Ask HN: What data store would you use for an instant messaging application?

As a side project, I have an idea for a particular instant messaging mobile app.

Each message will be tightly coupled to a specific geographic location, fwiw. And messages will be ephemeral, they'll be deleted completely from the server after a few hours (or maybe a week).

So I'm thinking that there'll be a backend API that the mobile apps will use to send/receive messages.

My question is: what kind of technology would you use for such an application? Where would you store those messages?