David, I did not mean to suggest having two versions of lucene-core.jar. I meant to imply only having lucene-core.jar v9.12.3 (the higher version which comes with Solr 9.10.1) to try with Solr 9.8.1 post the downgrade and remove the lower version lucene-core.jar...as a one time workaround in order to avoid having to recreate the indexes and still be able to achieve a downgrade. Not as a permanent future solution native to Solr.
Rahul On Wed, Feb 25, 2026 at 4:23 PM David Smiley <[email protected]> wrote: > Definitely "B"; "A" would hold Solr back! > And to help that along, I believe there are Lucene upgrade notes that Solr > has somewhere. Those notes should include a step to check for an > index/codec change, and if so then to ensure this is strongly announced in > the upgrade notes. > > Rahul: Lucene's current codec is held inside lucene-core.jar... and we > definitely can't have multiple lucene-core.jar. I think it's pretty shaky > ground to pursue taking the "write" support of an older JAR. There are > very low level API changes that change internally but are used by codecs, > like for PackedInt or whatever. > > On Wed, Feb 25, 2026 at 9:52 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 >
