unix sockets on nodeJs

Question

Quick and basic nodeJs question, I'm working with unix socket for inter-server communication between c++ application and my NodeJs server,

I've wrote my nodeJs server like so:

var net = require('net');
var unixSocketServer = net.createConnection('/tmp/unixSocket');
unixSocketServer.on('connect',function(){
    console.log('unix server socket connected on /tmp/unixSocket');
    ...
});

However I'm getting connection refuse error. I can understand that the c++ application haven't opened/connected to the socket yet. My questions are why does it matter? shouldn't the nodeJs server wait until 'connect' event emitted? Am I using nodeJs currently? am I missing something?

Answer 1

Ok, so there's some misunderstanding here. Let's start from what a 'unix socket' really is. I think you're thinking it's just a file-like item that acts like a server on its own through the OS/filesystem. That's not quite correct. While it is indeed bound through the filesystem, it isn't really a traditional file. Instead, it's just like an TCP/IP socket, except instead of binding an IP and port, a filepath is bound.

The key point there is that it's just a bound socket with a different type of address (and some extra capabilities, but that's outside the scope here). So that means that something has to bind the socket! In this case, we need a server, just like we would if we were communicating over a 'normal' port. If there isn't a server bound to the path, you get an error, just like you would when connecting to a port with no listener.

To create a server on a unix domain socket in node, it's pretty simple:

'use strict';

const net = require('net');
const unixSocketServer = net.createServer();

unixSocketServer.listen('/tmp/unixSocket', () => {
  console.log('now listening');
});

unixSocketServer.on('connection', (s) => {
  console.log('got connection!');
  s.write('hello world');
  s.end();
});

Note that there is one other difference between 'normal' sockets and unix domain sockets: After a server is done with the unix domain socket, it is not automatically destroyed. You must instead unlink /tmp/unixSocket in order to reuse that 'address'/path