Re: Independent AZs deployments vs. streched deployments
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I can’t speak for Consul nor Etcd, but cf-MySQL, in HA mode, can be deployed in a 2+1 mode where you’re still deploying an odd number of VMs, for consensus, but only deployed to two AZs.
This gives you some degree of protection to AZ failure, but yes, not the same resiliency as three.
Let’s say you have two AZs, the “major,” with two cluster nodes, and the “minor” with only one.
With two AZs, if there is a net split, the AZ with the majority of cluster nodes will continue to run.
If you suffer an AZ failure, however, you run a 50/50 chance of staying up. If the major AZ goes down, you’re down. In that respect you’re no better than running a single AZ. However you may have a faster path to recovery, in that you will have a seed to rebuild in the minor AZ possibly more quickly than rebuilding the major AZ.
This differs from traditional HA where there are two equal master nodes, one in each AZ. This does give the ability to fail back and forth between the two, but typically not automatically.
If you are interested in investing some effort, we could probably work together to establish the processes by which one could “recover” the minor AZ to run in non-HA mode. I think it should be possible to write yourself a “break glass in case of emergency” manifest which you could use to redeploy the minor AZ to restore availability.
On Fri, Jan 12, 2018 at 05:12 Juan Pablo Genovese <juanpgenovese@...> wrote: