Mike Drob created LUCENE-9630:
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Summary: Allow Shard Leader to give up leadership gracefully via
shard terms
Key: LUCENE-9630
URL: https://issues.apache.org/jira/browse/LUCENE-9630
Project: Lucene - Core
Issue Type: Bug
Reporter: Mike Drob
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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