150px-Nodejs_logo_lightNode.js is a platform built on Chrome’s JavaScript runtime for easily building fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices.

Node.js was created by Ryan Dahl starting in 2009

Traditional Web Servers

The “traditional” mode of web servers has always been one of the thread-based model. You launch Apache or any other web server and it starts receiving connections. When it receives a connection, it holds that connection open until it has performed the request for the page or whatever other transaction was sent. If it make take a few microseconds to retrieve a page from disk or write results to a database, the web server is blocking on that input/output operation. (This is referred to as “blocking I/O“.) To scale this type of web server, you need to launch additional copies of the server (referred to as “thread-based” because each copy typically requires another operating system thread).

Node.js Event Driven Mechanism

In contrast, Node.js uses an event-driven model where the web server accepts the request, spins it off to be handled, and then goes on to service the next web request. When the original request is completed, it gets back in the processing queue and when it reaches the front of the queue the results are sent back (or whatever the next action is). This model is highly efficient and scalable because the web server is basically always accepting requests because it’s not waiting for any read or write operations. (This is referred to as “non-blocking I/O” or “event-driven I/O“.)

To put it a bit more concretely, consider this process:

  1. You use your web browser to make a request for “/mypage.html” on a Node.js web server.
  2. The Node server accepts your request and calls a function to retrieve that file from disk.
  3. While the Node server is waiting for the file to be retrieved, it services the next web request.
  4. When the file is retrieved, there is a callback function that is inserted in the Node servers queue.
  5. The Node server executes that function which in this case would render the “/mypage.html” page and send it back to your web browser.

Here is a video on youtube  by Ryan Dahl on node.js.

You can have further information and download link from the node,hs official site. http://nodejs.org