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https://issues.apache.org/jira/browse/SOLR-15718?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17435493#comment-17435493
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Jan Høydahl commented on SOLR-15718:
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{quote}So if I understand this right then direct rolling upgrades from 8.5 to
9.x would not be possible anymore, correct?
{quote}
Yep, or to be precise, from 8.0 to 8.4.x it will not be possible to go directly
to 9.0 in a rolling upgrade, unless you pause collection api commands during
the upgrade. So it is not impossible, but you may end up with these exceptions
once a newly upgraded 9.0 node tries to read an overseer response from the
Overseer that is still on <=8.4. See PR, I added a line to 9.0 upgrade
prerequisites in ref-guide to highlight this potential limitation.
However, there may be other reasons to upgrade to 8.11 before a rolling upgrade
to 9.0 among the huge amount of 9.0 JIRAs. And most users will likely not
attempt this at all?
I have no problems leaving this code in for 9.x and changing this JIRA/PR to
target 10.0, if I perhaps underestimate the disadvantages. But with my current
understanding of the problem, I think it's a reasonable thing to do now.
> Remove backcompat feature solr.useUnsafeOverseerResponse
> --------------------------------------------------------
>
> Key: SOLR-15718
> URL: https://issues.apache.org/jira/browse/SOLR-15718
> Project: Solr
> Issue Type: Sub-task
> Reporter: Jan Høydahl
> Assignee: Jan Høydahl
> Priority: Major
> Time Spent: 1h 10m
> Remaining Estimate: 0h
>
> In SOLR-14095 (Solr 8.5) a system property {{solr.useUnsafeOverseerResponse}}
> was introduced to do rolling upgrades from 8.5 to 8.6 (which lets 8.6 fall
> back to Java-serialization).
> See [https://github.com/apache/solr/search?q=useUnsafeOverseerResponse]
> When upgrading to 9.0 I suppose we tell people to first upgrade to 8.11, so
> in 9.0 we don't need this. [~tflobbe] agree?
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