OLX / Craigslist Classifieds — Simple

Problem statement (interviewer prompt)

Design OLX / Craigslist: a classifieds platform where individuals post ads (text + photos), buyers search by category + location, and the two parties chat in-app to arrange a deal. No integrated checkout — focus on listings, search, moderation, and trust.

flowchart LR
  U([User])
  POST[Post Ad Service]
  MOD[Moderation]
  SRCH[Search]
  CHAT[In-app chat]
  ADS[(Listings)]
  U --> POST --> MOD --> ADS
  U --> SRCH --> ADS
  U --> CHAT

    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 POST,MOD,SRCH,CHAT service;
    class ADS datastore;