title: Are GCM subscription IDs and instance IDs the same thing? (subscribe to topics via web)
body: <p>I'm trying to set up GCM for HTML5 push notifications through Chrome. I want these subscriptions to use topics, so that I don't have to maintain a list of subscribed IDs myself. However, all of the topic documentation I can find refers to subscribing through the iOS and Android APIs, but not through the web.</p>
  
  <p>I found <a href="https://developers.google.com/instance-id/reference/server#create_a_relation_mapping_for_an_app_instance" rel="nofollow">documentation on relationship mapping</a> that makes it seem like I can control topic subscriptions by sending requests from my server, but it requires an instance ID. The <a href="https://developers.google.com/web/fundamentals/getting-started/push-notifications/step-06?hl=en" rel="nofollow">documentation I've been able to find for Chrome web notifications</a> talks about a subscription ID.</p>
  
  <p>Are they the same? i.e., could I send my subscription ID to my server, to then forward onto the GCM server API? Or are the instance ID and subscription ID two different things? They don't look the same in the examples provided (the instance ID start xxxxxx:yyyyy) so I'm concerned this won't work.</p>