CDN Service — Simple

Problem statement (interviewer prompt)

Design a CDN like Cloudflare or Fastly: serve cached content from globally distributed edge POPs, handle TLS termination, support purge by URL + surrogate-key + entire domain, run edge compute (workers), and front a slow origin with hit ratios >90%.

flowchart LR
  U([User])
  EDGE[Edge POP]
  MID[Mid-tier Shield]
  ORIG[Origin]
  U --> EDGE
  EDGE -. miss .-> MID
  MID -. miss .-> ORIG

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class U client;
    class EDGE edge;
    class MID,ORIG service;

See 00-fundamentals/cdn for the canonical mental model. This page covers the CDN as a product/service offering.