notification in social network application

I am currently working on application like social networking. In this application if one user sends message or add request to any other user,so how can i achieve that.

For example, There are two users,USER A & USER B. If USER A sends message to USER B, then the USER A should get the notification in his profile immediately.

So what should be the functionality at USER A side &a开发者_JAVA百科mp; USER B side. Please give me some idea for this.

I think you can achieve this using AJAX using the method onreadystatechange()

hope that helps