Firebase is Google's all-in-one backend, and it's especially popular for mobile apps. Its main database, Firestore, stores data as documents rather than tables, and once you pass the free daily allowance you pay for each read and write. Sign-in, file storage and hosting live in the same project, so you can get a lot running without managing a server.
✓Good for
Mobile apps that want sign-in and data in one place
Small apps that stay inside the free daily quotas
Live-updating screens without writing your own server
!Watch out for
Costs follow reads and writes, so an inefficient screen can get expensive
File storage and phone sign-in need the pay-as-you-go Blaze plan with a card
Document-style data is harder to report on, or to move to SQL later
Spark (free, no card) or Blaze (pay as you go, with the same free daily quotas)
Tiny bills for apps with modest traffic, because the free daily quotas carry over onto the paid plan. A natural fit for mobile apps that want sign-in and data from one place.
You pay per document read, so the bill follows how your app is written, not how many users you have. Firestore stores documents rather than tables, which makes reporting, or moving to SQL later, harder. File storage and phone sign-in need Blaze with a card on file.
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