System designHardsd-g701

Subject Webhook ordering retry protocolLevel Senior–Staff~45 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

Design the delivery engine for outbound webhooks where per-endpoint event ORDER matters (e.g. invoice.created must reach the customer before invoice.paid) and customer endpoints are flaky. 200M events/day, ~80k destination endpoints. You need at-least-once delivery, strict per-resource ordering, retries with backoff that don't reorder, and a way for slow endpoints not to block everyone. Focus on the ordering + retry guarantees, not the public API.

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.

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