Even when you configure one replica, we recommend three AZs. If an AZ disruption occurs in a three AZ domain, you only lose one-third of your capacity but if the disruption occurs in a two AZ domain, you lose half your capacity, which can be more disruptive. Also, in a three AZ domain, when an AZ is disrupted, Amazon OpenSearch Service can fall back to the two remaining AZs, and still support cross-AZ replication . In a two AZ domain, you lose cross-AZ replication if one AZ is disrupted, which can further reduce availability. For more details refer our documentation.