Confirming Dynamic Core Discovery in Solr 8 Legacy Cloud Mode and Changes in Solr 9

Question

I’m currently working with a Solr 8.11 SolrCloud setup that uses legacyCloud=true in clusterprops.json. I’ve noticed that in this setup, Solr dynamically creates collections and cores upon startup based on the folder structure and core.properties files, with the collection state stored in state.json in Zookeeper after using the zkcli.sh linkconfig command.

However, I’m planning an upgrade to Solr 9, and I’ve noticed some key differences:

• In Solr 8, with legacyCloud=true, collections and cores are dynamically discovered and registered, and I can manually manage the core folder structures. There is no clusterstate.json in my Zookeeper; instead, the state is created dynamically in state.json files.
• In Solr 9, legacyCloud mode has been deprecated, and manual core discovery doesn’t seem to work anymore. It appears that all core and collection management now requires the Collections API, and state.json files must be generated through the API, without manual intervention.

My questions are:

1. Is it true that Solr 8 with legacyCloud=true enabled dynamic core discovery and allowed Solr to register collections and cores based on manual folder structures and core.properties files?
2. Are there any references from the Solr source code, Jira tickets, or official documentation that confirm this behavior?
3. Can someone clarify if the deprecation of legacyCloud=true in Solr 9 forces the usage of the Collections API, making manual core discovery and state synchronization in Zookeeper no longer possible?
4. If manual core discovery is no longer supported in Solr 9 SolrCloud, is the only alternative to switch to a standalone (user-managed mode) setup to keep using manual core creation?
   • If so, can a Solr setup in user-managed mode still use Zookeeper for some features, or would it be entirely independent of Zookeeper?

I've been reading these articles:

• https://solr.apache.org/guide/solr/latest/configuration-guide/core-discovery.html#the-core-properties-file
• https://solr.apache.org/guide/solr/latest/configuration-guide/configuration-files.html
• https://solr.apache.org/guide/solr/latest/upgrade-notes/major-changes-in-solr-9.html
• https://issues.apache.org/jira/browse/SOLR-15549
• https://issues.apache.org/jira/browse/SOLR-5473

But it is not explicitly stated that manual core discovery on Solr startup is not available anymore on Solr version 9. Any references to code commits, Jira tickets, or official documentation that provide more clarity on these behaviors and changes would be extremely helpful.

Thanks in advance for your help!