I used sets of flat JSON files as our "database" in the Wunderlist iOS and macOS clients.

Worked like a charm, never had a problem with it.

It was actually put in as a placeholder until we had time to think about a real storage solution, but it turned out we never needed anything more sophisticated, and were actually the fastest and most reliable clients we had. In fact, every time I encountered a performance problem I was hopeful that I would finally have a good reason to do that real implementation, but it invariably turned out to be a simple bug.

- Cocoa has -writeToFile:atomically:, which writes a new file and then renames, so no write-corruption

- We were lucky that lists had just the right granularity for a single file to be read/written atomically

- We likely wrote (quite) a bit more data than absolutely necessary, but I/O tends to have large fixed overheads so medium files tend to take around the same time as small files

- We did not do anything with the data on disk except read it, so not a DB

- We really did use files, not JSON strings inside SQLite

- We flushed to disk asynchronously, but as quickly as possible