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https://issues.apache.org/jira/browse/SOLR-8531?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15090706#comment-15090706
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Jeff Wartes commented on SOLR-8531:
-----------------------------------
I was imagining a note
https://lucene.apache.org/solr/5_4_0/changes/Changes.html#v5.4.0.upgrading_from_solr_5.3
But I could understand that being driven off of an immutable release tag.
I haven't fully read the SOLR-7844 patch for comprehension, but the change to
ZkStateReader.java looks like the reason:
https://github.com/apache/lucene-solr/commit/65cb72631b0833f8ddcf34dfa3d4a91f2c5091c4#diff-8f54b814c3da916328992910b1ad9163
I don't immediately see the change being necessary, so I suspect it could be
reverted or made reverse-compatible without too much trouble.
If it's the former, then I'll presumably hit the same issue again in reverse
moving from 5.4 to 5.4.1, which could be ok now that I know to expect it.
> ZK leader path changed in 5.4
> -----------------------------
>
> Key: SOLR-8531
> URL: https://issues.apache.org/jira/browse/SOLR-8531
> Project: Solr
> Issue Type: Bug
> Components: SolrCloud
> Affects Versions: 5.4
> Reporter: Jeff Wartes
>
> While doing a rolling upgrade from 5.3 to 5.4 of a solrcloud cluster, I
> observed that upgraded nodes would not register their shards as active unless
> they were elected the leader for the shard.
> There were no errors, the shards were fully up and responsive, but would not
> publish any change from the "down" state.
> This appears to be because the recovery process never happens, because the ZK
> node containing the current leader can't be found, because the ZK path has
> changed.
> Specifically, the leader data node changed from:
> <collection>/leaders/<shard>
> to
> <collection>/leaders/<shard>/leader
> It looks to me like this happened during SOLR-7844, perhaps accidentally.
> At the least, the "Migrating to Solr 5.4" section of the README should get
> updated with this info, since it means a rolling upgrade of a collection with
> multiple replicas will suffer serious degradation in the number of active
> replicas as nodes are upgraded. It's entirely possible this will reduce some
> shards to a single active replica.
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