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MongoDB 4.2 using change streams with read concern majority disabled in a PSA deployment

Database Administrators Asked by Franz Huebner on January 6, 2022

I currently run a PSA (Primary, Secondary, Arbiter) deployment and thous only have two databearing nodes.
In case of a databearing node going down, read concern majority prevented the change stream to be read out of.
In MongoDB 4.2 it is now possible to disable the read concern majority for the replicaset and still use the change stream (See https://docs.mongodb.com/manual/release-notes/4.2/#change-stream).

I disabled read concern majority in my deployment and the change stream works as expected.
If I shut down the secondary node the change stream can’t be read out of again.

Is this the expected behaviour? I would have assumed that with 4.2 a majority is not necessary anymore to read the change stream.
If that is the case, is there any way to read the change stream in a PSA deployment with only a minority of data bearing nodes?

One Answer

This is not the expected behavior. Since you have the majority members of the replica set online, it should work

Just wanted to confirm:

  1. Are you using version 4.2?
  2. After installing/upgrading the replica set, have you set the feature compatibility version to 4.2 by executing the below command?

    db.adminCommand( { setFeatureCompatibilityVersion: 4.2 } )

Thank you!

Answered by Anban on January 6, 2022

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