I've noticed from my DNS logs that domains associated with a small minority of my 3rd-party apps are accessed approximately every half hour or every hour, even when the relevant app is not active, and even when the phone is locked.
My understanding in the past was that 3rd-party apps do not execute unless they are the active app, and that notifications all come through a centralized Apple service. What mechanism is being used to allow this communication, and what reasons are there for these apps to need to phone home so frequently when there is no app activity?