In my solution, think of notification service as just a delivery boy. It does not has smartness. All it understands is - get the data from the Queue assuming it has everything, communicate with Email GW and manage the sending of emails and handle failures.

In-app notifications is similar but a different beast. And one solution does not fit all. So there will be slight changes for in-app notification.

But the concern you raised is absolutely valid. And BTW, there is more to it -

1. like sending email in user specific language, who should be owning the conversion,

2. the example which you mentioned - profile pic URL of the user liking the comment,

3. how about attachments,

4. what if user has configured the notification settings off.

Will be covering some of these design level aspects in my next blog, with the trade-offs off-course. Stay tuned !!!