How can you build a real-time notification center?

Use Nodejs

They are implemented as, Comet, in which an active HTTP request in hidden iframe, receives a script tag with a call to predefined function with arguments that updates the page. lFrame will execute script as soon as it is received. This technique works well even in older version of IE. Disadvantage is, you need powerful server and you will need more servers to support more users as one server will support approximately 1000 comets. Web sockets, it is new concept introduced in HTML5, it is same as comet but it provides two way communication and you need to have modern web server and server side programming is different then that of comet. Technical limitations of more server for more users is still in place but it performs better than comet. And older versions of IE do not support and future versions are also doubtful. Polling, you can perform an AJAX request every few seconds to identify change, and update the Ul. Although it is network intensive process, it is still a good solution to keep small number of servers. It is not real time but it is close to realtime.

Here is a fairly technical answer for question similar to this one.

As pointed out in the other answers, there are several techniques available to enable real-time notifications, which go under several different umbrella terms, such as "Real-Time Web". Since there are many caveats in choosing the best technique for each user (WebSockets, HTTP Streaming, HTTP Long Polling, and all their different implementation flavors) in order to make sure it works through any existing proxy, firewall and antivirus software, the best approach is to use a ready-made abstraction layer, rather than building it form scratch. You can find many different solutions, both free and commercial. I am biased, as the creator of Lightstreamer, but I suggest you to check out our product ( http://www.lightstreamer.com ), which has been around for 12 years now and has been used in many critical applications (from finance to aerospace and military).

I would recommend using Web Sockets if you would like to take advantage of the new technology. Web Sockets will allow you to deliver notifications to the users browser only when a new one is available, although an active connection will be maintained with the server. You could also use AJAX, but this method will make a request to the server after every few seconds asking for any new update. Personally, I like the one which employs Web Sockets, rest depends on the traffic of the website and the servers running behind it. And yes, both of them have security risks involved, but then what doesn't involve risk?

If you'd rather not spend time building this, we just released a JS plugin that'll power a notification center in your web-app: http://www.knowtify.io/inbox  (we call it InBox). You can set it up in a few hours and start sending alerts/messages to your users immediately. We just came out of closed beta a few weeks back and still working on the kinks... let us know what you think. You can see our API docs here: http://api.knowtify.io/v1.0/docs/get-started-with-inbox

you can use pubnub.. try this link http://www.pubnub.com/ ... Just use it in the right way..