System designHardsd-g502

Subject Adaptive bitrateLevel Senior–Staff~45 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design server-side ad insertion (SSAI) for an ad-supported streaming service that stitches personalized ads into the video stream at the manifest/segment level so the ads can't be blocked and play seamlessly (no buffering at the ad boundary). Each viewer gets different ads (targeting), so you can't serve one shared manifest; you must decide ad breaks, call an ad decisioning service per viewer, splice ad segments into the content timeline, and track that ads were actually watched — for live and VOD, at millions of concurrent streams.

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.