Notification System — Simple

Problem statement (interviewer prompt)

Design a notification platform that sends 1B+ messages/day across push (iOS/Android/Web), SMS, and email. Support templates, user preferences (channel + quiet hours + frequency cap), at-least-once delivery with idempotency, and delivery/open tracking.

flowchart LR
  P[Producer Service]
  Q[[(Queue)]]
  W([Notification Workers])
  R[Routing<br/>by channel + prefs]
  PUSH((APNS / FCM))
  SMS[[Twilio / SNS]]
  EM((SES / SendGrid))
  USER([(User prefs / device tokens)])
  P --> Q --> W --> R
  USER -.lookup.-> R
  R --> PUSH
  R --> SMS
  R --> EM

    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 P,R service;
    class Q,USER datastore;
    class SMS queue;
    class W compute;
    class PUSH,EM external;