Yes 200k SLOC is huge (modern development practices notwithstanding). SQLite creates temporary files at whim - nine different kinds! https://sqlite.org/tempfiles.html
I know how to atomically write a JSON file. But when I read, for example:
"The temporary files associated with transaction control, namely the rollback journal, super-journal, write-ahead log (WAL) files, and shared-memory files, are always written to disk. But the other kinds of temporary files might be stored in memory only and never written to disk. Whether or not temporary files other than the rollback, super, and statement journals are written to disk or stored only in memory depends on the SQLITE_TEMP_STORE compile-time parameter, the temp_store pragma, and on the size of the temporary file..."
My eyes have completely glazed over. If I add this to my app, what will it actually do? How can I even know?
Are you sure? I’ve had a lot of trouble getting that to work reliably myself across multiple OSes. (In hindsight I wish I’d used SQLite!) This article gives a good explanation of the many difficulties:
https://danluu.com/deconstruct-files/
My eyes have completely glazed over. If I add this to my app, what will it actually do? How can I even know?
Well, fundamentally it’s very hard to get it exactly right, and I imagine that’s why the implementation is a little involved.
But you could a) read through those docs, lengthy though they are, and/or b) trust the many testimonials saying SQLite is very, very robust and reliable.
(SQLite does this for you automatically, BTW.)
"Ensuring data reaches disk"
Unless you're on nfs. Remote file locking is hard, and I don't think that any nfs implementation has gotten to the point where you can trust SQLite on it.
SQLite does updates in place, which I would trust far less than a rename call.
No, and anyone who says yes is lying. (Lockless NFS exists and is no fun.)
> Well, fundamentally it’s very hard to get it exactly right, and I imagine that’s why the implementation is a little involved
SQLite has set itself the horrible task of updating files in-place. I know of two reliable, simpler alternatives:
1. Appending to files through O_APPEND
2. Rewriting files through rename()
If SQLite has different magic syscalls then I would very much like to learn.
(Haven’t googled, but if that’s possible, I don’t see why write would have that limitation)
Do you have any other resources regarding these types of low level "gotchas"?
I remember PostgreSQL having such an issue two years ago for example.
Be really, really, *really*, unambiguously sure about whether your data was written or not, AND have high confidence that I/O errors (eg, power loss) in the middle of does of deletes won't scramble (or truncate) existing data.
What you're looking at is the complexity required to solve for the wonderful tornado of "but it's my data really written???". But you don't have to deal with SQLite's implementation details in order for it to do its thing, which is what makes it so awesome (given is public domain status, what's more!).
neither XML, JSON nor zip solve the problems SQLite does, though; if you use plain old files, you need to make sure any changes you make actually end up on the disk, consistently. This is not easy to do. It also solves any consistency issues that might stem from someone reading the data while you're writing it.
On top of being just better, having a relational model for your data gives you much more freedom to use said data; you'll be able to do things efficiently that might require restructuring your JSON or XML format. Personally, I love SQLite-based application formats because I can explore them with SQL, which is often much easier than trying to make sense of a custom JSON or XML schema.