I may be wrong, but I think you wrote somewhere that you're looking at the WAL size increasing to know if something was committed. Well, the WAL can be truncated, what then? Or even, however unlikely, it could be truncated, then a transaction comes and appends just enough to it to make it the same size.

If SQLite has an API it guarantees can notify you of changes, that seems better, in the sense that you're passing responsibility along to the experts. It should also work with rollback mode, another advantage. And I don't think wakes you up if a large transaction rolls back (a transaction can hit the WAL and never commit).

That said, I'm not sure what's lighter on average. For a WAL mode database, I will say that something that has knowledge of the WAL index could potentially be cheaper? That file is mmapped. The syscalls involved are file locks, if any.