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https://issues.apache.org/jira/browse/RATIS-2272?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ivan Andika updated RATIS-2272:
-------------------------------
    Description: 
RATIS-1298 added support for Ratis listener (non-voting member). Ratis listener 
cannot be promoted to the leader even when it already catch up with the leader.

So currently to add a voting-member, Ratis only supports adding a Raft 
follower. However, the issue is that Raft follower is immediately a voting 
member, which will include the quorum requirement of the Raft group, although 
it might take a while for the new Raft follower to catch up to the leader. This 
might cause availability issues in cases like network partition, split brain, 
etc.

As discussed in RATIS-1298, we can support Raft learner as a next step. This 
will help to resolve availability issues. 

This is a long-term effort. However, this would increase the stability of the 
Raft membership add.

Resources
 * Raft dissertation (Section 4.2.1)
 * [https://etcd.io/docs/v3.5/learning/design-learner/]

  was:
RATIS-1298 added support for Ratis listener (non-voting member). Ratis listener 
cannot be promoted to the leader even when it already catch up with the leader.

So currently to add a voting-member, Ratis only supports adding a Raft 
follower. However, the issue is that Raft follower is immediately a voting 
member, which will include the quorum requirement of the Raft group, although 
it might take a while for the new Raft follower to catch up to the leader. This 
might cause availability issues in cases like network partition, split brain, 
etc.

As discussed in RATIS-1298, we can support Raft learner as a next step. This 
will help to resolve availability issues. 

This is a long-term effort. I would expect that the current Raft listener in 
RATIS-1298 will be evolved to Raft learner since the architecture is already 
similar and the new Raft listener, will be based on the design on RATIS-2273 
due to the eventually consistency model.

Resources
 * Raft dissertation (Section 4.2.1)
 * [https://etcd.io/docs/v3.5/learning/design-learner/]


> Support Raft learner
> --------------------
>
>                 Key: RATIS-2272
>                 URL: https://issues.apache.org/jira/browse/RATIS-2272
>             Project: Ratis
>          Issue Type: New Feature
>          Components: server
>            Reporter: Ivan Andika
>            Assignee: Ivan Andika
>            Priority: Major
>
> RATIS-1298 added support for Ratis listener (non-voting member). Ratis 
> listener cannot be promoted to the leader even when it already catch up with 
> the leader.
> So currently to add a voting-member, Ratis only supports adding a Raft 
> follower. However, the issue is that Raft follower is immediately a voting 
> member, which will include the quorum requirement of the Raft group, although 
> it might take a while for the new Raft follower to catch up to the leader. 
> This might cause availability issues in cases like network partition, split 
> brain, etc.
> As discussed in RATIS-1298, we can support Raft learner as a next step. This 
> will help to resolve availability issues. 
> This is a long-term effort. However, this would increase the stability of the 
> Raft membership add.
> Resources
>  * Raft dissertation (Section 4.2.1)
>  * [https://etcd.io/docs/v3.5/learning/design-learner/]



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