Index. Kelondro? .. vs lucene (solr) ? which in use, why, advantage?

I don’t understand that either.
After several years of using YaCy, I’m still not sure, what is exactly the role of Kelondro vs solr, which data are stored where and whether both are necessary.

I noticed, @Sviatoslav even wrote scripts to delete kelondro files as unnecessary. But I’m still curious, if kelondros have some function, or if I can switch them off with no lose (and how?).

@Orbiter , could you please explain? Thanks!