For many years[1], SolrCloud has supported a "collection" query parameter for both routing a request to a collection as an alternative to "/solr/collectionNameHere/handler" [2],[3], as well as for doing distributed-search across a number of collections[4]. It's actually a comma delimited list of collections. AFAICT, this parameter is undocumented. I want to deprecate this parameter in Solr 9.9, removing it in 10. It will still be possible to accomplish the desired effect by simply putting the collection list into the path, e.g. "/solr/col1,col2,col3/select" which alas isn't documented although there's a mention of it in CHANGES.txt for SOLR-11444.
I could see an argument for keeping it on the distributed-search side but most importantly I don't want it to be considered as part of request routing. [1] At least since 2017, probably earlier when SolrCloud collection aliases first appeared. [2] SolrJ side is SolrRequest.getCollection's default implementation and is consulted by CloudSolrClient. [3] Solr server side is HttpSolrCall (also in a V2 subclass) [4] Distributed-search reference is in CloudReplicaSource ~ David Smiley Apache Lucene/Solr Search Developer http://www.linkedin.com/in/davidwsmiley
