Piping data from child to parent in nodejs

Question

I have a nodejs parent process that starts up another nodejs child process. The child process executes some logic and then returns output to the parent. The output is large and I'm trying to use pipes to communicate, as suggested in documentation for child.send() method (which works fine BTW).

I would like someone to suggest how to properly build this communication channel. I want to be able to send data from parent to child and also to be able to send data from child to parent. I've started it a bit, but it is incomplete (sends message only from parent to child) and throws an error.

Parent File Code:

var child_process = require('child_process');

var opts = {
    stdio: [process.stdin, process.stdout, process.stderr, 'pipe']
};
var child = child_process.spawn('node', ['./b.js'], opts);

require('streamifier').createReadStream('test 2').pipe(child.stdio[3]);

Child file code:

var fs =  require('fs');

// read from it
var readable = fs.createReadStream(null, {fd: 3});

var chunks = []; 

readable.on('data', function(chunk) {
    chunks.push(chunk);
});

readable.on('end', function() {
    console.log(chunks.join().toString());
})

The above code prints expected output ("test 2") along with the following error:

events.js:85
      throw er; // Unhandled 'error' event
            ^
Error: shutdown ENOTCONN
    at exports._errnoException (util.js:746:11)
    at Socket.onSocketFinish (net.js:232:26)
    at Socket.emit (events.js:129:20)
    at finishMaybe (_stream_writable.js:484:14)
    at afterWrite (_stream_writable.js:362:3)
    at _stream_writable.js:349:9
    at process._tickCallback (node.js:355:11)
    at Function.Module.runMain (module.js:503:11)
    at startup (node.js:129:16)
    at node.js:814:3

---

Full Answer:

Parent's code:

var child_process = require('child_process');

var opts = {
    stdio: [process.stdin, process.stdout, process.stderr, 'pipe', 'pipe']
};
var child = child_process.spawn('node', ['./b.js'], opts);

child.stdio[3].write('First message.\n', 'utf8', function() {
    child.stdio[3].write('Second message.\n', 'utf8', function() {

    });
}); 

child.stdio[4].pipe(process.stdout);

Child's code:

var fs =  require('fs');

// read from it
var readable = fs.createReadStream(null, {fd: 3});

readable.pipe(process.stdout);
fs.createWriteStream(null, {fd: 4}).write('Sending a message back.');

Answer 1

Your code works, but by using the streamifier package to create a read stream from a string, your communication channel is automatically closed after that string is transmitted, which is the reason you get an ENOTCONN error.

To be able to send multiple messages over the stream, consider using .write on it. You can call this as often as you like:

child.stdio[3].write('First message.\n');
child.stdio[3].write('Second message.\n');

If you want to use this method to send multiple discrete messages (which I believe is the case based on your remark of using child.send() before), it's a good idea to use some separator symbol to be able to split the messages when the stream is read in the child. In the above example, I used newlines for that. A useful package for helping with this splitting is event-stream.

Now, in order to create another communication channel from the child in the parent, just add another 'pipe' to your stdio.

You can write to it in the child:

fs.createWriteStream(null, {fd: 4}).write('Sending a message back.');

And read from it in the parent:

child.stdio[4].pipe(process.stdout);

This will print 'Sending a message back.' to the console.