Hello, I think I saw something similar on Solr 8.x. If you decide on B) option, on top of the backup, users may need to save the new/updated indexed data after the upgrade, as it will be lost with the rollback.
Maybe the tool proposed in this thread "Automatic upgrade of Solr indexes over multiple versions" can add in the future the downgrade option and be option C). Kind Regards, Alejandro Arrieta On Wed, Feb 25, 2026 at 11:53 AM Jan Høydahl <[email protected]> wrote: > Hi, > > At a customer we upgraded from Solr 9.8.1 to 9.10.1 recently to stay on > top of CVEs etc. > Today we had the need to roll back Solr due to some crash situation. > But after downgrading to 9.8.1 the nodes don't get up with errors about > missing codec "Lucene912". > > Par of Solr/Lucene's back-compat promise is that we won't do breaking > chagnes in minor versions. > But this kind of codec changes between minor versions breaks the ability > to downgrade - you need a full reindex. > I find a related discussion in Lucene about this > https://github.com/apache/lucene/issues/14623#issuecomment-2866257196 > > We currently do not clearly document this limiation in RefGuide. > > In my eyes we (Solr) have two choices: > > A) Stay at the same lucene minor version throughout a Solr major version > and keep back-compat also for downgrades > B) Start doumenting in release notes when a new lucene version will > prevent future downgrade, and advise users to back up their index before > upgrading if they want the ability to roll back. > > Thoughts? > > Jan
