CDN — Simple

Problem statement (interviewer prompt)

Design a global content delivery layer that replicates origin assets to edge POPs worldwide. Hot objects must be served in <50ms p99 from the nearest edge; cache invalidation must propagate in seconds; and origin egress should drop by 90%+ for static workloads.

A CDN replicates origin content to globally distributed edge POPs so clients hit a nearby cache instead of the origin.

flowchart LR
  U1([User EU])
  U2([User US])
  U3([User APAC])
  E1[Edge POP EU]
  E2[Edge POP US]
  E3[Edge POP APAC]
  O[Origin]
  U1 --> E1
  U2 --> E2
  U3 --> E3
  E1 -. miss .-> O
  E2 -. miss .-> O
  E3 -. miss .-> O

    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 U1,U2,U3 client;
    class E1,E2,E3 edge;
    class O service;