how does whatsapp service gets restarted even if i force stop app?

Question

I am running whatsapp (we could call it appX from now on) in device A. I go to manage applications -> force close so appX gets closed and i no longer see appX as running ser

Answer 1

its nothing magical here appX uses push notifications via android GCM platform https://developer.android.com/google/gcm/index.html in GCM the app registers for a braodcast reciever and the broadcast receiver starts the service on getting the push notification. Android GCM is dependent on google play services that are available on android 2.2 and beyond that therefore you didn't see the message on 2.1 device