Scanned this thread, apologies if I missed something, but here's a few thoughts:
To get better advice make it clear if you are running Solr in Cloud mode (a.k.a. self managed) or Legacy (a.k.a user managed). Some ways to know which quickly: 1. Is there an associated Zookeeper cluster? If yes, then you are in cloud mode if not then *probably* legacy (there's a way to run zookeeper embedded, but that's not the normal setup). 2. In the admin UI do you see the word 'Cloud' in the left navigation bar? If yes, cloud, if no, legacy *Key concept: Solr is (normally) just a server providing access to an index of your data. It allows you to find a link, or id for a "document" but does not (normally) serve as a repository for your data.* This has some implications: 1. Solr is typically paired with one or more data repositories (database, file system, sharepoint, etc) 2. Solr normally cannot reindex data all by itself. Re-indexing is the process of re-reading the repository, and creating a fresh index. 3. Solr is just an index, and does not manage the process of reading the data from sources (Exceptions like Data import handler[DIH] and streaming expressions exist, but DIH went away in 9.x and these are exceptions not the rule) 4. Typically *something* outside of solr sends documents to solr. Re-indexing is normally the process of re-triggering something to send the documents again. 5. This is unlike a database which contains both the data (the table) and an index (PK/FK/index) of the data. 6. Versus a database, Solr's benefit is that it is an index of the *words* in the text of the document rather than entire string values. Thus (exceptional cases excluded) things you do to or in solr don't "trigger reindexing". I have implied that sometimes solr can be the store for your data, which is technically true. Unfortunately, this is tricky to get right, may negatively impact performance, and results in long term data loss if done wrong, so it's rarely recommended. I hope you haven't inherited this type of problem! Upgrading Solr across a single minor version is often simple, but occasionally requires work. Always read release notes and test the result before going live. Upgrading across major versions is always work. Lucene (and therefore solr) requires that you reindex data with each major version. There are stopgap tools to allow an upgrade of an existing index, but that is a temporary measure that only works for N to N+1 and you are expected to re-index before N+2. - Gus -- http://www.needhamsoftware.com (work) https://a.co/d/b2sZLD9 (my fantasy fiction book)
