I am upgrading Mongo replicaset from 3.4 → 4.2. Last week I did the upgrade fro 3.6 → 4.0 and the next step is to upgrade from 4.0 → 4.2.
During the upgrade fro 3.6 to 4.0, we faced some issues due to read concern majority.
After we stepped down the primary, there was some issue in the old primary and took a lot of time for this instance to be back up and running. now since we have read concern majority and w:1 , writes were successful, but the reads were failing because the majority was not satisfied.
And after sometime there was performance issue on the new primary and some writes also started timing out and the overall response time increased.
In the next upgrade i.e fro 4.0 to 4.2, we might face this issue again. is there a way to do this upgrade without read and write failures?