Designing a CDN: a system design walkthrough for interviews

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A CDN design is framed not as a simple file server but as a globally distributed read-through cache that serves requests from nearby edge servers. The core challenges are routing each user to a healthy nearby edge and keeping cached content fresh and invalidated correctly. Because geography and caching are essentially the same problem, the design aims to answer most requests from a close cache hit while keeping cache misses cheap and safe for the origin. The walkthrough follows the structure a candidate typically draws in a roughly 45-minute interview round, covering requirements, estimates, data model, high-level design, and deep dives. It also highlights a frequent follow-up scenario: a sudden viral surge that causes a cache-miss storm against a single origin.

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