How would you design a system that sends order confirmation emails reliably, even if the email provider is down for a while?

What the interviewer is looking for

• This question tests distributed systems & scalability. Discusses queues, retries, idempotency, caching and failure modes, and reasons about when added complexity is justified.
• Strong answers: Decoupling with a queue or outbox pattern, retries with backoff, idempotency to avoid duplicates, dead letter handling, and monitoring the backlog.
• Likely follow-up: How do you make sure a customer never gets the same email twice?

Another way it may be asked: Customers must always get their order email, but the email service sometimes fails. How would you build this?

A structure to follow: Requirements, sketch, deep-dive, trade-offs

1. Step 1

   Requirements

   Ask who uses it, what it must do, and which qualities matter most: speed, reliability, cost, consistency.

2. Step 2

   Sketch

   Draw the main parts and the path one request takes through them before adding detail.

3. Step 3

   Deep-dive

   Pick the riskiest part (storage, queueing, failure handling) and go deeper there.

4. Step 4

   Trade-offs

   Name what you gave up for each choice, and what you would do first if it needed to grow.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Naming tools before establishing requirements.
• Drawing every box at the same depth instead of going deep where it matters.
• Presenting one design as the only answer, with no trade-offs.

How to practise it

Time yourself: two minutes on requirements, five on the sketch, five on one deep-dive, two on trade-offs.

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