A few weeks ago I criticized a proposal by Antirez for a hypothetical linearizable system built on top of Redis WAIT and a strong coordinator. I showed that the coordinator he suggested was physically impossible to build, and that anybody who tried to actually implement that design would run into serious problems. I demonstrated those problems (and additional implementation-specific issues) in an experiment on Redis’ unstable branch.

Antirez’ principal objections, as I understand them, are:

1. Some readers mistakenly assumed that the system I discussed was a proposal for Redis Cluster.
2. I showed that the proposal was physically impossible, but didn’t address its safety if it were possible.
3. The impossible parts of the proposed system could be implemented in a real asynchronous network by layering in additional constraints on the leader election process.

In a recent blog post, antirez detailed a new operation in Redis: WAIT. WAIT is proposed as an enhancement to Redis’ replication protocol to reduce the window of data loss in replicated Redis systems; clients can block awaiting acknowledgement of a write to a given number of nodes (or time out if the given threshold is not met). The theory here is that positive acknowledgement of a write to a majority of nodes guarantees that write will be visible in all future states of the system.

As I explained earlier, any asynchronously replicated system with primary-secondary failover allows data loss. Optional synchronous replication, antirez proposes, should make it possible for Redis to provide strong consistency for those operations.

WAIT means that if you run three nodes A, B, C where every node contains a Sentinel instance and a Redis instance, and you “WAIT 1” after every operation to reach the majority of slaves, you get a consistent system.

WAIT can be also used, by improving the failover procedure, in order to have a strong consistent system (no writes to the older master from the point the failure detection is positive, to the end of the failover when the configuration is updated, or alternative, disconnect the majority of slaves you can reach during the failure detection so that every write will fail during this time).

In response to my earlier post on Redis inconsistency, Antirez was kind enough to help clarify some points about Redis Sentinel’s design.

Previously on Jepsen, we explored two-phase commit in Postgres. In this post, we demonstrate Redis losing 56% of writes during a partition.

Redis is a fantastic data structure server, typically deployed as a shared heap. It provides fast access to strings, lists, sets, maps, and other structures with a simple text protocol. Since it runs on a single server, and that server is single-threaded, it offers linearizable consistency by default: all operations happen in a single, well-defined order. There’s also support for basic transactions, which are atomic and isolated from one another.

Because of this easy-to-understand consistency model, many users treat Redis as a message queue, lock service, session store, or even their primary database. Redis running on a single server is a CP system, so it is consistent for these purposes.