How does the cluster module work in Node.js? - node.js

Question

Can someone explain in detail how the core cluster module works in Node.js?
How the workers are able to listen to a single port?
As far as I know that the master process does the listening, but how it can know which ports to listen since workers are started after the master process? Do they somehow communicate that back to the master by using the child_process.fork communication channel? And if so how the incoming connection to the port is passed from the master to the worker?
Also I'm wondering what logic is used to determine to which worker an incoming connection is passed?

Answer 1

I know this is an old question, but this is now explained at nodejs.org here:
The worker processes are spawned using the child_process.fork method,
so that they can communicate with the parent via IPC and pass server
handles back and forth.
When you call server.listen(...) in a worker, it serializes the
arguments and passes the request to the master process. If the master
process already has a listening server matching the worker's
requirements, then it passes the handle to the worker. If it does not
already have a listening server matching that requirement, then it
will create one, and pass the handle to the worker.
This causes potentially surprising behavior in three edge cases:
server.listen({fd: 7}) -
Because the message is passed to the master,
file descriptor 7 in the parent will be listened on, and the handle
passed to the worker, rather than listening to the worker's idea of
what the number 7 file descriptor references.
server.listen(handle) -
Listening on handles explicitly will cause the
worker to use the supplied handle, rather than talk to the master
process. If the worker already has the handle, then it's presumed that
you know what you are doing.
server.listen(0) -
Normally, this will cause servers to listen on a
random port. However, in a cluster, each worker will receive the same
"random" port each time they do listen(0). In essence, the port is
random the first time, but predictable thereafter. If you want to
listen on a unique port, generate a port number based on the cluster
worker ID.
When multiple processes are all accept()ing on the same underlying
resource, the operating system load-balances across them very
efficiently. There is no routing logic in Node.js, or in your program,
and no shared state between the workers. Therefore, it is important to
design your program such that it does not rely too heavily on
in-memory data objects for things like sessions and login.
Because workers are all separate processes, they can be killed or
re-spawned depending on your program's needs, without affecting other
workers. As long as there are some workers still alive, the server
will continue to accept connections. Node does not automatically
manage the number of workers for you, however. It is your
responsibility to manage the worker pool for your application's needs.

Answer 2

NodeJS uses a round-robin decision to make load balancing between the child processes. It will give the incoming connections to an empty process, based on the RR algorithm.
The children and the parent do not actually share anything, the whole script is executed from the beginning to end, that is the main difference between the normal C fork. Traditional C forked child would continue executing from the instruction where it was left, not the beginning like NodeJS. So If you want to share anything, you need to connect to a cache like MemCache or Redis.
So the code below produces 6 6 6 (no evil means) on the console.
var cluster = require("cluster");
var a = 5;
a++;
console.log(a);
if ( cluster.isMaster){
worker = cluster.fork();
worker = cluster.fork();
}

Answer 3

Here is a blog post that explains this

Answer 4

As an update to #OpenUserX03's answer, nodejs has no longer use system load-balances but use a built in one. from this post:
To fix that Node v0.12 got a new implementation using a round-robin algorithm to distribute the load between workers in a better way. This is the default approach Node uses since then including Node v6.0.0