Slack / Discord — Simple

Problem statement (interviewer prompt)

Design Slack/Discord: workspaces with public + private channels, threads, reactions, DMs, mentions, bots, slash commands, voice rooms, and full-history search. Handle 5M concurrent WS connections, 3000 msgs/s sustained, and workspaces from 5 to 500k members.

flowchart LR
  C([Client])
  WS[WebSocket Gateway]
  CH[Channel Service]
  MSG[(Messages)]
  IDX[(Search index)]
  PUSH[Push]
  C --> WS --> CH --> MSG
  CH -. fan-out .-> WS --> C
  MSG --> IDX
  CH --> PUSH

    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 WS edge;
    class CH,PUSH service;
    class MSG,IDX datastore;