Design a real-time auction and bidding system
PremiumReal-timeMedium
- Single writer per item
- Bid ordering
- Optimistic concurrency
Quick answer
This real-time interview question asks you to design a real-time auction and bidding system. Aim for a design that delivers updates to many users with low delay, then explain connections, fan-out, ordering and trade-offs. Interviewers want to see how you keep latency low, handle reconnects and stay correct under load.
The problem
Design a real-time auction and bidding system like eBay. Bidders place bids on items with an end time, and the highest valid bid wins; the hard part is the last two minutes, when hundreds of bidders hit one item at once and every watcher must see the current price and leader update live.
Each item has a start price, a minimum increment and an end time. Payment, listings, search, seller tools and fraud detection are out of scope.
What must a real-time auction and bidding system do?
- Place a bid: A bidder submits an amount; it is accepted if it is at least the current price plus the increment and the auction is still open, otherwise rejected with a reason.
- Live price: Watchers see the current price, the leading bidder's alias and the bid count within 1 second of a change.
How well must it do it?
- Consistency: Exactly one leading bid at any moment; two bids of the same amount cannot both lead.
- Latency: A bidder learns their bid's result within 500 ms; watchers see the new price within 1 second.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Single writer per item
- Bid ordering
- Optimistic concurrency
- Anti sniping
- WebSocket
- Event log
How you’ll practise
- Requirements
- Estimate
- Connections
- Message flow
- Delivery guarantees
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any real-time design round.
- Clarifies latency, ordering and delivery guarantees
- Chooses a connection model with reasons
- Explains fan-out to many receivers
- Handles reconnects, missed messages and duplicates
- Plans for hot spots and sudden traffic spikes
- States trade-offs between consistency and speed
Questions people ask about this interview question
- Which design type is this question?
- It is a real-time question. You design a real-time auction and bidding system and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with who sends and receives updates, how fast they must arrive and what order matters. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team