I created a Push Notification Server (PNS) by using Google Cloud Messaging (GCM). With this architecture, my server has to know the token associated to a user in order to send him push notifications.
I extended the class InstanceIDListenerService which is correctly notified when a token has been refreshed (I tested it with adb). When its method onTokenRefresh() is called I take the new token and, so far, all is ok.
My problem is the following: what if I fail to send this new token to my server? (a network error, a lack of connection, a sudden device shutdown, or something else...). Do I have to store somewhere (preferences?) that server does not know my new token and retry when possible? Is there a way to let the OS to perform this operation?
You may use GCM NetworkManager to queue, schedule, check network connections and... , its very reliable and useful, it handles all of your desired tasks.
Also you have to save your token some where is your device, may be on your shared preferences, till you make sure your server recives it. Please note that if you want to use other features of GCM like topic messaging, you have to save and use token for all topic registrations.