[LewisVerstappen]

> Replication in Redis is cheap. If your data is small and latency is a concern (eg happened to me with an adserver), then you can locate read-only Redis replicas everywhere. The speed of querying off of your local machine is not to be underestimated.

Do you face any consistency issues with doing this?

[wokwokwok]

It’s not a daft question, because redis clusters do have consistently issues (https://redis.io/docs/manual/scaling/#redis-cluster-consiste...), but for simple replication it is not really an issue.

[btilly]

No. Replication time was measured in hundredths of a second, and Redis operations are atomic. So all queries got a consistent view of the data, and the lag to update was very reasonable.

[YetAnotherNick]

It depends on definition of consistency, but it is not strongly consistent in theoretical terms[0]. But the ordering of update is guaranteed to be same, so if master is guaranteed to be internally consistent, so is the replica. And that property is enough for almost all usecases, except for maybe transactions.

[0]: https://redis.io/docs/manual/scaling/#:~:text=Redis%20Cluste....

[adra]

During partions you may as well throw the play book away. You could have minority writes on both sides of the cluster and a big nadda to reconcile the two when they're mended. Redis is a great ssytem for what its built for and for the trade-offs that it makes to keep itself fast and lean. Redis is not CP and it will probably never care to support it. If data resiliency and correctness is important to you, Redis alone isn't sufficient. Several years ago, we tried sentinels mostly to avoid large costly rebuilds when an instance went down, and though it usually worked just fine, we certainly had single network disruptions large enough to throw off the cluster enough that required a manual rebuild.

[btilly]

I don't trust Redis clustering to actually work. I only trust the single threaded, single server. Potentially with lots of replicas.

See the following for why I don't trust their various attempts to scale to a truly distributed system.

https://aphyr.com/posts/283-jepsen-redis https://aphyr.com/posts/307-jepsen-redis-redux https://jepsen.io/analyses/redis-raft-1b3fbf6

[anonymousDan]

So in other words, potentially yes since there is some lag :)?

[btilly]

For that application, there really wasn't. The results of the read were not used for writes, and the latency from when information was published to available was on par with the time a request to the master would have taken. The time from data published to available was faster than the time to switch tabs in a browser and manually check.

But your requirements will depend on the application. Financial transactions need explicit locking logic and atomic operations. Such as is provided by SELECT ... FOR UPDATE in SQL. So another application could have more demanding requirements. Which is why, in addition to answering whether I encountered problems, I gave the actual performance characteristics. So that anyone planning their application can know whether this is a good enough solution for you.