javascript - Node.js and Mutexes

Question

I'm wondering if mutexes/locks are required for data access within Node.js. For example, lets say I've created a simple server. The server provides a couple protocol methods to add to and remove from an internal array. Do I need to protect the internal array with some type of mutex?

I understand Javascript (and thus Node.js) is single threaded. I'm just not clear on how events are handled. Do events interrupt? If that is the case, my app could be in the middle of reading the array, get interrupted to run an event callback which changes the array, and then continue processing the array which has now been changed by the event callback.

Answer 1

Locks and mutexes are indeed necessary sometimes, even if Node.js is single-threaded.

Suppose you have two files that must have the same content and not having the same content is considered an inconsistent state. Now suppose you need to change them without blocking the server. If you do this:

fs.writeFile('file1', 'content', function (error) {
    if (error) {
        // ...
    } else {
        fs.writeFile('file2', 'content', function (error) {
            if (error) {
                // ...
            } else {
                // ready to continue
            }
        });
    }
});

you fall in an inconsistent state between the two calls, when another function in the same script may be able to read the two files.

The rwlock module is perfect to handle these cases.