Consul.IO - Why does Consul Cluster need at least a quorum of server nodes to be active

Question

I spent a little bit time to study about Consul. I had read about Consensus Protocol. I saw that if Consul cluster want to be available, it needs a quorum of servers node to elect leader. I wonder that when consul cluster has less than a quorum of nodes running, why does it not use the remain running nodes instead? Can anyone answer for me? Thank you so much

Answer 1

A quorum is required to ensure that you'll never have inconsistent results. Otherwise a cluster of eight nodes (A B C D E F G H) might have a networking fail and become partitioned into two clusters (A B C D) (E F G H). Those two clusters then make two separate decisions that are incompatible (say update the same property of some configuration). When the partition is healed there's no way to merge the two changes.

If a quorum is required then you know that any quorum has at least one copy of every decision made; thus any conflicts will be spotted by at least one node and disallowed.