Vectors are the headline of Solr 9, but not the only change. These are the other additions, and the ones that change how you run or call Solr.
Java 11 is the floor, Java 17 the recommendation
Solr 9 requires Java 11 as a minimum and is continuously tested against Java 11, 17 and 21. The reference guide names JRE 17 as the recommended version.
Lucene 9 underneath
Solr 9 is the first release built and shipped independently of Lucene, on Lucene 9. That is where the HNSW vector index, the faster BKD point trees and the smaller segment format come from.
Certificate authentication
A new Certificate Authentication Plugin allows end-to-end x509 client certificates for both authentication and authorization, alongside improved PKI internode security and ZooKeeper 3.7 with TLS.
Rate limiting and task management
Update and search requests can be throttled on usage metrics, and long-running tasks can be declared cancellable and trackable. Both are node-level protections that did not exist in Solr 8.
Node roles and pluggable replica placement
SolrCloud gained explicit overseer and data roles, and a pluggable Replica Placement API that replaced the removed autoscaling framework.
SQL in the Admin UI
The Admin UI can run SQL queries directly. The SQL engine itself moved into the sql module, so existing configurations keep working once the module is enabled.
Six new language stemmers
Snowball stemmers for Hindi, Indonesian, Nepali, Serbian, Tamil and Yiddish, plus a new Norwegian normalisation filter.
SolrJ on HTTP/2
CloudSolrClient now speaks HTTP/2 with a new builder. The older HttpSolrClient and LBHttpSolrClient without HTTP/2 support are deprecated, and the previous CloudSolrClient was renamed CloudLegacySolrClient.
Leader and follower, everywhere
The master and slave vocabulary is gone from configuration, metrics and the Admin UI. If you monitor Solr KPIs by metric name, those names changed.
Solr 9
Everything else Solr 9 added (this page)