Up and Running with Node.js
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.
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.
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.