🚀 System Design Interview: How Would You Send 1 Million Notifications Without Crashing Your System?

Suneel Kumar

6 min read

·

Aug 6, 2026

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What looks like a simple “Send Notification” button can become one of the most challenging distributed systems problems you’ll encounter.

Imagine you’re interviewing for a backend engineering role.

The interviewer smiles and asks:

“Design a notification service capable of sending one million notifications without overwhelming the system.”

At first glance, it sounds straightforward.

Create an API.
Store the notification.
Loop through users.
Send it.

Done, right?

Not quite.

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The moment you start thinking about real-world scale, the problem becomes much more interesting.

• What if one million users receive the notification at exactly the same moment?
• What happens if Firebase rejects thousands of requests?
• How do you retry failed notifications without sending duplicates?
• How do you prevent your database from melting under sudden traffic?
• What if email, SMS, and push notifications all have different…