Notification Delivery

Functional requirements

• Accept an event and deliver it to the right recipients
• Support several channels: in-app, email, push
• Respect per-user channel preferences and quiet hours
• Collapse repeated notifications about the same subject
• Let a user read their in-app history

Non-functional requirements

• In-app delivery should feel immediate; email may take longer
• A user must never receive the same notification twice
• A failing channel provider must not block the others
• Preferences must be honoured even under retry

Scale assumptions

Worked estimates for this brief, not measurements of any real system. Disagree with them out loud — that is the exercise.

Retention

90 days of in-app history

Events per day

20 million

Average fan-out

~8 recipients per event

Peak multiplier

~5x, driven by scheduled digests

Deliveries per day

~160 million (~1,600/second average)

Suggested API surface

• POST/v1/events

  Publish an event for fan-out

• GET/v1/notifications

  The signed-in user's in-app feed

• POST/v1/notifications/read

  Mark one or many as read

• PUT/v1/preferences

  Per-channel and quiet-hour settings

Decisions you should be able to defend

• When fan-out happens
• How duplicates are prevented
• Channel isolation
• Very large fan-outs