Will an even number of master nodes cause split brain situation with 7.x version of ES?

[ted.fed (ted)]

Due to some limitation with our provisioning system, it is difficult to provision an odd number of nodes.
For some other technologies, we have provisioned zookeepers with an even number of nodes for 2-3 years now and they never ran into split-brain situations.
So I wanted to check if the same can be expected from ES master node clusters too.

Example:
If we have a 6 node master cluster, then does the quorum occur at 3 nodes or 4 nodes ?

I would expect the quorum formula to be: ceil(N/2) as there in zookeeper too. But a confirmation would be much appreciated.

[Christian_Dahlqvist]

@ted.fed

So I wanted to check if the same can be expected from ES master node clusters too.

Even number of nodes is fine, as long as it is greater than 2.

@ted.fed

If we have a 6 node master cluster, then does the quorum occur at 3 nodes or 4 nodes ?

4 nodes would be required for quorum as a strict majority is required.

[ted.fed (ted)]

Thanks Christian.
Can you pls also point to some documentation link for the same?
It would be good to see some official recommendation too.

[Christian_Dahlqvist]

The docs describe the need for a majority of master eligible nodes to be present. Having an even number of nodes in a cluster is not uncommon, although they are typically distributed across 3 availability zones.

[ted.fed (ted)]

Thank you Christian !

[DavidTurner]

@Christian_Dahlqvist

Even number of nodes is fine, as long as it is greater than 2.

To be clear, even with 2 master-eligible nodes there is no risk of split-brain. The worst thing with a cluster with just 2 master-eligible nodes is you lose half-or-more of them (i.e. one) and the cluster stops working until they come back.