System designHardsd-g276

Subject Messaging queuesLevel Senior–Staff~50 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

Design a webhook-delivery system that reliably delivers event notifications to millions of customer-owned HTTP endpoints. You generate ~500k events/sec; each customer endpoint must receive its events in order, at-least-once, with retries and exponential backoff when their server is down or slow. A single broken/slow endpoint must not back up delivery for everyone else, and customers need a way to replay missed events. How do you deliver at scale while isolating slow/failing destinations and guaranteeing ordering + retries?

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.