HDDS-9279. Basic implementation of OM Follower read

[symious]

What changes were proposed in this pull request?

This is another implemetation of OM FollowerRead.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-9279

How was this patch tested?

Test with new Freon class: FollowerReader.

[szetszwo]

@symious , Thanks for working on this!

A quick comment: For OM, any Ratis confs can be set by adding the ozone.om.ha prefix. In this case, the existing confs are

• ozone.om.ha.raft.server.read.option
• ozone.om.ha.raft.server.read.leader.lease.enabled

So, the new confs are not needed.

[symious]

the new confs are not needed.

@szetszwo Thank you for the review, configs removed.

[szetszwo]

+1 the change looks good. It is very simple and elegant!

[ivandika3]

Overall LGTM +1 since it is disabled by default so there should not be any regression. @symious Please help to address #9222 (comment) first.

Regarding my other comment #5288 (comment), we currently don't have clear root cause of the performance regression (e.g. whether it's inherent to the algorithm or implementation issues). I think we can merge this first and improve it from Ozone (e.g. client failover implementation) or Ratis (e.g. ReadIndex performance improvements) in follow up patches.

[ivandika3]

@symious Thanks for the patch and @szetszwo for the review.

[symious]

@ivandika3 @szetszwo Thank you for the review and merge.

[peterxcli]

Suggested change

	" DEFAULT - Directly query statemachine (non-linearizable). " +
	" Only the leader can serve read requests. " +
	" DEFAULT - Directly query statemachine. " +
	" Only the leader can serve read requests. " +

IIUC, the current DEFAULT is still linearizable. To avoid confusion, let’s rename the modes to something more explicit—for example:

• leader-only
• follower-linearizable
• follower-stale-read (as introduced in HDDS-13954. Add localLease for followerRead #9320)

Then we can add our own parsing that maps these OM-level configs to the corresponding Ratis read options, instead of relying directly on RaftServerConfigKeys.Read.option(...).

[symious]

Currently this configuration is using directly from Ratis. If changed, OM Ratis Server won't start normally.

The thing is the localLease is not a 1:1 mapping, if locallease condition check doesn't fit, it will fallback to ReadIndex which is linearizable.