Switch Datacenter

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Introduction

A 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.

Schedule for 2017 switch

  • Deployment server:
  • Traffic:
  • MediaWiki 5-minute read-only test: (?)
  • ElasticSearch:
  • Media storage/Swift:
  • Services:
  • MediaWiki: (requires read-only mode)

Switching back

  • MediaWiki: (requires read-only mode)
  • Services, ElasticSearch, Traffic, Swift, Deployment server: (after the above is done)

Schedule for Q3 FY2015-2016 rollout

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

Switching back

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


Per-service switchover instructions

MediaWiki-related

Phase 1 - preparation

  1. (days in advance) Warm up databases; see MariaDB/buffer_pool_dump.
  2. Stop jobqueues in the active site
  3. Stop all jobs running on the maintenance host

Phase 2 - read-only mode

  1. Deploy mediawiki-config with all shards set to read-only (set ro on active site)

Phase 3 - lock down database masters, cache wipes

  1. Set active site's databases (masters) in read-only mode except parsercache ones (which are dual masters) standalone es1 servers (which are always read only) and misc servers (for now, as they are independent from mediawiki and do not have yet clients on codfw).
    • Check with: sudo salt -C 'G@mysql_role:master and G@site:eqiad and not G@mysql_group:misc and not G@mysql_group:parsercache' cmd.run 'mysql --defaults-file=.my.cnf --batch --skip-column-names -e "SELECT @@global.read_only"'
    • Change with: sudo salt -C 'G@mysql_role:master and G@site:eqiad and not G@mysql_group:misc and not G@mysql_group:parsercache' cmd.run 'mysql --defaults-file=.my.cnf --batch --skip-column-names -e "SET GLOBAL read_only=1"'
  2. Wipe new site's memcached to prevent stale values — only once the new site's read-only master/slaves are caught up. Run: salt -C 'G@cluster:memcached and G@site:codfw' cmd.run 'service memcached restart' salt -C 'G@cluster:memcached and G@site:eqiad' cmd.run 'service memcached restart'
  3. Warm up memcached and APC
    apache-fast-test wiki-urls-warmup1000.txt <new_active_dc>
    

Phase 4 - switch active datacenter configuration

  1. Switch the datacenter in puppet, by setting $app_routes['mediawiki']
  2. Switch the datacenter in mediawiki-config ($wmfMasterDatacenterMerge)

Phase 5 - apply configuration

  1. Redis replication
    • stop puppet on all redises salt 'mc*' cmd.run 'puppet agent --disable'; salt 'rdb*' cmd.run 'puppet agent --disable'
    • switch the Redis replication on all Redises from the old site(codfw) the new site (eqiad) at runtime with switch_redis_replication.py
    • Run switch_redis_replication.py memcached.yaml codfw eqiad
    • Run swicth_redis_replication.py jobqueue.yaml codfw eqiad
    • Verify those are now masters by running check_redis.py memcached.yaml,jobqueue.yaml
    • Alternatively via puppet:
    • run salt 'mc1*' cmd.run 'puppet agent --enable; puppet agent -t'; salt 'rdb1*' cmd.run 'puppet agent --enable; puppet agent -t'
    • verify eqiad Redises now think they're masters
    • run salt 'mc2*' cmd.run 'puppet agent --enable; puppet agent -t'; salt 'rdb2*' cmd.run 'puppet agent --enable; puppet agent -t'
    • verify codfw redises are replicating
  2. RESTBase (for the action API endpoint)
    • run salt -b10% -G 'cluster:restbase' cmd.run 'puppet agent -t; service restbase restart'
  3. Misc services cluster (for the action API endpoint)
    • run salt 'sc*' cmd.run 'puppet agent -t'
  4. Parsoid (for the action API endpoint)
  5. Switch Varnish backend to appserver.svc.$newsite.wmnet/api.svc.$newsite.wmnet
  6. Point Swift imagescalers to the active MediaWiki
    • Merge https://gerrit.wikimedia.org/r/284401
    • restart swift in eqiad: salt -b1 'ms-fe1*' cmd.run 'puppet agent -t; swift-init all restart'
    • restart swift in codfw: salt -b1 'ms-fe2*' cmd.run 'puppet agent -t; swift-init all restart'

Phase 6 - database master swap

  1. Database master swap for every core (s1-7), External Storage (es2-3, not es1) and extra (x1) database
    • Check with: sudo salt -C 'G@mysql_role:master and G@site:codfw and not G@mysql_group:misc and not G@mysql_group:parsercache' cmd.run 'mysql --defaults-file=.my.cnf --batch --skip-column-names -e "SELECT @@global.read_only"'
    • Change with: sudo salt -C 'G@mysql_role:master and G@site:eqiad and not G@mysql_group:misc and not G@mysql_group:parsercache' cmd.run 'mysql --defaults-file=.my.cnf --batch --skip-column-names -e "SET GLOBAL read_only=0"'

Phase 7 - Undo read-only

  1. Deploy mediawiki-config eqiad with all shards set to read-write

Phase 8 - post read-only

  1. Start the jobqueue in the new site
  2. Start the cron jobs on the maintenance host in codfw
  3. Re-enable puppet on all eqiad and codfw databases masters
    • sudo salt -C 'G@mysql_role:master or pc*' cmd.run "puppet agent --enable"
  4. Update DNS records for new database masters
  5. Update tendril for new database masters
  6. Run the script to fix broken wikidata entities on the maintenance host of the active datacenter: sudo -u www-data mwscript extensions/Wikidata/extensions/Wikibase/repo/maintenance/rebuildEntityPerPage.php --wiki=wikidatawiki --force

Phase 9 - verification and troubleshooting

  1. Make sure reading & editing works! :)
  2. Make sure recent changes are flowing (see Special:RecentChanges, EventStreams, RCStream and the IRC feeds)
  3. Make sure email works (exim4 -bp on mx1001/mx2001, test an email)

Media storage/Swift

Ahead of the switchover, originals and thumbs

  1. MediaWiki: Write synchronously to both sites with https://gerrit.wikimedia.org/r/#/c/282888/https://gerrit.wikimedia.org/r/284652
  2. Cache->app: Change varnish backends for swift and swift_thumbs to point to new site with https://gerrit.wikimedia.org/r/#/c/282890/https://gerrit.wikimedia.org/r/284651
    1. Force a puppet run on cache_upload in both sites: salt -v -t 10 -b 17 -C 'G@cluster:cache_upload and ( G@site:eqiad or G@site:codfw )' cmd.run 'puppet agent --test'
  3. Inter-Cache: Switch new site from active site to 'direct' in cache::route_table for upload https://gerrit.wikimedia.org/r/#/c/282891/https://gerrit.wikimedia.org/r/284650
    1. Force a puppet run on cache_upload in new site: salt -v -t 10 -b 17 -C 'G@cluster:cache_upload and G@site:codfw' cmd.run 'puppet agent --test'
  4. Users: De-pool active site in GeoDNS https://gerrit.wikimedia.org/r/#/c/283416/https://gerrit.wikimedia.org/r/#/c/284694/ + authdns-update
  5. Inter-Cache: Switch all caching sites currently pointing from active site to new site in cache::route_table for upload https://gerrit.wikimedia.org/r/#/c/283418/https://gerrit.wikimedia.org/r/284649
    1. Force a puppet run on cache_upload in caching sites: salt -v -t 10 -b 17 -C 'G@cluster:cache_upload and G@site:esams' cmd.run 'puppet agent --test'
  6. Inter-Cache: Switch active site from 'direct' to new site in cache::route_table for upload https://gerrit.wikimedia.org/r/#/c/282892/https://gerrit.wikimedia.org/r/284648
    1. Force a puppet run on cache_upload in active site: salt -v -t 10 -b 17 -C 'G@cluster:cache_upload and G@site:eqiad' cmd.run 'puppet agent --test'

Switching back

Repeat the steps above in reverse order, with suitable revert commits

ElasticSearch

Point CirrusSearch to codfw by editing wmgCirrusSearchDefaultCluster InitialiseSettings.php. The usual default value is "local", which means that if mediawiki switches DC, everything should be automatic. For this specific switch, the value has been set to "codfw" to switch Elasticsearch ahead of time.

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.

Traffic

GeoDNS user routing

Inter-Cache routing

Cache->App routing

Specifics for Switchover Test Week

After switching all applayer services we plan to switch successfully, we'll switch user and inter-cache traffic away from eqiad:

  • The Upload cluster will be following similar instructions on the 14th during the Swift switch.
  • Maps and Misc clusters are not participating (low traffic, special issues, validated by the other moves)
  • This leaves just the text cluster to operate on below:
  1. Inter-Cache: Switch codfw from 'eqiad' to 'direct' in cache::route_table for the text cluster.
  2. Users: De-pool eqiad in GeoDNS for the text cluster.
  3. Inter-Cache: Switch esams from 'eqiad' to 'codfw' in cache::route_table for the text cluster.
  4. Inter-Cache: Switch eqiad from 'direct' to 'codfw' in cache::route_table for the text cluster.

Before reversion of applayer services to eqiad, we'll revert the above steps in reverse order to undo them:

  1. Inter-Cache: Switch eqiad from 'codfw' to 'direct' in cache::route_table for all clusters.
  2. Inter-Cache: Switch esams from 'codfw' to 'eqiad' in cache::route_table for all clusters.
  3. Users: Re-pool eqiad in GeoDNS.
  4. Inter-Cache: Switch codfw from 'direct' to 'eqiad' in cache::route_table for all clusters.

Services

  • RESTBase and Parsoid already active in codfw, using eqiad MW API.
  • Shift traffic to codfw:
    • Public traffic: Update Varnish backend config.
    • Update RESTBase and Flow configs in mediawiki-config to use codfw.
  • During MW switch-over:

Other miscellaneous