Redis primary/secondary without replication

Question

I am new to Redis. I read their documentation on Sentinel and Replication in which they talk about how the replicas try to remain in sync with the master as much as possible, but it is still possible that if the master fails after a successful write, the replica might not receive that write. If Sentinel then marks this replica as the new master, it is possible that the replica serves stale data.

If I cannot afford to lose consistency and prefer it over availability, how can I turn off replication so that when Sentinel marks a new replica as master, all the first requests would be cache misses and my cache can slowly warm up instead of returning potentially stale data?

Also, is that a good idea? Are there other good alternatives?

Answer 1

I cannot afford to lose consistency and prefer it over availability

It's not clear that redis automated failover is appropriate for your application. It looks like each client would need to carefully keep track of server availability.

Suppose you have a few clients, a master, M1, and three replicas, R2, R3, R4. Client C5 writes a new bank account balance to M1, which immediately permanently fails, and R2 is promoted to become master M2. Master did not obtain an acknowledge from a replica before replying to client. No paxos-like consensus protocol happens between servers prior to the reply being sent to C5.

C5 could remember counters / timestamps embedded in each write request, forget the write payload, and detect stale reads. But client C6 could not, unless you supply such data quickly and reliably outside the protocol. Nathan Fritz observes that your app could issue a write and then a PUBLISH event, and monitor multiple replicas with a LISTEN for that event, delaying its report of success to end user. Consider incorporating derecho into your app if the solid promises of virtual synchrony are necessary. Production releases of redis are targeted at a different part of the problem space than your primary interest.