Honestly, at this point, if I had a design that required making atomic changes to files, I'd redo the design to use SQLite. The other way around sounds crazy to me.

"Why use spray paint when you can achieve the same effect by ejecting paint from your mouth in a uniform high-velocity mist?" If you happen to have developed that particular weird skill, by all means use it, but if you haven't, don't start now.

That probably sounds soft and lazy. I should learn to use my operating system's filesystem APIs safely. It would make me a better person. But honestly, I think that's a very niche skill these days, and you should consider if you really need it now and if you'll ever benefit from it in the future.

Also, even if you do it right, the people who inherit your code probably won't develop the same skills. They'll tell their boss it's impossibly dangerous to make any changes, and they'll replace it with a database.

For the simple case, it isn't necessarily that fragile. Write the entire database to a temp file, then after flushing, move the temp file to overwrite the old file. All Unix filesystems will ensure the move operation is atomic. Lots of "we dump a bunch of JSON to the disk" use cases could be much more stable if they just did this.

Doesn't scale at all, though - all of the data that needs to be self-consistent needs to be part of the same file, so writes goes through the roof if you're only doing small updates on a giant file. Still gotta handle locking if there is risk of a stray process messing it up. And doing this only handles part of ACID.