Hard Design: Notification System

You need to send a promotional notification to 50 million users at 9am in their local timezone. How do you architect this?

— Tests your understanding of this concept.

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This question is from the Design: Notification System topic (System Design Cases).

What is the difference between FCM and APNS?

What happens when you try to send a push notification to an invalid/expired device token?

How do you implement 'quiet hours' — not sending notifications between 10pm and 8am in user's local time?

Your email notification service sends 2 million emails/day. SendGrid has a 4-hour outage. How do you handle this?

What legal requirement mandates that marketing emails include an unsubscribe link?