Upgrade Single Config Server to Replica Set Recently we wanted to upgrade our Config Server to Replica Set. Since it is a development environment, we only have one Config Server running. 

However, the official mongoDB documentation "Upgrade Config Servers to Replica Set" is written for three Config Servers. 

How can we upgrade a configuration with only one Config Server?
Add a the following line in config file 

replication:
   replSetName: csReplSet

restart your instance 
login and use admin 
initiate to replica 
add members
configure priority 
check your replica cluster.