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Re: MySQL Cluster 8 - 4 nodes
Posted by: Mauritz Sundell
Date: January 04, 2023 07:32AM

Hi Chris

If two of four data nodes from different node groups fail at once, the surviving nodes can not by themself decide whether they lost connection with the other still alive nodes (split-brain) or if they are dead.

In that case the data nodes use the arbitrator, by default one of the management nodes. If they get ok from the arbitrator they will stay up, else shutdown themself. If they have no contact with the arbitrator they will shutdown.

The arbitrator node should run on a separate machine than data nodes, and if different networks are used for different data nodes the arbitrator should also be on a separate network than the data nodes.

Note that arbitrator is only needed in the special cases there the data nodes can not rule out split-brain by themself.

For example in your case, if first one node die and then later another, the arbitrator would not be needed since the surviving nodes can at each failure decide that the disappeared node can not form a cluster itself.

Or if three replicas is used, and one replica set disappear the arbitrator is not needed either, the surviving nodes can deduce it can form a bigger cluster than the disappeared nodes can.

Mauritz

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