Redis High Availability Options

This article outlines the various Redis HA options and the tradeoffs between them as well as touching how service discovery might be used. There are two main options (other than not doing Redis HA)

  • Redis with Sentinel - Requires quite a few components but is currently more production ready and does not require client modification.
  • Redis Cluster - Results in a simpler architecture (from an admin point of view) but, according to the Redis developers, not yet production ready and requires client (app library) support. So your application needs to be modified in order to understand Redis clustering (primarily the redirects)   I will explain it all below as I understand it - feedback welcome!

Redis With Sentinel and Twemproxy

Credit: The ideas for Redis + Sentinal come from this excellent blog post written by Karl Stoney.

Consistent Hashing

This set up uses consistent hashing to spread data across many master instances and HA is achieved by a) using this consistent hashing (via Twemproxy) to ensure data is placed and retrieved from the correct master and b) having slaves attached each master instance for eventual failover (via Sentinel).

Architecture

The most promising architecture requires quite a few components to achieve high availability but I can’t think of a better way to do it at the moment; small proof-of-concept tests show that it works. The stack could be something like this (picture here) for a given environment.

  1. Redis servers (obviously) - at least two instances to be master and slave
  2. Redis sentinel - Triggers master/slave changes in order to ensure availability (at least two instances)
  3. Twemproxy - Transparently sends requests to the current master (at least two instances)
  4. Twemproxy agent or smitty - Restarts twemproxy to send requests to the new master when a failover happens (runs alongside twemproxy querying Sentinel for failover events)
  5. Load balancer pair - Some kind of highly available (e.g. using VRRP or Pacemaker load balancer at the TCP level to protect against twemproxy failures. Typically haproxy or a commercial appliance can fulfil this role.

(Again, a picture says a thousand words)

Depending on your application and tolerance of technological footprint you could possibly get away with just one master and a set of slaves but if your application uses Redis in a serious way you’ll want to spread the data across multiple masters.

Service Discovery

The role of Consul/Service Discovery Registry in all this is that it informs the various components as follows

  • Tells Sentinel where to find the Redis masters for the environment it’s in
  • Tells Twemproxy Agent (or Smitty) where to find the Sentinel host for the environment it’s in
  • Tells the applications where to find either the LB or Twemproxy (if not using an LB)

Redis Cluster

Redis cluster does not use consistent hashing but rather shards the data across all nodes participating in the cluster; the concept of masters and slaves are still used with Redis cluster but in a different way. This is explained in detail on the Redis cluster tutorial but I’ll go over it here for pass on the info and also to cement the knowledge in my head.

Cluster Sharding

During bootstrap you set up some Redis servers in “cluster mode” and then kick them off all at once with a special “cluster create” command specifying the amount of replicas per master. A replica being the “slave” - so if you have 4 masters then you’ll get 4 slaves for a total of 8 instances. So you’d need to provision 8 instances ahead of time by starting them up in that “cluster mode”.

They will then divide themselves up with a fixed set of “slots” (16384). So if you have 4 master nodes they’ll each get 4096 slots as follows

  • Node A : 0 - 4095
  • Node B : 4096 - 8191
  • Node C : 8192 - 12288
  • Node D : 12289 - 16384

Handling Failures

If one of the master nodes fail then a slave is automatically promoted to master and the show goes on. Upon recovery the former master will rejoin the cluster and continue as a slave. The downside being, that if you add a new master, you have to manually re-shard the data by moving slots from existing instances to the new instance. Maybe they’ll make this transparent in the future.

Client Support

How does a client know who to connect to? The nice thing is that, because the client must be aware of the cluster, it can connect to any instance and it will receive redirects telling it where to write to for a given “set” command (or where to fetch from for a given “get”). From an architectural perspective we can treat the Redis cluster more like a “pool” of servers rather than a set of distinct components.

Service Discovery

As for service discovery - you could have a Consul agent on each cluster node advertising it as available for taking requests. As long as the client can understand the Redis clustering protocol additions then you can connect to a slave or master and send read or write requests.

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