API Gateway — Simple

Problem statement (interviewer prompt)

Design a single entry point that fronts dozens of internal microservices for external clients. It must handle TLS termination, authentication, rate limiting, request routing by host/path, response aggregation, and surface a consistent API contract — all with <5ms p99 added latency.

A single entry point that fronts internal services, handling auth, rate limiting and routing.

flowchart LR
  C([Client])
  GW[API Gateway]
  S1([User Service])
  S2[Order Service]
  S3[Payment Service]
  C --> GW
  GW --> S1
  GW --> S2
  GW --> S3

    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,S1 client;
    class GW edge;
    class S2,S3 service;