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https://issues.apache.org/jira/browse/SOLR-6236?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14096040#comment-14096040
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Timothy Potter commented on SOLR-6236:
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I wasn't able to get a test working that showed a replica not being able to
become the leader after it loses its ZK session and its leader goes down around
the same time (on trunk). Every time I ran that scenario, the replica became
the leader, which is a good thing. I've heard about cases in the field where
this happens, so am still trying to simulate it in a test environment.
Basically, I've tried expiring the ZK session on the replica and then killing
the Jetty hosting the leader and the replica always becomes the leader as
expected.
Also, I'm reworking / rethinking this patch as the previous approach works fine
in a test environment but won't work in general. The problem is when a replica
is trying to decide if it should force itself to be the leader, it doesn't
really take the state of other replicas into account. It just kind of assumes
the others are in a bad state since it can't recover. So in one case, a replica
could decide to not force itself thinking another replica will do it, which
might not ever happen. Conversely, it could decide to force itself when a
better candidate is maybe just being slow at becoming the leader. Mainly, I
think these are areas that need more investigation before this approach is
vetted out. I definitely like giving operators the ability to "force_leader" by
updating the leader-initiated recovery status for a replica, but I'm not so
sure about a replica doing that itself (without the intervention of a human
operator).
> 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
> Assignee: Timothy Potter
> 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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