Published: Modified: V1.9
node.js

Threading in node.js with the cluster module

JavaScript has a reputation of being a single threaded language, for the most part it is if I only take into consideration core javaScript by itself. However when taking into account the full breath of what there is to work with in client side javaScript, as well as a node.js environment, it would appear that javaScripts reputation of being single threaded language is wrong, or at best a half truth.

In client side javaScript there is of course webworker which to some extent can be used to achieve threading in client side javaScript. Yes there are some limitations as doing anything with the DOM can only be done in the main thread, not the webworker context. Still this web worker constructor can be used to spin up a whole other event loop as a separate process on the guest operating system, and messages can be passed to and from it from the main thread of a page. What happens in the web worker context will not end up holding up the main thread of the page then as it is running in a whole other event loop, with a whole other thread.

However this post is about the cluster module in a node.js environment, which is one of several options when it comes to starting up more than one event loop when it comes to working wit sever side javaScript.

1 - nodejs cluster basics and What is threading?

So a computer might have a single processor, and this single processor might only have a single core. On a system like this threading can be achieved by way of time slicing, that is switching between instructions very fast giving the illusion that they are being executed in parallel.

On modern computers a computer might have more than one processor, and each processor might have more than one core. On a system like this time slicing could be done, however true parallel exaction of instructions can be achieved as well.

As I understand it, threads are part of what makes up a process that runs on an operating system. There can be one or more threads per process, and then more than one process running on the operating system of a computer. All the threads might be running on the same core by way of time slicing, or on separate cores in parallel.

1.1 - Why do so many people say javaScript is single threaded, are they wrong?

Maybe, maybe not, for me it looks like a bit of a half truth, and of course it depends on the environment. In any javaScript environment if I stick to core javaScript by itself, using just setInterval, and setTimeout only by themselves, then I what I am making is very much single threaded. However in modern client side javaScript there is now using the WebWorker constructor, and in node.js there is the cluster module that can be used to pull of something that can be considered more in line with true threading.

In client side javaScrpt something that is written in core javaScript can be offloaded into a web worker, but anything that has to do with the DOM is still very much single threaded.

When it comes to server side javaScript there are ways of taking advantage of a computers many cores. This post will be covering use of the cluster module which can be used to fork a script for each core on the system. In addition there is the child-process module as well that can be used to launch anything that exists on the operating system, such as another script with node.js, or any other CLI tool written in any language for that matter, including tools that tack advantage of multi core systems.

1.2 - How can true threading be done with javaScript in node.js?

There are two built in modules that are of interest which are:

  • The child-process module
  • The cluster module

The child-process module is what can be used to run a command on the operating system, it can be some other software tool written in another language, or another script launched with node.js, in ether case it will run as it’s own independent process separate from the main script that launched it.

This post however is on the cluster module that can be used to fork a single script into more than one process.

1.3 - Basic example of the node.js cluster module

For a basic usage example I made a script that launches a fork for every core on the system that it runs, and each fork just counts to ten, and kills itself. Another useful core module in node.js is the os module that gives me information about the system that the node.js script is running on.

So I made a basic.js file in a test folder that looks like this:

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// using the node.js built in cluster module
let cluster = require('cluster'),
 
// also using the node.js built in os (operating system) module
os = require('os'),
 
// I can get a list of the systems cpus like this:
cpus = os.cpus(),
 
// standard start message
startmess = function () {
 
    let pid = process.pid,
    processType = cluster.isMaster ? 'Master' : 'Worker',
    worker = cluster.worker || {
        id: 0
    };
 
    console.log(processType + ' started on pid: ' + pid + ' worker.id: ' + worker.id);
 
};
 
// if this is the master
if (cluster.isMaster) {
 
    startmess();
 
    // fork this script for each cpu
    let i = 0,
    len = cpus.length;
    while (i < len) {
 
        cluster.fork();
 
        i += 1;
 
    }
 
    // on exit
    cluster.on('exit', function (worker, code, sig) {
 
        console.log('he\'s dead Jim');
        console.log(worker.id);
 
    });
 
} else {
 
    // else it is a fork
    startmess();
 
    let c = 0,
    rate = 1000,
    lt = new Date(),
    worker = cluster.worker;
 
    // end process after 1 sec
    setInterval(function () {
 
        console.log('worker # : ' + worker.id + ' : ' + c);
 
        if (c === 10) {
 
            cluster.worker.kill();
 
        }
 
        c += 1;
 
    }, 100);
 
}

As you can see I am still using setInterval to call a function more than once, but it is being used in it’s own separate process launched with cluster.fork, so it is not the same thing as just emulating threading which would be the case if I where using setInterval by itself.

2- Conclusion

In the future I might expand on this post, and others like it. There is a lot more to write about with clustering alone in node.js. However there is also a lot more to write about when it comes to this sort of thing in general and not just with nodejs. There only major built in module in nodejs to be aware of is the child process module that can be used to start a whole other process in general, not just with javaScript alone. The child process module can be used to start another process with the node binary, but it can also be used to call any binary on the system also.