CODEWITHSUNDEEP
javascriptnode.jsfileasynchronousio
R.G.
R.G.

Reputation: 391

How does NodeJS handle async file IO?

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!

Upvotes: 4

Views: 395

Answers (1)

robertklep
robertklep

Reputation: 203231

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.

Upvotes: 3

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