Requests for comment/Multi datacenter strategy for MediaWiki

This proposal is in regards to Wikimedia configuration changes and MediaWiki support for having "reads" (GET/HEAD requests) be routed to both datacenters, with writes (POST requests) going to a single datacenter. Having writes be routable to multiple datacenters is not in the scope of this particular proposal.

Background
Historically, MediaWiki has designed for single-cluster master/slave database setups. Some modules (FileBackend, Search, BagOStuff, ExternalStore, ...) support distributed stores with no master/slave distinction. In any, case it was not designed for cross-DC sites serving the same MediaWiki instance. In order to avoid single DC disaster scenarios, the Wikimedia Foundation has opted to have multiple datacenters, with a full copy of the data. Caching proxy servers exist at these and at separate sites to handle latency.

Problem
The use of proxy servers only handles basic page, file, and asset (JS/CSS) requests by logged-out users. It does not cover various special pages or actions, just standard page views and asset delivery. Once users log in, page views always go to the active datacenter. The standby datacenter only makes use of proxy cache servers, the rest of them are just backups (Swift, Redis, MariaDB, ...). There is currently no way to (keep?) the caches there warm. Also, since no traffic goes to the standby datacenter, its ability to handle traffic and perform correctly is not well tested, leaving room for surprises on failover. Switch-over time is not terribly well optimized and involves a lot of puppet work. This should be optimized in any case, but it would be nice to have read requests "Just Work" if a DC has a power outage or other internal issues.

Proposal
Rather than active/standby, a master/slave setup could be established by sending actual traffic to all DCs. This could be extended to more than just two datacenters. Read requests could go to the DC closest to the cache proxy nodes handling them. Write requests would always be routed to the "master" DC, where the DB masters reside. The basic implementation is laid out in the next sections.

Request routing
Varnish will follow some rules for selecting a backend on cache miss:


 * GET/HEAD/OPTIONS goes to master DC
 * POST goes to the closest DC
 * Any request with a valid "datacenter_preferred" cookie is routed to the master DC
 * A response header should indicate the DC for debugging

HTTP GET/HEAD indempotence
Currently, some MediaWiki GET requests (like rollback) do DB writes. Sometimes opportunistic updates (e.g. cascading protection, protected page list purging) happen on read requests. These should be changed to use a local job queue to avoid slow cross-DC database master queries. Each DC would have it's own local jobqueue with runners for "enqueue jobs". These jobs simply enqueue the job to the master DC. All other job types would normally only run on the master DC.

Data store usage/replication

 * MariaDB (main wiki tables): master DBs are in the primary DC, slaves in the slave DC
 * MariaDB (externalstore): master DBs are in the primary DC, slaves in the slave DC
 * Swift: use a global swift cluster with read/write affinity set (http://docs.openstack.org/developer/swift/admin_guide.html)
 * CirrusSearch: clusters maintained in both DCs with a "MultiEnqueueJob" queue in the primary DC that forks jobs into both DCs (cirrus jobs could run in both DCs)
 * MediaWiki sessions: redis masters in the master DC and slaves in the slave DC

Job queuing
The would be two local job queues with runners:
 * a) The "EnqueueJob" type that pushes jobs to the master DC
 * b) The "MultiEnqueueJob" type that pushes jobs to both DCs

Some job types might also be run locally if needed (mostly to work around systems with poor geo-replication). Most jobs that do actual work will do so on the master DC.

Locking/PoolCounter

 * PoolCounter daemons would be local to each DC
 * LockManager would reside in the master DC, since the slave DCs shouldn't need it anyway
 * Random callers using $wgMemc->lock or $wgMemc->add can usually get DC local locks

Memory stashes
MediaWiki somtimes stashes values (like upload statuses, prepared edits in ApiEditStash, StatCounter deltas) in memcached for later use. The stash strategy will be:


 * memcached: stashed values could go exclusively to the primary DC memcached cluster

Live activity state
Some things need to convey real-time state between DCs. For example, FlaggedRevs shows the (last) user current viewing a pending changes diff at Special:PendingChanges (and the API exposes this). This essentially means there is state that needs to be mutated and replicated by users simply viewing content. As long as the state changes use POST requests (e.g. via AJAX), then a master/slave strategy could be used, with the data going in Redis. Using async AJAX also means that this doesn't slow anything down.

Caching and purges
MediaWiki makes fairly heavy use of caching. The cache strategy will be:


 * memcached: cached values would go in DC-specific memcached clusters with an interface for broadcasting deletes (WANObjectCache) via daemons
 * varnish: the HTCP purger could be subscribed to relay updates, but could possibly be consolidated into the memcached purger daemons
 * MariaDB parsercache: these caches would be cluster-local

Logging and metrics

 * Logging/metrics in Elastic: these could be DC-local?
 * graphite/gdash: these could be DC-local?

Consistency

 * Sessions using redis replication creates a slight opportunity for stale data. To avoid this, when sessions mutate (on POST), a "datacenter_preferred" cookie can be sent to the user (lasting only a few seconds). This would sticky them to the active DC for more than enough time for replication to complete. This also means that the DB positions stored in ChronologyProtecter will make it through, preserving the "session consistency" we try to maintain now.