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https://issues.apache.org/jira/browse/RATIS-1298?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17278439#comment-17278439
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Rui Wang commented on RATIS-1298:
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I mean because when a normal follower join Raft ring, it will increase leader's
burden. Thus if we make the listener/learner catch up from leader, the behavior
will be the same as follower thus I don't think this is a "regression" for
Ratis. That's probably the reason why I will call "catch up from follower" is
an optimization but not mandatory requirement.
I would also argue that if we allow a follower join the Ratis ring but we don't
concern the performance, then we shouldn't concern listener/learner. Otherwise
we should start to change the Ratis implementation to let any new followers
catch up from another follower first, then connect with the leader.
For etcd: https://etcd.io/docs/v3.3.12/learning/learner/. Seems that it does
not concern about catching up from leader in its practice.
> Support non-voting members (Listeners/Learners)
> -----------------------------------------------
>
> Key: RATIS-1298
> URL: https://issues.apache.org/jira/browse/RATIS-1298
> Project: Ratis
> Issue Type: New Feature
> Components: server
> Reporter: Rui Wang
> Assignee: Rui Wang
> Priority: Major
>
> It's time to add the non-voting member, or leaner that is proposed in Raft
> thesis 4.2.1 (you can find a copy at [1]).
> The leaner is useful to maintain high availability when new servers join Raft
> ring (details at thesis 4.2.1). For Ozone SCM HA effort, we have also
> discussed the possibility to utilize leaner as the tool to replace SCM nodes
> online.
> [1]: https://github.com/ongardie/dissertation
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