Search code examples
node.jsfile-iofilesystemstransactionalacid

Transactionally writing files in Node.js

I have a Node.js application that stores some configuration data in a file. If you change some settings, the configuration file is written to disk.

At the moment, I am using a simple fs.writeFile.

Now my question is: What happens when Node.js crashes while the file is being written? Is there the chance to have a corrupt file on disk? Or does Node.js guarantee that the file is written in an atomic way, so that either the old or the new version is valid?

If not, how could I implement such a guarantee? Are there any modules for this?

Solution

  • What happens when Node.js crashes while the file is being written? Is there the chance to have a corrupt file on disk? Or does Node.js guarantee that the file is written in an atomic way, so that either the old or the new version is valid?

    Node implements only a (thin) async wrapper over system calls, thus it does not provide any guarantees about atomicity of writes. In fact, fs.writeAll repeatedly calls fs.write until all data is written. You are right that when Node.js crashes, you may end up with a corrupted file.

    If not, how could I implement such a guarantee? Are there any modules for this?

    The simplest solution I can come up with is the one used e.g. for FTP uploads:

    1. Save the content to a temporary file with a different name.
    2. When the content is written on disk, rename temporary file to destination file.

    The man page says that rename guarantees to leave an instance of newpath in place (on Unix systems like Linux or OSX).