Question

I never have my hands on coding. I got a doubt regarding mongodb replica sets

below is the situation

I have an alert monitoring application.
It is using mongodb with replica set with 3 nodes.
Applications Java code base keep connecting to the primary and doing some transactions.

Now my question is that,
if the primary server is down, how will it effect the application server.
I mean, would the app server writes error saying connection failed like errors.
OR
the replica set will pick one of the slaves automatically as master and provides the application server to do its activity. How will it happen…?

Thanks & Regards,
UDAY

Answer 1

The replica set will try to pick another server as the new primary. If you have three nodes, and one goes down, the other two will negotiate which one becomes the new master. If two go down, or somehow communication between the remaining breaks down, there will be no new master until the situation is recovered.

The official drivers support this automatic fail-over, as does the mongos routing server if you use it. So the application code does not need to do anything here.

I am not sure if there will be connection errors during the brief period of time this fail-over negotiation takes (you will probably get errors for a few seconds).