Node.js fs.writeFile() empties the file

Question

I have an update method which gets called about every 16-40ms, and inside I have this code:

this.fs.writeFile("./data.json", JSON.stringify({
    totalPlayersOnline: this.totalPlayersOnline,
    previousDay: this.previousDay,
    gamesToday: this.gamesToday
}), function (err) {
    if (err) {
        return console.log(err);
    }
});

If the server throws an error, the "data.json" file sometimes becomes empty. How do I prevent that?

Answer 1

Problem

fs.writeFile is not an atomic operation. Here is an example program which I will run strace on:

#!/usr/bin/env node
const { writeFile, } = require('fs');

// nodejs won’t exit until the Promise completes.
new Promise(function (resolve, reject) {
    writeFile('file.txt', 'content\n', function (err) {
        if (err) {
            reject(err);
        } else {
            resolve();
        }
    });
});

When I run that under strace -f and tidied up the output to show just the syscalls from the writeFile operation (which spans multiple IO threads, actually), I get:

open("file.txt", O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0666) = 9
pwrite(9, "content\n", 8, 0)            = 8
close(9)                                = 0

As you can see, writeFile completes in three steps.

1. The file is open()ed. This is an atomic operation that, with the provided flags, either creates an empty file on disk or, if the file exists, truncates it. Truncating the file is an easy way to make sure that only the content you write ends up in the file. If there is existing data in the file and the file is longer than the data you subsequently write to the file, the extra data will stay. To avoid this you truncate.
2. The content is written. Because I wrote such a short string, this is done with a single pwrite() call, but for larger amounts of data I assume it is possible nodejs would only write a chunk at a time.
3. The handle is closed.

My strace had each of these steps occurring on a different node IO thread. This suggests to me that fs.writeFile() might actually be implemented in terms of fs.open(), fs.write(), and fs.close(). Thus, nodejs does not treat this complex operation like it is atomic at any level—because it isn’t. Therefore, if your node process terminates, even gracefully, without waiting for the operation to complete, the operation could be at any of the steps above. In your case, you are seeing your process exit after writeFile() finishes step 1 but before it completes step 2.

Solution

The common pattern for transactionally replacing a file’s contents with a POSIX layer is to use these steps:

1. Write the data to a differently named file, fsync() the file (See “When should you fsync?” in “Ensuring data reaches disk”), and then close() it.
2. rename() (or, on Windows, MoveFileEx() with MOVEFILE_REPLACE_EXISTING) the differently-named file over the one you want to replace.

Using this algorithm, the destination file is either updated or not regardless of when your program terminates. And, even better, journalled (modern) filesystems will ensure that, as long as you fsync() the file in step 1 before proceeding to step 2, the two operations will occur in order. I.e., if your program performs step 1 and then step 2 but you pull the plug, when you boot up you will find the filesystem in one of the following states:

• None of the two steps are completed. The original file is intact (or if it never existed before, it doesn’t exist). The replacement file is either nonexistent (step 1 of the writeFile() algorithm, open(), effectively never succeeded), existent but empty (step 1 of writeFile() algorithm completed), or existent with some data (step 2 of writeFile() algorithm partially completed).
• The first step completed. The original file is intact (or if it didn’t exist before it still doesn’t exist). The replacement file exists with all of the data you want.
• Both steps completed. At the path of the original file, you can now access your replacement data—all of it, not a blank file. The path you wrote the replacement data to in the first step no longer exists.

The code to use this pattern might look like the following:

const { writeFile, rename, } = require('fs');

function writeFileTransactional (path, content, cb) {
    // The replacement file must be in the same directory as the
    // destination because rename() does not work across device
    // boundaries.

    // This simple choice of replacement filename means that this
    // function must never be called concurrently with itself for the
    // same path value. Also, properly guarding against other
    // processes trying to use the same temporary path would make this
    // function more complicated. If that is a concern, a proper
    // temporary file strategy should be used. However, this
    // implementation ensures that any files left behind during an 
    // unclean termination will be cleaned up on a future run.
    let temporaryPath = `${path}.new`;
    writeFile(temporaryPath, content, function (err) {
        if (err) {
            return cb(err);
        }

        rename(temporaryPath, path, cb);
    });
};

This is basically the same solution you’d use for the same problem in any langage/framework.