Android : How to keep data sync in android

Case : I am running a application in android, now how i do get know whether the server data has been changed as it can be changed from my website. Mechanism i can think of is: a) i should call server periodically and check whether the data has been changed recently by maintaining e-tag over there b) implement push notification and on getting a push notification i will call the server for the data change

Can there be something much than this, the other part is which one is the better approach(a,b) or both are obsolete

thanks for the coming help