Major Description
Part of moving forward to "the new way", after SOLR-4196 etc... I propose an additional parameter specified on the <core> node in solr.xml or as a parameter in the "discovery" mode core.properties file, call it configSet, where the value provided is a path to a directory, either absolute or relative. Really, this is as though you copied the conf directory somewhere to be used by more than one core.
Straw-man: There will be a directory <solr_home>/configsets which will be the default. If the configSet parameter is, say, "myconf", then I'd expect a directory named "myconf" to exist in <solr_home>/configsets, which would look something like
<solr_home>/configsets/myconf/schema.xml
solrconfig.xml
stopwords.txt
velocity
velocity/query.vm
etc.
If multiple cores used the same configSet, schema, solrconfig etc. would all be shared (i.e. shareSchema="true" would be assumed). I don't see a good use-case for not sharing schemas, so I don't propose to allow this to be turned off. Hmmm, what if shareSchema is explicitly set to false in the solr.xml or properties file? I'd guess it should be honored but maybe log a warning?
Mostly I'm putting this up for comments. I know that there are already thoughts about how this all should work floating around, so before I start any work on this I thought I'd at least get an idea of whether this is the way people are thinking about going.
Configset can be either a relative or absolute path, if relative it's assumed to be relative to <solr_home>.
Thoughts?