What exactly is a node in node.js?

Question

In Erlang I was able to immediately understand the notion of a 'node' - a self-contained Erlang VM. I could start a node on one machine with erl -name gandalf -setcookie abc, and another node on another machine (on the same LAN) with erl -name bilbo -setcookie abc. I could then spawn processes on gandalf which would communicate magically with other processes on bilbo. Now, since I also wanted to serve up a jazzy webpage with animated graphical results from my Erlang processes, I picked up some Javascript and learnt jQuery. Still a humble paduwan, but I sort of understand how Javascript fits into the scheme of things.

I recently came across node.js and an (evil) voice started whispering: 'This is it! Now you can do everything with Javascript! Forget Erlang and guards and periods, stick to a language that everyone uses'.

I've read the docs a bit, but I still don't understand what a node is in node.js. Do I have to run a http server and that becomes my node? What if I don't like http, or I don't care how gandalf talks to bilbo - that's what I like in Erlang. Maybe I nai:vely expect that node.js is erlang with Javascript sugar?

Answer 1

As I understand it, a Node.JS node is an instance of the V8 engine with the Node.JS runtime and event-loop running in it. While the Node.JS runtime gives you the ability to very quickly and simply begin processing HTTP requests, it's not mandatory; it is very good at handling most any kind of asynchronous I/O, really.

I don't know that much about Erlang, but my superficial understanding is that its great strength is high-concurrency computing. Node.JS doesn't specialize in that, per se. Its heart is "evented I/O", dealing neatly and cleanly with asynchronous I/O.

Answer 2

Node.js has much more in common with Twisted than Erlang/OTP. Node.js is just a single threaded SEDA event loop. Node.js has nothing compared to Erlang VM when it comes to distribution, hot code reloading, and scalability via processes, it isn't anything close to "Erlang with Javascript sugar"

Answer 3

there is no "node" in node.js

as mentioned, when you run

node my_script.js 

you are running one instance of V8 java script interpreter (which is using one core for its lifetime).

Answer 4

Maybe because of your Erlang knowledge you thought that somehow Node.js had something to do with "nodes" (as erlang nodes), but it's just the name.

The main idea with Node.js is that you defer all expensive I/O operations and assign callbacks to the result of those operations. The reason is that I/O blocks the (only) process that is running at the moment. Node.js will handle this for you, given that you are coding in the proper way.

An easy example of this is a database call:

result = SQL.query("EXPENSIVE SELECT HERE")
doSomething(result);
moreStuff(); // This line must wait until the previous ones are completed.

In node you would code this in a very different way:

SQL.query("EXPENSIVE SELECT HERE", function(result) {
  doSomething(result);
});
moreStuff(); // This line executes inmediately

If you have wrong code in your Node.js script, like:

while(true) { }

Then you are blocking the process and it won't be able to handle more requests than the current one, so in Node.js is mandatory to follow the above guidelines.