MongoDB replication Election on a three members replica set

manpreet Best Answer 2 years ago

I have deployed a MongoDB testing environment that consists of a primary, a secondary and an arbiter instances, for the sake of understanding the election procedure that occurs when the primary server instance is down. ServerA: Primary ServerB: Secondary ServerC: Arbiter

As per official documentation,replica sets can trigger an election in response to a variety of events, such as:

• Adding a new node to the replica set.
• initiating a replica set.
• performing replica set maintenance using methods such as rs.stepDown() or rs.reconfig().
• losing connectivity to the primary for more than the configured timeout

I have simulated the part where the connection is severed between ServerA and the replica set, and it seems that the ServerB is elected primary.

The part that I do not understand is that when ServerA is back online, it seems that it is automatically re-elected as "Primary", and I do not understand what is triggering the re-election. Please note that I am simulating the "Server Down" by shutting down the mongod instance on the server, and then running it ("Server Up").

Your help is appreciated.