How to debug Node.JS child forked process?

Question

How to debug Node.JS child forked process?

I'm trying to debug the child Node.JS process created using:

var child = require('child_process');
child .fork(__dirname + '/task.js');

The problem is that when running in IntelliJ/WebStorm both parent and child process start on the same port.

debugger listening on port 40893
debugger listening on port 40893

So it only debugs the parent process.

Is there any way to set IntelliJ to debug the child process or force it to start on a different port so I can connect it in Remote debug?

node.js
debugging
intellij-idea

Answer 1

Yes. You have to spawn your process in a new port. There is a workaround to debug with clusters, in the same way you can do:

Start your app with the --debug command and then:

var child = require('child_process');
var debug = typeof v8debug === 'object';
if (debug) {   
    //Set an unused port number.    
    process.execArgv.push('--debug=' + (40894));    
}    
child.fork(__dirname + '/task.js');

debugger listening on port 40894

Answer 2

It is a known bug in node.js that has been recently fixed (although not backported to v0.10).

See this issue for more details: https://github.com/joyent/node/issues/5318

There is a workaround where you alter the command-line for each worker process, although the API was not meant to be used this way (the workaround might stop working in the future). Here is the source code from the github issue:

var cluster = require('cluster');
var http = require('http');

if (cluster.isMaster) {
  var debug = process.execArgv.indexOf('--debug') !== -1;
  cluster.setupMaster({
    execArgv: process.execArgv.filter(function(s) { return s !== '--debug' })
  });
  for (var i = 0; i < 2; ++i) {
    if (debug) cluster.settings.execArgv.push('--debug=' + (5859 + i));
    cluster.fork();
    if (debug) cluster.settings.execArgv.pop();
  }
}
else {
  var server = http.createServer(function(req, res) {
    res.end('OK');
  });
  server.listen(8000);
}