title: Detecting IAP Cancellation
body: <p>I generally understand receipt validation on In App Purchases. Most libraries make this as simple as calling a method.</p>
  
  <p>I'm not so clear on how subscriptions work though -- specifically, how can I detect if the subscription is cancelled?</p>
  
  <p>Subscriptions are cancelled on Apple's iTunes interface only. How is my server supposed to know that a subscription was cancelled?</p>