Why can’t I use Elastic/solr to store data instead of a DB?

Assuming you're not trolling, ACID compliance

What is the query pattern for your data? I think so you can definitely store your data in Solr, depends on the kind of data and use case . Random reviews by folks on which you want to perform some key word based search, I think solr / ES is fantastic choice

Is your main use case search? Use elastic/solr to build the indexes. If not, use DB

Depending on your goals, you can. But my instinct is to reach for PostgreSQL and, if that doesn't work, reach for something else. I'm biased so I'd reach for elasticsearch for full text search instead. But I'm biased. PostgreSQL has good full text search. If postgresql doesn't work reach for the thing that solves that problem. Unless your org has some good reason to have a policy to reach one particular way. We store more stuff in elasticsearch than most folks would. Because we employ dozens of elasticsearch hackers. That's compelling. Like you said, sometimes you don't need ACID. Lots of times. Before you store things you are going to update in any systems go read aphyr's blog on them. If aphyr hasn't blogged about it probably don't use it. Unless you have some really compelling reason. But you aren't as good at finding bugs as aphyr. No one is. Keep in mind aphyr writes about the state of a system at one point in time. And he always finds problems. Things will have gotten better. They did for us. But reading his work will give you a sense of what can go wrong. And you should look for orgs to respond to his work. Small note: aphyr doesn't separate topics on his blog. He's a wonderful writer and very funny. And very into leather. And the person I trust most on distributed systems. You have to take all three.