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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.
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A mutex is a mutual exclusion object which creates a resource that can be shared between multiple threads of a program. The resource can be seen as a lock that only one thread can acquire. If another thread wants to acquire the lock, it has to wait until the lock is released.
Yes. It can. Race condition in Nodejs is feasible when you use cluster module to initialise multiple workers.
In languages with actual concurrency, this will lock up the waiting threads. In JavaScript, nothing will lock up, but the steps that lead to the completion of taskA will never be scheduled by the event loop. function taskA() { return mutex.
A race condition is a type of programming error that can occur when multiple processes or threads are accessing the same shared resource, for instance, a file on a file system or a record in a database, and at least one of them is trying to modify the resource. Let's try to present an example.
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.
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I'm wondering if mutexes/locks are required for data access within Node.js.
Nope! Events are handled the moment there's no other code to run, this means there will be no contention, as only the currently running code has access to that internal array. As a side-effect of node being single-threaded, long computations will block all other events until the computation is done.
I understand Javascript (and thus Node.js) is single threaded. I'm just not clear on how events are handled. Do events interrupt?
Nope, events are not interrupted. For example, if you put a while(true){} into your code, it would stop any other code from being executed, because there is always another iteration of the loop to be run.
If you have a long-running computation, it is a good idea to use process.nextTick, as this will allow it to be run when nothing else is running (I'm fuzzy on this: the example below shows that I'm probably right about it running uninterrupted, probably).
If you have any other questions, feel free to stop into #node.js and ask questions. Also, I asked a couple people to look at this and make sure I'm not totally wrong ;)
var count = 0; var numIterations = 100; while(numIterations--) { process.nextTick(function() { count = count + 1; }); } setTimeout(function() { console.log(count); }, 2); // //=> 100 // Thanks to AAA_awright of #node.js :)
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