Revert to Zookeeper AFTER Kraft migration

[jonsnowseven]

Hello!

Is there a guide to rollback to Zookeeper after Kraft migration is completed?

I checked that is possible to rollback while in migration but didn't find a walkthrough if the migration is complete.

[scholzj]

I don't think you can migrate from KRaft to ZooKeeper. I do not think Apache Kafka supports that.

[ppatierno]

No there is no way to do so. When you complete the migration to KRaft, you cannot revert back to ZooKeeper. You can rollback until the migration state is still in KRaftPostMigration where ZooKeeper is still up and running and KRaft controllers are in dual-write mode so they are still copying metadata to ZooKeeper. If you end the migration to the KRaft state, you are done. No revert back is possible. As Jakub said, it's not about Strimzi. It's about Kafka.

[jonsnowseven]

So what happens if I revert back the changes I did in the migration to Kraft (literally reverting the commits)?

[ppatierno]

Which commits?
When you end the migration you have strimzi.io/kraft: enabled annotation.
If you try to apply any other value from here (i.e. disabled) because you would like to go back to ZooKeeper, it will raise a warning that the annotation value is not allowed in the KRaft state and nothing will happen. As I said. There is no way to come back. Apache Kafka doesn't support it at all, leaving Strimzi out of the picture.

[jonsnowseven]

I did these steps by committing the changes to the repository where I have the strimzi template.

If I revert these changes (remove the node pools, the annotations), what happens?

[ppatierno]

Let's ask a simple question, because you still point me to the steps talking about rollback. Did you end or not the migration? Did you apply the strimzi.io/kraft: enabled annotation? Is your kafkaMetadataState field in the Kafka CR status at KRaft? If the answer is yes for my questions, you ended the migration, ZooKeeper is not even running anymore and you are in KRaft, no way to go back, whatever you do. Removing node pools will just break the cluster. Changing the annotation will just raise a warning as I said.

Instead .. if you are still in the KRaftPostMigration state as described in the blog post, you can still rollback by applying the strimzi.io/kraft: rollback annotation (and then the disabled value by following the procedure). It's not about undoing some commits (if you are using GitOps), it's about doing different commits.