Node's goal is to provide an easy way to build scalable network programs

In the "hello world" web server example below, many client connections can be handled concurrently. Node tells the operating system (through epoll, kqueue, /dev/poll, or select) that it should be notified when a new connection is made, and then it goes to sleep. If someone new connects, then it executes the callback. Each connection is only a small heap allocation.

var http = require('http');
http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World\n');
}).listen(1337, "");
console.log('Server running at');

This is in contrast to today's more common concurrency model where OS threads are employed. Thread-based networking is relatively inefficient and very difficult to use. See: this and this. Node will show much better memory efficiency under high-loads than systems which allocate 2mb thread stacks for each connection. Furthermore, users of Node are free from worries of dead-locking the process—there are no locks. Almost no function in Node directly performs I/O, so the process never blocks. Because nothing blocks, less-than-expert programmers are able to develop fast systems.

Node is similar in design to and influenced by systems like Ruby's Event Machine or Python's Twisted. Node takes the event model a bit further—it presents the event loop as a language construct instead of as a library. In other systems there is always a blocking call to start the event-loop. Typically one defines behavior through callbacks at the beginning of a script and at the end starts a server through a blocking call like EventMachine::run(). In Node there is no such start-the-event-loop call. Node simply enters the event loop after executing the input script. Node exits the event loop when there are no more callbacks to perform. This behavior is like browser javascript—the event loop is hidden from the user.

HTTP is a first class protocol in Node. Node's HTTP library has grown out of the author's experiences developing and working with web servers. For example, streaming data through most web frameworks is impossible. Node attempts to correct these problems in its HTTP parser and API. Coupled with Node's purely evented infrastructure, it makes a good foundation for web libraries or frameworks.

But what about multiple-processor concurrency? Aren't threads necessary to scale programs to multi-core computers? You can start new processes via child_process.fork() these other processes will be scheduled in parallel. For load balancing incoming connections across multiple processes use the cluster module.

See also: