What happens to the primary member in a MongoDB Server Cluster during the NDB patching...

According to the NDB documentation, the NDB patching process for MongoDB Server Cluster follows these steps1:

NDB identifies the primary member of the MongoDB Server Cluster and patches it first.

NDB triggers a failover to elect a new primary member from the remaining secondary members.

NDB patches the former primary member, which becomes a secondary member after the failover.

NDB patches the remaining secondary members one by one.

NDB verifies the patching status and the cluster health.

This process ensures that the MongoDB Server Cluster always has a primary member available to handle write operations, while minimizing the downtime and the impact on the cluster performance.

References: Nutanix Support & Insights, section “Patching a MongoDB Server Cluster”