Reddit / Quora / Stack Overflow — Simple

Problem statement (interviewer prompt)

Design Reddit (or Quora / Stack Overflow): users post questions/threads in communities, others comment in nested threads, everyone votes. Listings (hot/new/top) must be ordered by a ranking algorithm. Support 50M+ DAU and 2B+ page views/day.

flowchart LR
  U([User])
  W[Write API]
  POST[(Posts / Questions)]
  COMM[(Comments / Answers)]
  VOTE[Vote Service]
  RANK[Ranking Service]
  FEED[Feed / Listing API]
  SRCH[Search]
  U -->|post| W --> POST
  U -->|comment| W --> COMM
  U -->|vote| W --> VOTE
  U -->|read| FEED --> RANK
  RANK --> POST
  POST --> SRCH

    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 U client;
    class W,VOTE,RANK,FEED,SRCH service;
    class POST,COMM datastore;