Online Auction System (like eBay)¶

Quick Reference Guide for System Design Interviews

Problem Statement¶

Design an online auction platform where sellers can list items for bidding, buyers can place bids, and the system automatically manages auction lifecycles, determining winners and processing payments.

Requirements¶

Functional Requirements¶

Create auction listings
Place bids on items
Real-time bid updates
Automatic auction closure
Winner determination
Bid history

Non-Functional Requirements¶

Consistency: No overselling, correct winner
Availability: 99.99%
Latency: < 100ms for bid placement
Scale: 10M concurrent auctions, 1M bids/minute

High-Level Architecture¶

High-Level Architecture

Auction Data Model¶

-- Auctions table
CREATE TABLE auctions (
    auction_id      UUID PRIMARY KEY,
    seller_id       UUID NOT NULL,
    title           VARCHAR(500),
    description     TEXT,
    category        VARCHAR(100),
    starting_price  DECIMAL(12,2) NOT NULL,
    reserve_price   DECIMAL(12,2),  -- Minimum to sell
    buy_now_price   DECIMAL(12,2),  -- Instant purchase
    current_price   DECIMAL(12,2),
    bid_increment   DECIMAL(12,2) DEFAULT 1.00,
    start_time      TIMESTAMP NOT NULL,
    end_time        TIMESTAMP NOT NULL,
    status          VARCHAR(20) DEFAULT 'pending',
        -- pending, active, ended, sold, cancelled
    winner_id       UUID,
    winning_bid_id  UUID,
    created_at      TIMESTAMP DEFAULT NOW(),

    INDEX idx_status_end (status, end_time),
    INDEX idx_category (category),
    INDEX idx_seller (seller_id)
);

-- Bids table
CREATE TABLE bids (
    bid_id          UUID PRIMARY KEY,
    auction_id      UUID NOT NULL REFERENCES auctions(auction_id),
    bidder_id       UUID NOT NULL,
    amount          DECIMAL(12,2) NOT NULL,
    max_amount      DECIMAL(12,2),  -- For proxy bidding
    status          VARCHAR(20) DEFAULT 'active',
        -- active, outbid, winning, cancelled
    created_at      TIMESTAMP DEFAULT NOW(),

    INDEX idx_auction_amount (auction_id, amount DESC),
    INDEX idx_bidder (bidder_id)
);

-- Bid history (denormalized for fast reads)
CREATE TABLE bid_history (
    auction_id      UUID,
    bid_id          UUID,
    bidder_id       UUID,
    amount          DECIMAL(12,2),
    created_at      TIMESTAMP,

    PRIMARY KEY (auction_id, created_at, bid_id)
);