Pub/Sub Pattern — Simple

Problem statement (interviewer prompt)

Design a messaging substrate that decouples producers from consumers. It should support topic-based fan-out, per-key ordering, at-least-once delivery (with optional exactly-once), bounded retention, and consumer groups for parallel processing.

flowchart LR
  P1[Publisher A]
  P2[Publisher B]
  B((Broker / Topic))
  S1[Subscriber 1]
  S2[Subscriber 2]
  S3[Subscriber 3]
  P1 --> B
  P2 --> B
  B --> S1
  B --> S2
  B --> 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 P1,P2,S1,S2,S3 service;
    class B queue;

Publishers don't know subscribers; the broker fan-outs messages to all (or to a chosen group).