eBay / Auction System — Detailed

flowchart TB
  subgraph Clients
    SEL[Seller]
    BID[Bidder]
  end

  subgraph Edge
    CDN
    GW
  end

  subgraph Listing
    LSVC[Listing Service]
    LDB[(Listings)]
    IMG[Images / S3]
  end

  subgraph Search
    SRCH[Search]
    IDX[(Inverted index)]
    REC([Recommendation])
  end

  subgraph Auction[Auction Engine]
    AUC[Auction Service<br/>per-item shard]
    AUCDB[(Auction state<br/>strong consistency)]
    PROXY[Proxy bidding<br/>auto-rebid up to max]
    SNIPE[Anti-snipe extension]
    TICK[Closing clock]
    NTQ[[(Pending bids queue)]]
  end

  subgraph Pay
    PAYWIN[Winner payment service]
    PAYSTRIPE((Payment provider))
    HOLD[Authorization hold]
  end

  subgraph Notif
    WS[[WebSocket / SSE bid stream]]
    PUSH[Push / email]
  end

  subgraph Trust
    FRAUD[Fraud detection]
    DISP[Dispute / chargeback]
    SAFE[Buyer/seller protection]
  end

  subgraph Real
    LB[Auction LB<br/>sticky by listing_id]
  end

  SEL --> Listing
  Listing --> Search
  BID --> CDN --> GW --> LB --> Auction
  Auction --> AUCDB
  AUC --> PROXY
  AUC --> SNIPE
  AUC --> NTQ
  Auction --> Notif
  Auction -. winner .-> Pay
  Trust --- Pay

    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 LB edge;
    class SEL,BID,LSVC,SRCH,AUC,PROXY,SNIPE,TICK,PAYWIN,HOLD,PUSH,FRAUD,DISP,SAFE service;
    class LDB,IDX,AUCDB,NTQ datastore;
    class WS queue;
    class REC compute;
    class IMG storage;
    class PAYSTRIPE external;

Auction correctness

• All bids for a listing must serialize on a single owner (shard by listing_id).
• Each bid: validate amount ≥ current + increment, atomic CAS update, persist event.
• Replicate to followers; reads from followers may show stale leaderboard — that's acceptable.

Proxy bidding

• Bidder sets max_bid; system auto-rebids minimum required up to max.
• Stored as private field; visible to all is current high-bid only.

Anti-snipe

• Last-minute bid extends auction by N seconds.
• Prevents server-clock gaming + last-second sniping.

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	Sharding	horizontal partitioning across nodes	database-sharding
HLD	CAP / PACELC	C vs A under partition; L vs C otherwise	cap-pacelc
HLD	Leader/follower replication	sync/semi-sync/async replication, failover	replication-leader-follower
HLD	Realtime protocols	WS / SSE / polling / gRPC streaming	realtime-protocols
HLD	Search internals	inverted index, BM25, embeddings, ANN	search-internals
LLD	Structural patterns	Adapter, Decorator, Facade, Proxy, Composite	structural-patterns
LLD	Concurrency primitives	mutex, semaphore, RW lock, atomic, CAS	concurrency-primitives