Having worked with NodeJS for some time now, I've been wondering about how node handles file operations internally.
Considering the following pseudo code:
initialize http server
on connection:
modify_some_file:
on success:
print "it worked"
Let's consider two users A & B that try to access the page nearly simultaneously. Let's further assume A is the first one to connect, then the following happens:
And here's what I'm wondering about: Let's say, the file operation isn't done yet and B connects, what does node do? How and when does it access the file when it is still in the process of "being modified"?
I hope my question is somewhat clear ;) Looking forward to your answers!
AFAIK, Node won't care.
At least on Unix, it's perfectly legal to have multiple writers to the same file. Sometimes that's not a problem (say your file consists of fixed-size records, where writer #1 writes to record X and writer #2 writes to record Y, with X !== Y), and sometimes it is (same example: when both writers want to write to record X).
In Node, the problems are mitigated because I/O operations "take turns", but I think there's still potential of two writers getting in each others way. It's up to the programmer to make sure that doesn't happen.
With Node, you could use the *Sync() versions of the fs operations (but those will block your app during the operation), use append mode (which is only atomic up to certain write sizes I think, and it depends on your requirements if appending is actually useful), use some form of locking, or use something like a queue where write operations would be put onto the queue and there's a single queue consumer to handle the writes.