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WhatsApp / Messenger — Simple#

Problem statement (interviewer prompt)

Design WhatsApp/Messenger: 1:1 + group chat with E2E encryption, presence + typing + read receipts, voice/video calls, multi-device sync, and push notifications when offline. Handle 100B+ messages/day and 2B users.

flowchart LR
  A([User A])
  B([User B])
  WS[WebSocket Gateway]
  CH[Chat Service]
  MQ[[(Message Inbox)]]
  PRES[Presence]
  PUSH((APNS / FCM))
  A --> WS --> CH --> MQ
  CH --> WS
  WS --> B
  PRES --- WS
  CH -. offline .-> PUSH --> B

    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 A,B client;
    class WS edge;
    class CH,PRES service;
    class MQ datastore;
    class PUSH external;