URL Filter / Web Safe Browsing — Simple

Problem statement (interviewer prompt)

Design a URL / web-safe-browsing filter (Google Safe Browsing / Cisco Umbrella): clients query whether a URL is malicious; you must answer in <50ms while keeping a hash-prefix database on the client to avoid leaking browsing history.

flowchart LR
  C([Client])
  URL[URL]
  LOOK[Filter Service]
  HASH[Hash prefix lookup]
  DB[(Threat list)]
  C --> URL --> HASH --> LOOK
  LOOK --> DB
  LOOK --> C

    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 C client;
    class URL,LOOK,HASH service;
    class DB datastore;