doc: update Raft information in 6.0

[annastuchlik]

This commit updates the documentation about Raft in version 6.0.

• "Introduction": The outdated information about consistent topology updates not being supported is removed and replaced with the correct information.
• "Enabling Raft": The relevant information is moved to other sections. The irrelevant information is removed. The section no longer exists.
• "Verifying that the Raft upgrade procedure finished successfully" - moved under Schema (in the same document). I additionally removed the include saying that after you verify that schema on Raft is enabled, you MUST enable topology changes on Raft (it is not mandatory; also, it should be part of the upgrade guide, not the Raft document).
• Unnecessary or incorrect references to versions are removed.

Refs #18580

• No backport. This update is relevant in version 6.0.

[scylladb-promoter]

Docs Preview 📖

Docs Preview for this pull request is available here

Changed Files:

• architecture/_common/note-enabling-consistent-topology-changes
• architecture/raft

Note: This preview will be available for 30 days and will be automatically deleted after that period. You can manually trigger a new build by committing changes.

[kbr-scylla]

You must also perform it after going through the manual Raft recovery procedure (https://opensource.docs.scylladb.com/master/troubleshooting/handling-node-failures.html#manual-recovery-procedure)

every time one does the manual Raft recovery procedure (hopefully never, because it's for disaster scenarios -- loss of majority), one has to do this validation.

Perhaps we can preamble the statement with "Normally" or something. "Normally, you must perform ..."

[kbr-scylla]

We don't want to explain when this happens? What is the trigger for the schema changes feature to get enabled? If we don't explain it -- how will the user know when to actually perform this procedure?

[annastuchlik]

You've made a good point, obviously. This section doesn't make it clear. In previous releases, we added the information WHEN the procedure should be performed to the upgrade guides - with this section serving as a reference.

I've added the above sentence for the sake of clarification, but it might make the message more vague.

I could explain the details, but it would involve adding more release-specific information in additional include files.
I'd lean to taking the approach we had in previous releases - adding release-specific information in the upgrade guide, and removing the above sentence altogether.
@kbr-scylla Are you OK with that? We'll simply keep the version we had in previous releases.

[kbr-scylla]

But it is possible to also enable Raft in a, let's say, 5.2 cluster, by setting consistent_cluster_management: true (when it was previously false) and doing a rolling restart --- disconnected from the version upgrade procedure.

But this is 6.0 documentation, so I think we should assume that whoever is reading this article, is reading it in the context of running a 6.0 cluster or upgrading their cluster to 6.0.

So with that assumption, we could just say something like:
"If you disabled Raft in 5.4 (by setting consistent_cluster_management: false), the Raft-based schema changes feature will automatically get enabled once you finish rolling-upgrade to 6.0, and then you must perform the verification procedure."

[annastuchlik]

But this is 6.0 documentation, so I think we should assume that whoever is reading this article, is reading it in the context of running a 6.0 cluster or upgrading their cluster to 6.0.

Yes, we shouldn't consider 5.2 or any other version other than 6.0.

So with that assumption, we could just say something like: "If you disabled Raft in 5.4 (by setting consistent_cluster_management: false), the Raft-based schema changes feature will automatically get enabled once you finish rolling-upgrade to 6.0, and then you must perform the verification procedure."

I totally agree with the message. I just think that it should be added to the upgrade guide (I'm working on it now). Upgrade-related details on the Raft page are not helpful, they must be part of the upgrade procedure.

I'll just restore this section to what it was in previous releases, i.e., I'll remove the unfortunate sentence I've added above.

If you insist we should have something, it could be: "You may need to perform the following procedure on upgrade if you explicitly disabled the Raft-based schema changes feature in the previous ScyllaDB version. Please consult the upgrade guide." - without version numbers or links (which will cause issues on merge with enterprise).

Let me know what you think.

[annastuchlik]

@kbr-scylla I've applied the update. Even if not perfect, it still adds some clarity we didn't have in previous versions. I don't wan't to add any links here. Please have a look.