System designHardsd-g315

Subject Request coalescingLevel Senior–Staff~40 minCommon in Distributed systems interviewsIndustries Technology

Question

A CDN with 200 PoPs serves a popular live-event video manifest that's regenerated every 2 seconds. On each refresh, every PoP misses and hits the origin; with 200 PoPs each running many cache nodes, the origin sees a request storm every 2 seconds. Design request coalescing across the whole hierarchy so the origin sees roughly one request per refresh interval, not thousands. Cover within-PoP coalescing, cross-PoP coalescing (origin shield), and what happens on the failure/timeout path so a slow origin response doesn't pile up coalesced waiters.

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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