Redis

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Wikimedia infrastructure

Data centres and PoPs

Networking

HTTP Caching

MediaWiki


Media

Logs

Redis is used in Wikimedia production for:

  • changeprop (role::redis::misc)
  • stashing user sessions ($wgSessionCacheType) and the data stored in the main stash ($wgMainStash).
  • Maps
  • As a cache and queue backend in ORES
  • Receiver of sampled profile data from PHP, as part as the sampling/profiling pipeline (Arc Lamp).
  • stores lists of articles for new editors to edit as part of the GettingStarted MediaWiki extension

MediaWiki-Vagrant and MediaWiki-Vagrant in Cloud VPS are configured by default to use redis for $wgMainCacheType, $wgSessionCacheType, etc.

Role redis::misc (Redis 3.x)

The role redis::misc is for our general purpose master-slave cluster in eqiad and codfw DCs. Each rdb* node has 5 instances (ports 6378, 6379, 6380, 6381, 6382) because redis is sigle threaded.

Servers

Each master has its respected slave. Masters use odd numbers (e.g. rdb1005) and slaves an even one (e.g. rdb1006). Master-slave instances use the same ports e.g. rdb1005:6379 is the master of rdb1006:6379

eqiad:

  • rdb1005 (m) - rdb1006 (s)
  • rdb1009 (m) - rdb1010 (s)

codfw:

  • rdb2003 (m) - rdb2004 (s)
  • rdb2005 (m) - rdb2006 (s)

Services

Change propagation (or changeprop) is a service running on scb* servers listening to topics on Kafka for events, and translates them into http requests to various systems. It is also responsible for cache evictions to happen on all services like RESTBase. Changeprop talks to redis via Nutcracker.

Related puppet code

  • hieradata/role/common/redis/misc/master.yaml
  • hieradata/role/common/redis/misc/slave.yaml
  • modules/role/manifests/redis/misc/master.pp
  • modules/role/manifests/redis/misc/slave.pp

Other Info

Using Redis

Connecting

redis-cli is installed on all servers where redis-server is installed. This will leave you at a redis prompt where you can enter commands interactively.

Some useful commands

  • AUTH <somepass> authenticate
  • INFO status information, including:
# Replication
role:slave
master_host:10.64.0.24
master_port:6379
master_link_status:up
master_last_io_seconds_ago:0
<snip>
# Keyspace
db0:keys=9351936,expires=9291239,avg_ttl=0
  • KEYS <pattern-here> list of all keys matching the given pattern. Use this sparingly! This query could take seconds to complete
  • QUIT closes the connection.

Using Redis from other Services

Some services may require or be able to use Redis, and this Redis cluster is appropriate for that.

As noted above, each pair of Redis servers in each data center have five separate instances on different ports, a majority of which are not in use; the first step to using the Redis server in production service is to choose an unused instance/port pair which can be located by examining Hiera data for what is currently in use: a relatively straight forward way to do this is to use git grep '\Wrdb[12]' within a Puppet tree, which shows every use of an rdb address. A similar procedure may be used to find a port that is unallocated.

Once a port/host combination for each datacenter is chosen, it is as simple as referring to those from the Puppet state which will use them.

Using Redis from a service requires a password; the password may be obtained from the Hiera key ::passwords::redis::main_password in hieradata/role/common/redis/misc/master.yaml in the private repository. It is currently the convention to introduce a new private Hiera key to store the password for your service's use, however this is obviously inefficient and subject to change.

Other references

Commands are easy, they all depend on the data type (hash, set, list, etc). Here's a quick reference.

Configuration is likewise pretty straightforward with perhaps the exception of the snapshotting, aof and memory settings; here's the sample config file.

See also

  • memcached
  • nutcracker (AKA twemproxy), the proxy used by all application servers to contact memcached (but not redis as of 2015, except it does again as of 2016)