I've been asked to investigate an upgrade of Solr from version 4.x to 8.x. I got it up and running, and it seems to work fine, but in the process I had to remove deprecated sections from solrconfig.xml:
There doesn't seem to be any information on how to replace them, though. I found a list of deprecated Solr 4.x classes, with alternatives, but the ones I had to remove didn't have any.
A bit more worrisome is that in schema.xml I had to remove enablePositionIncrements, as it's deprecated. I can imagine that this will have a more direct impact on searches, but there doesn't seem to be an upgrade path for this functionality either.
All of Solr's expertise has left the company, so I'm on my own, here. Can anyone help me move forward?
For clustering support you'll have to do a bit of legwork yourself. According to the ticket documenting its removal, you can install the bundled version from 8.7 (the last version that had the component before it got removed awaiting inclusion in Solr 9 when Java 1.8 support can be dropped, iirc). There's also a backported version if you don't require 1.8 support - it has been linked in the ticket and in the comments.
Including Jörn Franke's comment in case the ticket disappears:
Just for information: If you need to update to Solr 8.11.1 or later due to the log4j issue and you still need clustering for business continuity then you can copy the clustering component from Solr 8.7.0 (https://archive.apache.org/dist/lucene/solr/8.7.0/solr-8.7.0.tgz).
Copy the contrib/clustering folder to the contrib folder of the 8.11.1 installation.
Copy dist/solr-clustering* to the dist folder of the 8.11.1 installation.
It might be not that elegant, but it seems to work and this is probably better than not upgrading to Solr 8.11.1 with a critical security fix.
Since version 9.0 is not out yet (and it might get back the clustering component?) this could be a temporary solution to apply a critical security fix and at the same time still have clustering if you need it.
The JSONUpdateRequestHandler is no longer required as the regular update handler handles JSON documents fine. You might have to change how you use the handler and update your paths.
The same is the case with the CSVRequestHandler.
Both are documented in Uploading data with index handlers in the reference guide.
The Admin handler is now implicit and shouldn't require any configuration (it's always included and available).