Ask HN: How are you building real-time web and mobile notifications?

I recently noticed that I've built multiple realtime notifications systems from scratch, often inspired by the Facebook notifications experience.

There have been a variety of features and combinations, but I've often needed:

- Context-based fallback from websockets to mobile and then to email when attempting to notify a particular user.

- Suppression of notifications if the user is online and actively engaged in the relevant context (e.g. in a specific conversation).

- Merging of multiple notifications about the same topic (e.g. "John and Jane liked your post").

- Retrieval of the latest notifications and read status (to show count badge).

- Support for desktop push notifications (service worker in Chrome/Firefox or Safari Notifications).

What do HN readers do when they need these features? Is there a product out there that I just haven't noticed, or is everyone rolling their own?

There seem to be a few services concerned with just the simple notification transport and channel subscriptions (e.g. Pusher), but that only solves part of the problem.