April 27th, 2015
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: firstname.lastname@example.org
Kyle Kingsbury (the same guy who does cool Clojure stuff like Reimann and teasers) has an epic writeup on Redis:
“In this post, we demonstrate Redis losing 56% of writes during a partition.”
I have trouble finding the pithy excerpt here, because it is all good. I guess I’ll just quote the opening where Kingsbury sets up the argument, and then you can read the rest for the way things fall apart:
Redis offers asynchronous primary->secondary replication. A single server is chosen as the primary, which can accept writes. It relays its state changes to secondary servers, which follow along. Asynchronous means that you don’t have to wait for a write to be replicated before the primary returns a response to the client. Writes will eventually arrive on the secondaries, if we wait long enough. In our application, all 5 clients will read from the primary on n1, and n2–n5 will be secondaries.
This is still a CP system, so long as we never read from the secondaries. If you do read from the secondaries, it’s possible to read stale data. That’s just fine for something like a cache! However, if you read data from a secondary, then write it to the primary, you could inadvertently destroy writes which completed but weren’t yet replicated to the secondaries.
What happens if the primary fails? We need to promote one of the secondary servers to a new primary. One option is to use Heartbeat or a STONITH system which keeps a link open between two servers, but if the network partitions we don’t have any way to tell whether the other side is alive or not. If we don’t promote the primary, there could be no active servers. If we do promote the primary, there could be two active servers. We need more nodes.
If one connected component of the network contains a majority (more than N/2) of nodes, we call it a quorum. We’re guaranteed that at most one quorum exists at any point in time–so if a majority of nodes can see each other, they know that they’re the only component in that state. That group of nodes (also termed a “component”) has the authority to promote a new primary.
Redis has a system called Sentinel, which, when configured correctly, will try to establish a quorum between Sentinel nodes, agree on which Redis servers are alive, and promote any which appear to have failed. If we colocate the Sentinel nodes with the Redis nodes, this should allow us to promote a new primary in the majority component (should one exist).
What are the consistency and availability properties of Sentinel? Antirez, the author of Redis, says:
Redis Cluster for instance is a system biased towards consistency rather than availability. Redis Sentinel itself is an HA solution with the dogma of consistency and master slave setups.“
So we expect this system to be CP. Nodes in the minority component will become unavailable during the partition, and the majority component will elect a new primary. The Sentinels will then order clients to abandon the old primary and reconnect to the new one.