System designHardsd-g496

Subject Video streamingLevel Senior–Staff~45 minCommon in Storage & CDN · Reliability & on-call · Distributed systems interviewsIndustries Technology, Software development

Question

Design the multi-CDN steering and failover system for a streaming service that delivers VOD + live across three commercial CDNs plus its own edge. You must route each viewer's segment requests to the CDN that's currently fastest/cheapest for them, detect a CDN degrading (a region browning out, error rate climbing) within seconds, and shift traffic away — all without breaking in-flight playback sessions or stampeding a healthy CDN. Scale: tens of millions of concurrent streams, a single live event can spike one region 10x.

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.