A quorum is Number of Dedicated Master Nodes / 2 + 1 (rounded down to the nearest whole number), which Amazon ES sets to discovery.zen.minimum_master_nodes when you create your domain. In this case, 2 / 2 + 1 = 2. Because one dedicated master node has failed and only one backup exists, the cluster does not have a quorum and cannot elect a new master. nearest whole number), which Amazon ES sets to discovery.zen.minimum_master_nodes when you create your domain. https://qbox.io/blog/split-brain-problem-elasticsearch