Server selection is not performed when replica has two members

Working with Data Drivers & ODMs
#1

i’m using mongoose

configured a replica set of 4 instances

i started inserting data into the replica set with interval 1 second

while the data is being inserted, i stepped down the primary server
after that a ‘disconnected’ event was fired and ‘reconnected’ is fired … after that the replica set has 3 servers it elects one of the 3 remaining to be the primary and the data continued to be inserted into the database
when the new primary is stepped down now i have 2 working secondaries servers in the replica set it doesn’t elect a new primary of the two secondaries to be the new primary instead of that it throws MongooseServerSelectionError

why not one of the two remaining servers is selected as primary?

#2

Hi Ahmed,

It’s because you have an even number of nodes in the replica set. This is not a recommended setup (see Replica Set Deployment Architectures). You should have an odd number of nodes in a replica set.

A replica set elects a new primary based on simple majority voting between the nodes. Offline nodes is considered an abstain. So in your scenario of 4 nodes:

  • 1 node offline: there are 3 out of 4 nodes that can vote, so a new primary is elected
  • 2 nodes offline: there are 2 out of 4 nodes that can vote, and they cannot form a simple majority anymore, so a new primary is not elected.

This scenario is outlined in Consider Fault Tolerance, where a 4 node replica set can tolerate only 1 node offline. More than this, then they cannot elect a new primary.

See Replica Set Elections for more details about election procedures in a replica set.

Best regards,
Kevin

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