MongoDB cluster upgrade from 3.4.x to 3.6.x to 4.0.x

Hi,

I’ve a 3.4 MongoDB cluster with tree nodes :

  • node01 : Primary
  • node02 : Secondary
  • node03 : Secondary

Currently I have upgraded the version on all nodes to 3.6.x but i’ve kept the featureCompatibility version to be 3.4 only.

I need to know what will be the impact of changing the featureCompatibility version to 3.6. It needs to be done as it is one of the prerequisite to upgrade the servers to 4.0.x

And hence I won’t be able to downgrade the feature compatibility version it makes it a difficult task if things go wrong.

Need more insights about the featureCompatibility version in MongoDB and what are the best practices around the same.

Also it’ll be helpful if I can get list of known issue people face while moving from 3.4 to 3.6.

Hi @Manan_Verma

Changing the FCV allows you to use the backwards incompatible changes for the release.
https://docs.mongodb.com/v4.0/release-notes/3.6-compatibility/#backwards-incompatible-features
https://docs.mongodb.com/v4.0/release-notes/4.0-compatibility/#feature-compatibility

Usually you are encouraged to run with the previous FCV for a while to ensure the server upgrade itself does not impact your deployment and allow for a quicker downgrade path.

The downgrade guide have step on how to downgrade if you have set the FCV. It is usually removing any reliance on the features that were introduced with the new version.

If you’re not using the features now, it is unlikely they will cause you issues.

Read the release notes and the upgrade guide thoroughly and make some backups for peace of mind.

I’d say good luck but upgrades are well documented and not that difficult.

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