# Design an online auction system like eBay: system design interview question

> Concurrent bids and fixed deadlines test price order, winner selection, and service behavior after failures.

Source: https://sysround.com/questions/design-online-auction-system-like-ebay
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design an online auction platform where sellers list items and buyers submit bids. Each auction closes at a set time and selects a winner."

## What to clarify first

- Auction format
- Bid rules
- Deadline and winner
- Daily scale and request mix
- Peak traffic
- Latency
- Consistency and availability
- Durability and retention
- Client retries
- Scope limits

## Scope of a strong answer

Core requirements:

- Create an auction for one item with a start price and a fixed close time
- View an auction and its current price
- Submit a bid and receive a clear decision
- Close an auction and record its final winner

Usually out of scope:

- Payments and shipment
- Proxy bids and reserve prices
- Bid withdrawal and deadline extensions
- Account creation
- Writes across multiple regions

## What interviewers probe

### Concurrent bids

Every accepted bid changes the price for later bids. Many requests compete for the same auction state.

### The auction deadline

A close task can arrive late while bids wait for access. The service must apply one deadline rule to both paths.

### An uncertain bid result

A timeout does not reveal whether the service accepted a bid. A failover can also expose an outdated auction state.

### Traffic for one auction

More database partitions do not remove contention within one auction. Public reads can also overwhelm the same item.

## Common mistakes

- Trusting the price that the buyer saw instead of the authoritative auction state
- Checking the deadline only when the close task runs
- Recording a bid separately from the auction's price change
- Treating a timeout as proof that the bid failed
- Using database partitions as the answer to contention within one auction
- Acknowledging bids before durable commit

## The bar by level

- **Senior:** Defines the decision order and deadline boundary. Handles retries, delayed close tasks, and crashes. Separates public reads from bids and calculates hot-auction capacity.
- **Staff+:** Sets explicit guarantees for overload and failover. Preserves those guarantees through batches, clock uncertainty, and primary changes. Explains why partitions cannot remove one auction's serial limit.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=online-auction-platform


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.