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Design an Online Auction System - Low-Level Design | OneNoughtOne

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Design an Online Auction System

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Design an object-oriented online auction system where sellers list items and bidders compete in real-time. The system supports multiple auction formats (English ascending, Dutch descending, sealed-bid), reserve prices, outbid notifications via the Observer pattern, and category-based search.

The auction lifecycle transitions through SCHEDULED → ACTIVE → CLOSED (or CANCELLED). Bids are validated against the current highest bid, and the winner is determined only if the reserve price is met.

Core Use Cases

Create an auction

Sellers list items with a starting price, reserve price, time window, and auction type

Place a bid

Bidders submit bids that must exceed the current highest bid (English auction)

Outbid notification

Notify a bidder immediately when they are outbid

Close auction

Automatically close when end time is reached; award to highest bidder if reserve is met

Cancel auction

Seller can cancel a scheduled or active auction

Search auctions

Browse auctions by category, status, or price range

Reserve price

Item is not sold if highest bid is below the hidden reserve price

Multiple auction types

Support English (ascending), Dutch (descending), and sealed-bid formats

Constraints

A bid must be strictly greater than the current highest bid
A seller cannot bid on their own auction
Bids are only accepted while the auction is active (within the time window)
The item is not sold if the reserve price is not met
Each auction has exactly one winning bidder (or none if reserve not met)

Assumptions

In-memory storage (no database persistence)
Single-threaded execution (concurrency is a follow-up)
No payment processing — bidding only
Time-based closure is triggered manually via close() for simulation

In Scope

Auction lifecycle (scheduled → active → closed/cancelled)
Bid validation and ranking
Reserve price mechanism
Observer-based notifications (email, SMS)
Multiple auction types (English, Dutch, sealed-bid)
Search by category and status

Out of Scope

Payment processing and escrow
Shipping and delivery logistics
Fraud detection and bid sniping prevention
Image upload and media management
User reputation / rating system

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

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Loading LLD design...

00:00:00

Design an Online Auction System

Advanced

Core Use Cases

Create an auction

Sellers list items with a starting price, reserve price, time window, and auction type

Place a bid

Bidders submit bids that must exceed the current highest bid (English auction)

Outbid notification

Notify a bidder immediately when they are outbid

Close auction

Automatically close when end time is reached; award to highest bidder if reserve is met

Cancel auction

Seller can cancel a scheduled or active auction

Search auctions

Browse auctions by category, status, or price range

Reserve price

Item is not sold if highest bid is below the hidden reserve price

Multiple auction types

Support English (ascending), Dutch (descending), and sealed-bid formats

Constraints

A bid must be strictly greater than the current highest bid
A seller cannot bid on their own auction
Bids are only accepted while the auction is active (within the time window)
The item is not sold if the reserve price is not met
Each auction has exactly one winning bidder (or none if reserve not met)

Assumptions

In-memory storage (no database persistence)
Single-threaded execution (concurrency is a follow-up)
No payment processing — bidding only
Time-based closure is triggered manually via close() for simulation

In Scope

Auction lifecycle (scheduled → active → closed/cancelled)
Bid validation and ranking
Reserve price mechanism
Observer-based notifications (email, SMS)
Multiple auction types (English, Dutch, sealed-bid)
Search by category and status

Out of Scope

Payment processing and escrow
Shipping and delivery logistics
Fraud detection and bid sniping prevention
Image upload and media management
User reputation / rating system

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

Loading canvas...

Design an Online Auction System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

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Design an Online Auction System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

Sign in to view submissions

Design an Online Auction System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you handle concurrent bids to prevent race conditions?

2How would you implement automatic bid (proxy bidding) up to a max amount?

3How would you add a bid sniping protection (extend auction on last-second bids)?

4How would you implement a Dutch (descending price) auction?

5How would you add a reputation/trust score for sellers and buyers?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions

Design an Online Auction System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you handle concurrent bids to prevent race conditions?

2How would you implement automatic bid (proxy bidding) up to a max amount?

3How would you add a bid sniping protection (extend auction on last-second bids)?

4How would you implement a Dutch (descending price) auction?

5How would you add a reputation/trust score for sellers and buyers?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions