[ ] Separate Subscription graphs (I suggest one graph per subscription method) from Overview graph
[ ] Overview graph:
[ ] Depict possible proprietary subscription APIs somewhere. In the case of Google Calendar, a client (or maybe even the Push Director?) can subscribe to Google Calendar API: Push notifications, which would then act as an Application Server, but with a proprietary protocol (it will send POST /push messages, but in their format).
POST /push
messages, but in their format).