[jira] [Commented] (SOLR-6236) Need an optional fallback mechanism for selecting a leader when all replicas are in leader-initiated recovery.

["David Smiley (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-6236?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17694600#comment-17694600 ] 

David Smiley commented on SOLR-6236:
------------------------------------

Closing because we don't have LIR anymore thanks to SOLR-11702 (ZK shard terms).

> Need an optional fallback mechanism for selecting a leader when all replicas are in leader-initiated recovery.
> --------------------------------------------------------------------------------------------------------------
>
>                 Key: SOLR-6236
>                 URL: https://issues.apache.org/jira/browse/SOLR-6236
>             Project: Solr
>          Issue Type: Improvement
>          Components: SolrCloud
>            Reporter: Timothy Potter
>            Priority: Major
>         Attachments: SOLR-6236.patch
>
>
> Offshoot from discussion in SOLR-6235, key points are:
> Tim: In ElectionContext, when running shouldIBeLeader, the node will choose to not be the leader if it is in LIR. However, this could lead to no leader. My thinking there is the state is bad enough that we would need manual intervention to clear one of the LIR znodes to allow a replica to get past this point. But maybe we can do better here?
> Shalin: Good question. With careful use of minRf, the user can retry operations and maintain consistency even if we arbitrarily elect a leader in this case. But most people won't use minRf and don't care about consistency as much as availability. For them there should be a way to get out of this mess easily. We can have a collection property (boolean + timeout value) to force elect a leader even if all shards were in LIR. What do you think?
> Mark: Indeed, it's a current limitation that you can have all nodes in a shard thinking they cannot be leader, even when all of them are available. This is not required by the distributed model we have at all, it's just a consequence of being over restrictive on the initial implementation - if all known replicas are participating, you should be able to get a leader. So I'm not sure if this case should be optional. But iff not all known replicas are participating and you still want to force a leader, that should be optional - I think it should default to false though. I think the system should default to reasonable data safety in these cases.
> How best to solve this, I'm not quite sure, but happy to look at a patch. How do you plan on monitoring and taking action? Via the Overseer? It seems tricky to do it from the replicas.
> Tim: We have a similar issue where a replica attempting to be the leader needs to wait a while to see other replicas before declaring itself the leader, see ElectionContext around line 200:
> int leaderVoteWait = cc.getZkController().getLeaderVoteWait();
> if (!weAreReplacement)
> { waitForReplicasToComeUp(weAreReplacement, leaderVoteWait); }
> So one quick idea might be to have the code that checks if it's in LIR see if all replicas are in LIR and if so, wait out the leaderVoteWait period and check again. If all are still in LIR, then move on with becoming the leader (in the spirit of availability).
> {quote}
> But iff not all known replicas are participating and you still want to force a leader, that should be optional - I think it should default to false though. I think the system should default to reasonable data safety in these cases.
> {quote}
> Shalin: That's the same case as the leaderVoteWait situation and we do go ahead after that amount of time even if all replicas aren't participating. Therefore, I think that we should handle it the same way. But to help people who care about consistency over availability, there should be a configurable property which bans this auto-promotion completely.
> In any case, we should switch to coreNodeName instead of coreName and open an issue to improve the leader election part.



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