How can I execute a node.js module as a child process of a node.js program?

Question

Here's my problem. I implemented a small script that does some heavy calculation, as a node.js module. So, if I type "node myModule.js", it calculates for a second, then returns a value. Now, I want to use that module from my main Node.JS program. I could just put all the calculation in a "doSomeCalculation" function then do:

var myModule = require("./myModule");
myModule.doSomeCalculation();

But that would be blocking, thus it'd be bad. I'd like to use it in a non-blocking way, like DB calls natively are, for instance. So I tried to use child_process.spawn and exec, like this:

var spawn = require("child_process").spawn;
var ext = spawn("node ./myModule.js", function(err, stdout, stderr) { /* whatevs */ });
ext.on("exit", function() { console.log("calculation over!"); });

But, of course, it doesn't work. I tried to use an EventEmitter in myModule, emitting "calculationDone" events and trying to add the associated listener on the "ext" variable in the example above. Still doesn't work.

As for forks, they're not really what I'm trying to do. Forks would require putting the calculation-related code in the main program, forking, calculating in the child while the parent does whatever it does, and then how would I return the result?

So here's my question: can I use a child process to do some non-blocking calculation, when the calculation is put in a Node file, or is it just impossible? Should I do the heavy calculation in a Python script instead? In both cases, how can I pass arguments to the child process - for instance, an image?

Artefact

"Good afternoon"? It could be morning (or even worse!) somewhere else :) – Aristotelianism

Answer 1

I think what you're after is the child_process.fork() API.

For example, if you have the following two files:

In main.js:

var cp = require('child_process');
var child = cp.fork('./worker');

child.on('message', function(m) {
  // Receive results from child process
  console.log('received: ' + m);
});

// Send child process some work
child.send('Please up-case this string');

In worker.js:

process.on('message', function(m) {
  // Do work  (in this case just up-case the string
  m = m.toUpperCase();

  // Pass results back to parent process
  process.send(m.toUpperCase(m));
});

Then to run main (and spawn a child worker process for the worker.js code ...)

$ node --version
v0.8.3

$ node main.js 
received: PLEASE UP-CASE THIS STRING

Suddenly

The call should better be cp.fork(__dirname + '/worker'); to make it work if you run main.js from another location. – Joost

Answer 2

It doesn't matter what you will use as a child (Node, Python, whatever), Node doesn't care. Just make sure, that your calculcation script exits after everything is done and result is written to stdout.

Reason why it's not working is that you're using spawn instead of exec.

Aristotelianism