You can optionally configure how a single-primary cluster elects a new primary, for example to prefer one instance as the new primary to fail over to. Use the memberWeight option and pass it to the dba.createCluster() and Cluster.addInstance() methods when creating your cluster. The memberWeight option accepts an integer value between 0 and 100, which is a percentage weight for automatic primary election on failover. When an instance has a higher percentage number set by memberWeight, it is more likely to be elected as primary in a single-primary cluster. When a primary election takes place, if multiple instances have the same memberWeight value, the instances are then prioritized based on their server UUID in lexicographical order (the lowest) and by picking the first one.

Setting the value of memberWeight configures the group_replication_member_weight system variable on the instance. Group Replication limits the value range from 0 to 100, automatically adjusting it if a higher or lower value is provided. Group Replication uses a default value of 50 if no value is provided. See Single-Primary Mode for more information.

For example to configure a cluster where ic-3 is the preferred instance to fail over to in the event that ic-1, the current primary, leaves the cluster unexpectedly use memberWeight as follows:

dba.createCluster('cluster1', {memberWeight:35})
var mycluster = dba.getCluster()
mycluster.addInstance('icadmin@ic2', {memberWeight:25})
mycluster.addInstance('icadmin@ic3', {memberWeight:50})