A master node is a node that can become master, not an elected master node in a specific cluster. By default, a node can be elected to become master in a cluster unless node.master is set to false.

Add zen.discovery.minimum_master_nodes setting, controlling how many total master nodes (a node can be explicitly set to be, or not to be, a master node) should be in the cluster a specific node "sees". Below this number, the relevant node will get back into trying to form a cluster with enough master nodes. A cluster will not be formed if a node does not see minimum_master_node or above.

For example, in a 5 node cluster, one can set the minimum_master_nodes to 3, meaning that if a network disconnection happens of a node (or 2), then they will not form their own cluster, but instead, get back into trying to join the bigger cluster.

This can also help in a cluster where there are specific master nodes (lets say 3), and the rest are just data nodes (non master). Setting then the minimum_master_nodes to 2 can help with the master nodes not forming their own cluster, and the data nodes making sure they are connected to at least 2 master nodes.