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<h1>Node's goal is to provide an easy way to build scalable
network programs</h1>
<p>In the "hello world" web server example
below, many client connections can be handled concurrently.
Node tells the operating system (through <code>epoll</code>,
<code>kqueue</code>, <code>/dev/poll</code>, or
<code>select</code>) 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.</p>
<pre>
var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Hello World\n');
}).listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');</pre>
<p>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: <a
href="http://www.kegel.com/c10k.html">this</a> and <a
href="http://bulk.fefe.de/scalable-networking.pdf">this</a>.
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.</p>
<p>Node is similar in design to and influenced by systems like
Ruby's <a href="http://rubyeventmachine.com/">Event
Machine</a> or Python's <a
href="http://twistedmatrix.com/">Twisted</a>. 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 <code>EventMachine::run()</code>. 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.</p>
<p>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 <a
href="https://github.com/joyent/http-parser/tree/master">parser</a>
and API. Coupled with Node's purely evented infrastructure, it
makes a good foundation for web libraries or frameworks.</p>
<p>But what about multiple-processor concurrency? Aren't
threads necessary to scale programs to multi-core computers?
You can start new processes via <code><a
href="http://nodejs.org/api/child_process.html#child_process.fork">child_process.fork()</a></code>
these other processes will be scheduled in parallel. For load
balancing incoming connections across multiple processes use
<a href="http://nodejs.org/api/cluster.html">the
cluster module</a>.</p>
<p>See also:</p>
<ul>
<li><a href="http://s3.amazonaws.com/four.livejournal/20091117/jsconf.pdf">Slides from JSConf 2009</a></li>
<li><a href="http://nodejs.org/jsconf2010.pdf">Slides from JSConf 2010</a></li>
<li><a href="http://www.yuiblog.com/blog/2010/05/20/video-dahl/">Video from a talk at Yahoo in May 2010</a></li>
</ul>
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