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OLX / Craigslist — Detailed#

flowchart TB
  subgraph Apps
    BR
    MOB
  end

  subgraph Edge
    CDN
    GW
  end

  subgraph Post[Post & Manage]
    POST[Post Ad Service]
    IMG[Image upload]
    OBJ[(S3)]
    CAT[Category / Taxonomy]
    MOD[Moderation pipeline<br/>ML + human]
    NSFW
    DUPE[Duplicate detector]
  end

  subgraph Search
    SRCH[Search API]
    IDX[(Inverted index<br/>geo + filters)]
    REL([Relevance ranker])
    SAVED[Saved searches]
    ALERT[Alert notifier]
  end

  subgraph Engage
    CHAT[Chat Service]
    WS[WebSocket]
    PUSH
    EMAIL
    REPORT[Report / abuse]
  end

  subgraph Monetize
    PROMO[Featured listing]
    PAY[Payment]
    SUB[Subscription / shops]
  end

  subgraph Trust
    BAN[Ban list]
    KYC[Light KYC]
    PHONE([Phone verify])
  end

  Apps --> CDN --> GW
  GW --> Post --> ADS[(Listings)]
  Post --> Moderation:::nope
  GW --> Search --> ADS
  Search --> SAVED --> ALERT --> Engage
  GW --> Engage --> CHAT
  CHAT --> WS
  Engage --> REPORT --> Trust
  Monetize --- GW

    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 PHONE client;
    class POST,IMG,CAT,MOD,DUPE,SRCH,SAVED,CHAT,WS,REPORT,PROMO,PAY,SUB,BAN,KYC service;
    class IDX,ADS datastore;
    class REL compute;
    class OBJ storage;
    class ALERT obs;

Moderation pipeline#

  • On create: ML classifies for prohibited content (weapons, drugs, fraud signals).
  • Image NSFW model.
  • Duplicate detection via perceptual hash + text similarity.
  • Borderline cases → human queue.
  • Heavy geo filter — index by city / S2 cell + categorical filters.
  • Saved searches with periodic alert evaluation.

Glossary & fundamentals#

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag Concept What it is Page
HLD CDN edge caching for static assets cdn
HLD Realtime protocols WS / SSE / polling / gRPC streaming realtime-protocols
HLD Geo indexing Geohash, Quadtree, S2, H3, R-tree geo-indexing
HLD Search internals inverted index, BM25, embeddings, ANN search-internals