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Comment by zytek | Hacker News Reader

Agree. From 6+ years of experience it seems that we got fouled by the multi-az promise of being able to survive datacenter outage.

You can survive datacenter (AZ) outage IF you have separate stacks per AZ and don't mix traffic. If you have Kafka cluster spread out in 3 AZ don't get surprised if you just LOWERED your availability because any issue in one AZ makes your stack unstable. And issues in single AZ are quite common.

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A properly configured kafka cluster across 3 AZs _should_ be able to survive the loss of a single AZ. Obviously you should do testing and DR exercises to make sure _your_ cluster and application work in that scenario.

That's a really interesting point. The startup I currently work for only uses a single AZ due to financial concerns (and some performance as well), but I assume we'll have to move to more AZs for reliability. Would you advise the same for clusters of RDS and Elasticache? I'm wondering how you would even go about having two separate data sources, how would this be manageable?

Active-passive with database replication and manual failover is the usual solution.

I've found that for RDS, a writer instance and a hot standby reader instance with automatic failover work pretty well. When a failover happens, you're usually looking at about 30 seconds of downtime, which is "good enough" for most purposes.

30 seconds is pretty good. I worked on an "enterprise" system running AIX and HACMP (IBM's HA software.) A failover event would take minutes... and this was on the same local network.

Before assuming that your reliability would be increased by adding more AZs, verify where the problems of reliability comes from in the first place. I find more times than not, the down times comes from people applying changes, not when you just leave things running like they are. It's only if the AZ or underlying machines has troubles, that you should start thinking of expanding to other AZs

> From 6+ years of experience it seems that we got fouled by the multi-az promise of being able to survive datacenter outage.

You have quite a misunderstanding ...

AWS' "multi-az promise" has always been that they will try to take only one AZ down at a time within a region.

It was never "blend your AZ usage so we can't take one down."

If you don't have a wiki page with some HA architecture diagrams for each of your systems, then you probably don't have HA. Hint: at every company that I've worked at, I drew the first diagrams. Something to think about.

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