title: When do GCM Tokens Expire and What is the InstanceID?
body: <p>Since GCM keeps getting updated, most of the resources I have searched seem outdated or unclear. Basically, I am confused over when the tokens and ID's expire. (For reference, I am working with Android.)</p>
  
  <p>From what I understand (and please correct me if I'm wrong), my server has an API Key and a Sender ID. Using the Sender ID I can have my client request a Token via the InstanceID stored locally on my client. I'm already a bit confused here. The InstanceID is assigned the moment my app goes online? Does it ever change? What about when the app upgrades or is uninstalled and reinstalled (or the device is restored)? By calling InstanceID.getInstance will I always retrieve the same InstanceID, or will it eventually expire and give me a new one? Is there any value to storing the string you retrieve by calling getID()? The docs seem to indicate that you actually retrieve a new InstanceID when you call getID(), so that complicates things even more. (For reference, I'm referring to: <a href="https://developers.google.com/instance-id/">https://developers.google.com/instance-id/</a>)</p>
  
  <p>Using the InstanceID, my client can request a Token from the GCM servers, which it then sends to my app server. My app server stores this Token, and can use this to send messages to the GCM servers, which will then send the message to the device. The device uses the stored InstanceID to actually receive these messages, I believe. So having a class that extends GcmListenerService will allow me to receive these messages with onMessageReceived? I don't have to do anything special (other than defining it in the AndroidManifest)? I don't have to actually tell it to use the InstanceID? It just magically knows?</p>
  
  <p>When do these ID's and Tokens expire? Do they expire? I store the Token as a string on the server, but if at any point one of these expires, how do I know they have expired? I can always generate a new InstanceID and Token, that much seems easy, but then do the old ones stay active? How do I wipe the old tokens from the server? There seems to be an easy way to do this with APNS on the iOS side of things, where you can retrieve a list of all the expired tokens and just wipe them from your database.</p>