In fact, that node that is now unavailable will have its vote removed from the
cluster by the remaining nodes.The dynamic quorum feature may seem to introduce a possible problem to clustering,
considering that the whole point of the votes and quorum is to protect the cluster
from becoming split-brain, with multiple partitions offering services at the same time.
With dynamic quorum in place and votes being removed from the cluster when nodes
go into maintenance or fail, you may think, “Couldn’t the cluster split and both parts
make quorum?” The answer is no. There are still rules for how dynamic quorum can
remove votes and keep quorum.
To be able to deterministically remove the vote of a cluster node, the remaining nodes
must have quorum majority. For example, if I had a three-node cluster and one of the
nodes fails, the remaining two nodes have quorum majority, two out of three votes,
and therefore are able to remove the vote of the failed node, which means that the
cluster now has two votes. Let’s go back to our five-node cluster, which experiences a
network failure. One partition has three nodes, and the other partition has two nodes.
The partition with three nodes has quorum majority, which means that it keeps
offering services and can therefore remove the votes of the other two nodes. The
partition with two nodes does not have quorum majority, so the cluster service will
shut down. The partition with three nodes now has a total vote count of three, which
means that partition can now survive one of the three nodes failing, whereas without
dynamic quorum, another node failure would have caused the cluster to shut down.
This is shown in Figure 7.4.