Automatically hit a server every few seconds and receive data?

I have a local node.js server which has some new data every few seconds and also i have a iPhone app by using which right now i am able to get the JSON data from server and able to display it in my application.

But in current process user has to hit a option to get the data from server but i want to hit the server every 30 seconds automatically without user interaction and if any new data is there i want to get the data and display it.

So what is the best way to do this ? Also is there any iOS Library is there which will help me to do this?