Firebase

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Do not rebuild this — but understand what you are actually buying, because it is one specific thing.

**Offline sync that resolves conflicts**: a mobile application writes while disconnected, and when the connection returns the changes merge without the developer reasoning about it. That is genuinely difficult, and it is why this platform is chosen for mobile work.

WHEN TO KEEP PAYING
- A mobile application that must work offline and sync afterwards
- A team that needs something running this month

WHEN TO BUILD, AND WHAT IT LOOKS LIKE
- A web application, or a mobile one that is online most of the time. Then a small server with SQLite covers it, and the shape is under Appwrite and PocketBase elsewhere in this catalogue
- If you do need offline, the honest answer is an existing local-first sync library rather than writing merge semantics yourself

FIRST, LOOK AT THE BILL
- Priced per read, per write and per gigabyte transferred. A listener on a collection that re-reads on every change can cost more than a server would
- Audit your reads before deciding anything. Most surprising invoices in this category are one query in a loop
- No aggregate queries, so counting things means maintaining counters, and those counters are where consistency bugs live

WHAT TO WATCH WHATEVER YOU CHOOSE
- **Security rules are your entire authorisation layer**, and they are easy to get wrong in a way that exposes a whole collection. Write tests for them — the emulator supports it — and test the negative case: sign in as one user and try to read another's document
- The client talks to the database directly, so any logic that must not be bypassed belongs in a rule or a function, never in the application
- Vendor lock-in is real here: the query model, the rules and the functions do not port. Decide early whether you would ever need to leave, and keep a schema you could actually export

THE EXIT TO KEEP OPEN
Export your data on a schedule to storage you control, in a format you can read without the platform. That single job is the difference between a decision and a marriage.

THE ONE-LINE VERSION
Buy it for offline mobile sync. Audit your reads, test your security rules, and keep an export running.