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https://issues.apache.org/jira/browse/SOLR-15029?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17244119#comment-17244119
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Mike Drob commented on SOLR-15029:
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The existing code for increasing shard terms works when you ask it to exclude
the leader. That was a pleasant surprise, and I've added a unit test and some
documentation for this locally. I don't think this breaks the original term
increment rules too badly, and doesn't even require a new method signature,
since it is still only the leader doing the increment.
After that, I think we can issue a FORCELEADER command, we might need to add a
flag to it to be more strict on who it will accept as a leader. Currently it
looks like it would allow an out of sync replica to become leader (useful when
you are just trying to get _something_ online), but in this case we want to
explicitly limit to the highest shard term.
> Allow Shard Leader to give up leadership gracefully via shard terms
> -------------------------------------------------------------------
>
> Key: SOLR-15029
> URL: https://issues.apache.org/jira/browse/SOLR-15029
> Project: Solr
> Issue Type: Improvement
> Reporter: Mike Drob
> Assignee: Mike Drob
> Priority: Major
>
> Currently we have (via SOLR-12412) that when a leader sees an index writing
> error during an update it will give up leadership by deleting the replica and
> adding a new replica. One stated benefit of this was that because we are
> using the overseer and a known code path, that this is done asynchronous and
> very efficiently.
> I would argue that this approach is too heavy handed.
> In the case of a corrupt index exception, it makes some sense to completely
> delete the index dir and attempt to sync from a good peer. Even in this case,
> however, it might be better to allow fingerprinting and other index delta
> mechanisms take over and allow for a more efficient data transfer.
> In an alternate case where the index error arises due to a disconnected file
> system (possible with shared file systems, i.e. S3, HDFS, some k8s systems)
> and the required solution is some kind of reconnect, then this approach has
> several shortcomings - the core delete and creations are going to fail
> leaving dangling replicas. Further, the data is still present so there is no
> need to do so many extra copies.
> I propose that we bring in a mechanism to give up leadership via the existing
> shard terms language. I believe we would be able to set all replicas
> currently equal to leader term T to T+1, and then trigger a new leader
> election. The current leader would know it is ineligible, while the other
> replicas that were current before the failed update would be eligible. This
> improvement would entail adding an additional possible operation to terms
> state machine.
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