Saturday, December 29, 2018

Force a mongodb in a replica set to be a primary

There was one situation, where our production mongo server suddenly becomes secondary, causing any write and read to the server to fail. Searching in the mongo documentation, we found an easy solution. Below are the steps.

First step:
Check that your replica is running fine
mongo> rs.status()

Second step:
Freeze all mongo node in the replica that you do not want to be primary for lets say, 120 seconds. Access the mongo shell, and run below command, do this for all nodes that you do not want to be primary
mongo> rs.freeze(120) 

Third step:
Demote the current primary, so that other node that has not been frozen, will take over as primary. Run this in mongo shell, to demote the node from being a primary, for 120 seconds
mongo> rs.stepdown(120)

That's it, run rs.status() again to make sure that your desired server is now a primary.


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