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https://issues.apache.org/jira/browse/SOLR-12281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17433910#comment-17433910
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Jan Høydahl commented on SOLR-12281:
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Ok, got it. This means the current Solr Ref-Guide documentation is still
correct.
I'll remove the blocker on this, and keep it open as an umbrella to improve the
documentation regarding back-compat. I believe several other RefGuide pages can
also need some clarification.
> Document the new index back-compat guarantees
> ---------------------------------------------
>
> Key: SOLR-12281
> URL: https://issues.apache.org/jira/browse/SOLR-12281
> Project: Solr
> Issue Type: Task
> Components: documentation
> Affects Versions: 8.0
> Reporter: Jan Høydahl
> Priority: Blocker
> Fix For: 8.1, main (9.0)
>
>
> Refer to discussions in LUCENE-8264. Since 7.x a lucene segment will carry
> information about *first created* version, and in 8.0 Lucene will refuse to
> read an index created pre-7.0, even if the index is "upgraded" and rewritten
> in 7.x format.
> This new policy breaks with our historic N-1 guarantee, in that you now will
> be *forced* to reindex from source every 2 major versions, starting from 8.0.
> There is a chance that the requirement will be relaxed in 9.0 so that 9.x
> will be able to read 7.x indices but that I guess depends on what breaking
> index changes are made before 9.0.
> This all must be documented clearly both in CHANGES and in RefGuide. Now we
> have wordings that you can upgrade 6->7 and then 7->8.
> Related to this we should also touch parts of our documentation that deals
> with stored/docvalues, and add a recommendation to keep things stored because
> you *will* need to reindex.
> Probably this also affects backup/restore in that if you plan to upgrade to
> 8.x you will need to make sure that none of your segments are created
> pre-7.x. And perhaps it affects CDCR as well if one cluster is upgraded
> before the other etc.
> Then finally -- and this is beyond just documentation -- we should look into
> better product support for various re-index scenarios. We already have the
> streaming {{update()}} feature, backup/restore, manual /export to json etc.
> But what about:
> * A nice Admin UI on top of the streaming capabilities, where you enter the
> URL of the remote (old) system and then you get a list of collections that
> you can select from, and click "GO" and go for lunch, then when you return
> everything is reindexed into the new cluster.
> * Have the install script warn you when doing an in-place upgrade from 7.x
> to 8.0?
> * If Solr 8.x is started and detects a pre-7.0 index, then log big ERROR,
> and show a big red "UPGRADE" button in the Admin UI. Clicking this button
> would trigger some magic code that re-builds all collections from source (if
> stored/DV fields can be found in the old indices).
> I've put this as a blocker issue for now, in that we at least need some
> documentation updates before 8.0, and ideally some tools as well.
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