Notification Service
mediumOpen designCommonly asked~45 minHLD roundSDE-2 and upAmazonUberSwiggyRazorpayFlipkartQueues & AsyncResilience
The question, as asked
“We need to send notifications — push, SMS, and email — across the company. Design that service.”
Scope it before you design it
The prompt is vague on purpose. These are the clarifying questions a strong candidate asks first — answer them out loud before you touch the architecture.
- Which channels — push, SMS, email, in-app? Each has a different provider, cost, latency and failure mode, and SMS costs real money per message.
- Transactional or promotional, or both? This is the single most important question: an OTP and a marketing blast have opposite requirements and should not share a path.
- Who triggers a send — other internal services, or a campaign tool? An API for services and a bulk campaign interface are different front doors onto the same pipeline.
- What's the biggest single campaign? 'Send to all 50 million users' is a fanout problem hiding inside what looks like a messaging problem.
- Do users have channel preferences and quiet hours? Almost always yes, and in most jurisdictions honouring opt-out is a legal requirement, not a feature.
- Do we need delivery tracking — sent, delivered, opened? That's a whole feedback pipeline, often larger in volume than the sends themselves.
- What are the latency expectations per type? An OTP is useless after 30 seconds; a weekly digest can be hours late.
Your answer, first
Commit something before anything unlocks. Saved on this device as you go.
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Framework hint
Sketch or write your approach above first. The hint gives you the structure, not the answer.