (Maintenances -> Schedule Maintenance) This is not covered by the switchdc script.
  2. Add a scap lock on the deployment server scap lock --all --time 1H "Datacenter Switchover - T12345" This is not covered by the switchdc script.
  3. Disable puppet on maintenance hosts in both eqiad and codfw:
  4. Reduce the TTL on appservers-rw, api-rw, jobrunner, videoscaler, parsoid-php to 10 seconds: Make sure that at least 5 minutes (the old TTL) have passed before moving to Phase 1, the cookbook should force you to wait.
  5. Optional Warm up APC running the mediawiki-cache-warmup on the new site clusters. The warmup queries will repeat automatically until the response times stabilize:
    • The global "urls-cluster" warmup against the appservers cluster
    • The "urls-server" warmup against all hosts in the appservers cluster.
    • The "urls-server" warmup against all hosts in the api-appservers cluster.
  6. Set downtime for Read only checks on mariadb masters changed on Phase 3 so they don't page.
Stop for GO/NOGO

Phase 1 - stop maintenance

  1. Stop maintenance jobs in both datacenters and kill all the periodic jobs (systemd timers) on maintenance hosts in both datacenters:
Stop for final GO/NOGO before read-only.
The following steps until Phase 7 need to be executed in quick succession to minimize read-only time

Phase 2 - read-only mode

  1. Go to read-only mode by changing the ReadOnly conftool value:

Phase 3 - lock down database masters

  1. Put old-site core DB masters (shards: s1-s8, x1, es4-es5) in read-only mode and wait for the new site's databases to catch up replication:

Phase 4 - switch active datacenter configuration

  1. Switch the discovery records and MediaWiki active datacenter:
    • Flip appservers-rw, api-rw, jobrunner, videoscaler, parsoid-php to pooled=true in the new site. Since both sites are now pooled in etcd, this will not actually change the DNS records for the active datacenter.
    • Flip WMFMasterDatacenter from the old site to the new.
    • Flip appservers-rw, api-rw, jobrunner, videoscaler, parsoid-php to pooled=false in the old site. After this, DNS will be changed for the old DC and internal applications (except mediawiki) will start hitting the new DC

Phase 5 - DEPRECATED - Invert Redis replication for MediaWiki sessions

Phase 6 - Set new site's databases to read-write

  1. Set new-site's core DB masters (shards: s1-s8, x1, es4-es5) in read-write mode:

Phase 7 - Set MediaWiki to read-write

  1. Go to read-write mode by changing the ReadOnly conftool value:
You are now out of read-only mode

Phase 8 - Restore rest of MediaWiki

  1. Restart Envoy on the jobrunners that are now inactive, to trigger changeprop to re-resolve the DNS name and connect to the new DC:
    • A steady rate of 500s is expected until this step is completed, because changeprop will still be sending edits to jobrunners in the old DC, where the database master will reject them.
  2. Start maintenance in the new DC:
    • Run puppet on the maintenance hosts, which will reactivate systemd timers in both datacenters in the primary DC
    • Most Wikidata-editing bots will restart once this is done and the "dispatch lag" has recovered. This should bring us back to 100% of editing traffic.
  3. End the planned maintenance in StatusPage

Phase 9 - Post read-only

  1. Set the TTL for the DNS records to 300 seconds again:
  2. Update DNS records for new database masters deploying eqiad->codfw; codfw->eqiad This is not covered by the switchdc script. Please use the following to SAL log !log Phase 9.5 Update DNS records for new database masters
  3. Run Puppet on the database masters in both DCs, to update expected read-only state: This will remove the downtimes set in Phase 0.
  4. Make sure the CentralNotice banner informing users of readonly is removed. Keep in mind, there is some minor HTTP caching involved (~5mins)
  5. Wait for scap lock to expire This is not covered by the switchdc script.
  6. Re-order noc.wm.o's debug.json to have primary servers listed first, see T289745 and backport it using scap. This will test scap2 deployment This is not covered by the switchdc script.
  7. Update maintenance server DNS records eqiad -> codfw This is not covered by the switchdc script.

Phase 10 - verification and troubleshooting

This is not covered by the switchdc script

  1. Make sure reading & editing works! :)
  2. Make sure recent changes are flowing (see Special:RecentChanges, EventStreams, and the IRC feeds) curl -s -H 'Accept: application/json' | jq .
  3. Make sure email works (sudo -i; sudo exim4 -bp | exiqsumm | tail -n 5 on mx1001/mx2001 it should fluctuate between 0m and a few minutes, test an email)

Audible indicator

Put Listen to wikipedia in the background during the switchover. Silence indicates read-only, when it starts to make sounds again, edits are back up.



General context on how to switchover

CirrusSearch talks by default to the local datacenter ($wmgDatacenter). No special actions are required when disabling a datacenter.

Manually switching CirrusSearch to a specific datacenter can always be done. Point CirrusSearch to codfw by editing wgCirrusSearchDefaultCluster ext-CirrusSearch.php.

To ensure coherence in case of lost updates, a reindex of the pages modified during the switch can be done by following Recovering from an Elasticsearch outage / interruption in updates.



It is relatively straightforward for us to depool an entire datacenter at the traffic level, and is regularly done during maintenance or outages. For that reason, we tend to only keep the datacenter depooled for about a week, which allows us to test for a full traffic cycle (in theory).

General information on generic procedures

See Global traffic routing.


GeoDNS (User-facing) Routing:

  1. gerrit: C+2 and Submit commit
  2. <any authdns node>: authdns-update
  3. SAL Log using the following !log Traffic: depool eqiad from user traffic

(Running authdns-update from any authdns node will update all nameservers.)


Same procedure as above, with reversion of the commit specified: GeoDNS.



General procedure

All services, are active-active in DNS discovery, apart from restbase, that needs special treatment. The procedure to fail over to one site only is the same for every one of them:

  1. reduce the TTL of the DNS discovery records to 10 seconds
  2. depool the datacenter we're moving away from in confctl / discovery
  3. restore the original TTL

All of the above is done using the sre.discovery.datacenter cookbook in the case of a global switchover


# Switch all services to codfw
$ sudo cookbook sre.discovery.datacenter depool eqiad --all --reason "Datacenter Switchover" --task-id T12345

This will depool all active/active services, and prompt you to move or skip active/passive services.


# Repool eqiad
$ sudo cookbook sre.discovery.datacenter pool eqiad --all --reason "Datacenter Switchback" --task-id T12345

This will repool all active/active services, and prompt you to move or skip active/passive services.

Special cases


If it is needed to exclude services, using the old is still necessary until exclusion is implemented.

# Switch all services to codfw, excluding parsoid and cxserver
$ sudo cookbook --exclude parsoid cxserver -- eqiad codfw

Single service

If you are switching only one service, using the old is still necessary

# Switch the service "parsoid" to codfw-only
$ sudo cookbook --services parsoid -- eqiad codfw

apt needs a puppet change


Restbase is a bit of a special case, and needs an additional step, if we're just switching active traffic over and not simulating a complete failover:

  1. pool restbase-async everywhere
    sudo cookbook sre.discovery.service-route --reason T123456 pool --wipe-cache $dc_from restbase-async
    sudo cookbook sre.discovery.service-route --reason T123456 pool --wipe-cache $dc_to restbase-async
  2. depool restbase-async in the newly active dc, so that async traffic is separated from real-users traffic as much as possible.
    sudo cookbook sre.discovery.service-route --reason T123456 depool --wipe-cache $dc_to restbase-async

When simulating a complete failover, keep restbase pooled in $dc_to for as long as possible to test capacity, then switch it to $dc_from by using the above procedure.

Manual switch

These services require manual changes to be switched over and have not yet been included in service::catalog

    • The DNS discovery name planet.discovery.wmnet needs to be switched from one backend to another as in example change gerrit:891369. No other change is needed.
    • In puppet hieradata the rsync_src and rsync_dst hosts need to be flipped as in example change gerrit:891382.
    • FIXME: manual rsync command has to be run
    • The DNS discovery name peopleweb.discovery.wmnet needs to be switched from one backend to another as in example change gerrit::891381.



Main document: MariaDB/Switch Datacenter

Other miscellaneous

Upcoming and past switches

2023 switches



  • Recap
  • Read only: 1 minute 59 seconds

Switching back:




2021 switches


Switching back:


2020 switches

  • Services: Monday, August 31st, 2020 14:00 UTC
  • Traffic: Monday, August 31st, 2020 15:00 UTC
  • MediaWiki: Tuesday, September 1st, 2020 14:00 UTC

Switching back:

  • Traffic: Thursday, September 17th, 2020 17:00 UTC
  • MediaWiki: Tuesday, October 27th, 2020 14:00 UTC
  • Services: Wednesday, October 28th, 2020 14:00 UTC

2018 switches

  • Services: Tuesday, September 11th 2018 14:30 UTC
  • Media storage/Swift: Tuesday, September 11th 2018 15:00 UTC
  • Traffic: Tuesday, September 11th 2018 19:00 UTC
  • MediaWiki: Wednesday, September 12th 2018: 14:00 UTC

Switching back:

  • Traffic: Wednesday, October 10th 2018 09:00 UTC
  • MediaWiki: Wednesday, October 10th 2018: 14:00 UTC
  • Services: Thursday, October 11th 2018 14:30 UTC
  • Media storage/Swift: Thursday, October 11th 2018 15:00 UTC

2017 switches

  • Elasticsearch: elasticsearch is automatically following mediawiki switch
  • Services: Tuesday, April 18th 2017 14:30 UTC
  • Media storage/Swift: Tuesday, April 18th 2017 15:00 UTC
  • Traffic: Tuesday, April 18th 2017 19:00 UTC
  • MediaWiki: Wednesday, April 19th 2017 14:00 UTC (user visible, requires read-only mode)
  • Deployment server: Wednesday, April 19th 2017 16:00 UTC

Switching back:

  • Traffic: Pre-switchback in two phases: Mon May 1 and Tue May 2 (to avoid cold-cache issues Weds)
  • MediaWiki: Wednesday, May 3rd 2017 14:00 UTC (user visible, requires read-only mode)
  • Elasticsearch: elasticsearch is automatically following mediawiki switch
  • Services: Thursday, May 4th 2017 14:30 UTC
  • Swift: Thursday, May 4th 2017 15:30 UTC
  • Deployment server: Thursday, May 4th 2017 16:00 UTC

2016 switches

  • Deployment server: Wednesday, January 20th 2016
  • Traffic: Thursday, March 10th 2016
  • MediaWiki 5-minute read-only test: Tuesday, March 15th 2016, 07:00 UTC
  • Elasticsearch: Thursday, April 7th 2016, 12:00 UTC
  • Media storage/Swift: Thursday, April 14th 2016, 17:00 UTC
  • Services: Monday, April 18th 2016, 10:00 UTC
  • MediaWiki: Tuesday, April 19th 2016, 14:00 UTC / 07:00 PDT / 16:00 CEST (requires read-only mode)

Switching back:

  • MediaWiki: Thursday, April 21st 2016, 14:00 UTC / 07:00 PDT / 16:00 CEST (requires read-only mode)
  • Services, Elasticsearch, Traffic, Swift, Deployment server: Thursday, April 21st 2016, after the above is done

Monitoring Dashboards

Aggregated list of interesting dashboards