About seven years ago, I got a contracting gig for a website that wanted a "search engine". I remember thinking "Solr/Lucene is old, not pure-functional, and therefore awful!" and decided to build my own. Somehow I even managed to convince the client that this was a good idea.

I ended up trying to reinvent Solr for the client, realizing after about two days of trying to reinvent stemming and indexing, that this was stupid to do on someone else's time, and called the client to tell them that I'm moving to Solr, and I got the project done before-schedule as a result.

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I think for 99% of usecases (involving search), Lucene/Solr/ES is perfectly fine. However, I do absolutely hate that some companies have decided to make it their primary database.

EDIT: I just want to make it clear, I think it's totally valid to try and reinvent Solr for fun, or if that's something you're paid specifically to do; nothing is perfect, and I am actually a big fan of the "if it works, break it and make it better!" mentality.