My question is this I implementing a system to clear our servers of expired GCM registration tokens.
Once a registrationID is rejected by GCM as not registered or expired can it ever become valid again ? For APNS in iOS this can occur, but it cannot happen for GCM correct?
Once a client updates its registration tokens with GCM and our servers try to send notifications downstream it can receive a result with a canonical ID listing the new registration ID. How long is the old registration ID valid?