Error handling using fork - node.js

Question

In my below code error is not getting catched by Parent.js which is thrown by processChildOne.js
// Parent.js
var cp = require('child_process');
var childOne = cp.fork('./processChildOne.js');
var childTwo = cp.fork('./processChildTwo.js');
childOne.on('message', function(m) {
// Receive results from child process
console.log('received1: ' + m);
});
// Send child process some work
childOne.send('First Fun');
childTwo.on('message', function(m) {
// Receive results from child process
console.log('received2: ' + m);
});
// Send child process some work
childTwo.send('Second Fun');
// processChildOne.js
process.on('message', function(m) {
var conn = mongoose.createConnection('mongodb://localhost:27017/DB');
conn.on('error', console.error.bind(console, 'connection error:'));
// Pass results back to parent process
process.send("Fun1 complete");
});
If processChildOne.js fails, how to throw error to parent so that processChildOne.js and processChildTwo.js both should be killed. How can we keep track of how many child processes have executed and how many still are in pending.
Thanks in advance

Answer 1

I think whats going on, your child process is not really throwing an error, its writing to console.error, so there's no 'error' to catch in the parent process.
You may want to throw an error explicitly in the child, or an error will get thrown anyway by whatever library.. With this, I got the same problem you mention..
node.js
var cp = require('child_process').fork('./p1.js');
cp.on('message', function(){
console.log('ya', arguments);
})
p1.js
console.error('bad stuff man')
But this at least threw the error as expected
p1.js
throw "bad stuff man";
This worked for catching the error in the client and sending to parent process.
node.js
var cp = require('child_process').fork('./p1.js');
cp.on('message', function(){
console.log('error from client', arguments[0]);
})
p1.js
try{
throw "bad stuff man"
} catch(e){
process.send(e);
}
or for catching ALL errors in the client process and sending them to parent..
p1.js
process.on('uncaughtException', function(e){
process.send(e);
})
throw "bad stuff man";
For spawning multiple processes, and keeping track of the number, you should be able to do this..
node.js
var numprocesses = 5, running = 0;
for(var i = numprocesses; i--;){
var cp = require('child_process').fork('./p1.js');
cp.on('message', function(pid){
console.log('error from client', pid, arguments[0]);
})
cp.on('exit', function(){
console.log('done');
running--;
console.log('number running', running, ', remaining', numprocesses-running);
})
running++;
}
p1.js
process.on('uncaughtException', function(e){
process.send(process.pid + ': ' + e);
})
// simulate this to be a long running process of random length
setTimeout(function(){}, Math.floor(Math.random()*10000));
throw "bad stuff man";