Design an inventory system for quick commerce
PremiumBackend systemHard
- Stock reservations
- Dark stores
- Flash sales
Quick answer
This backend system interview question asks you to design an inventory system for quick commerce. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design the inventory system for quick commerce, the 10-minute grocery delivery model that ships from small dark stores in each neighbourhood. It tracks stock per store, reserves items at checkout and assigns a delivery partner. The hard part is concurrency: two orders for the last unit, and thousands of buyers in a flash sale.
Each customer is served by the nearest dark store within 3 km. A cart does not hold stock; checkout reserves it, payment confirms it, and picking takes it off the shelf. Stores restock several times a day.
What must an inventory system for quick commerce do?
- Reserve at checkout: Reserve every item in the cart at one store in one step; if any item is short, reserve nothing and say which items are short.
- Last unit: When two orders want the last unit at a store, exactly one gets it and the other is told at once.
How well must it do it?
- No overselling: Reserved plus picked units never exceed what is on the shelf, under any concurrency.
- Fast checkout: Reservation answers in under 150 ms at p99, including during a flash sale.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Stock reservations
- Dark stores
- Flash sales
- Conditional updates
- Hot keys
- Delivery partner assignment
How you’ll practise
- Requirements
- Estimate
- API and data
- High-level design
- Deep dives
- 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 backend system design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a backend system question. You design an inventory system for quick commerce and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. 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