eBay / Auction System#

Q: How does eBay prevent two bidders from winning the same auction?

Bids per listing are serialized through a single owner shard or a row level lock. Each bid is compared against the current high bid and accepted or rejected atomically before notifications fire.

Q: What is proxy bidding in eBay?

A bidder enters a maximum price. The system bids the minimum needed to keep them on top, up to that maximum, so they don't need to manually re-bid every time someone outbids them.

Q: How does eBay handle anti sniping near auction end?

If a bid arrives in the last few seconds, the auction is extended by a fixed window. This prevents last instant snipes from winning without giving others a chance to respond.

Q: How are auction expiries scheduled at scale?

Each auction's end time goes into a delay queue or scheduled job store. A worker pool picks up expired auctions, determines the winner, charges payment, and notifies both parties.

Q: How does eBay search rank live listings?

Listings are indexed in a search engine with relevance signals like keyword match, seller reputation, ship time, price, and personalization. Ending soon and best match rankers are exposed as sort options.

Problem statement (interviewer prompt)

Design an online auction system (eBay): listings with bidding, proxy bidding (auto-rebid up to a max), live leaderboard, anti-snipe extensions, winner determination, and payment + shipping handoff. Strictly serialise bids per listing.

flowchart LR
  S[Seller]
  L[Listing Service]
  A[Auction Engine]
  B[Bidder]
  PAY[Payment]
  S --> L
  B --> A
  L --> A
  A --> 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 S,L,A,B,PAY service;

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

Quick reference#

Functional#

List item (auction / Buy It Now).
Bid placement, proxy bidding.
Live leaderboard, countdown.
Winner determination + payment + shipping.
Disputes & buyer/seller protection.

Non-functional#

Strict ordering of bids per listing.
p99 bid placement < 500 ms.
Active listings: tens of millions.

Capacity#

1B+ listings/yr; tens of millions live concurrently.
Bid storm at auction close: 1000s bids/s on hot items.

Schema#

listings(id, seller, type, start, end, current_high_bid, status)
bids(id, listing_id, bidder, amount, max_amount, ts) partition by listing_id
payments(...) similar to Amazon

Trade-offs#

Single-owner per listing is required for correctness.
Eventual consistency on viewer leaderboard is fine.
Anti-snipe extension keeps fairness but breaks fixed-time scheduling.

Refs#

eBay engineering blog (Cassandra rollouts, auction architecture talks).
"Designing Data-Intensive Applications" ch.7 (transactions).
ByteByteGo "Design an auction".

FAQ#

How does eBay prevent two bidders from winning the same auction?#

Bids per listing are serialized through a single owner shard or a row level lock. Each bid is compared against the current high bid and accepted or rejected atomically before notifications fire.

What is proxy bidding in eBay?#

A bidder enters a maximum price. The system bids the minimum needed to keep them on top, up to that maximum, so they don't need to manually re-bid every time someone outbids them.

How does eBay handle anti sniping near auction end?#

If a bid arrives in the last few seconds, the auction is extended by a fixed window. This prevents last instant snipes from winning without giving others a chance to respond.

How are auction expiries scheduled at scale?#

Each auction's end time goes into a delay queue or scheduled job store. A worker pool picks up expired auctions, determines the winner, charges payment, and notifies both parties.

How does eBay search rank live listings?#

Listings are indexed in a search engine with relevance signals like keyword match, seller reputation, ship time, price, and personalization. Ending soon and best match rankers are exposed as sort options.