Design a coupon and offers engine
PremiumMachine codingHard
- Eligibility rules
- Usage limits
- Stacking groups
Quick answer
This machine coding interview question asks you to design a coupon and offers engine. 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
Build a coupon and offers engine, like the offers screen of a food delivery app that shows which codes apply and picks the best one. It checks who may use each offer, chooses the best combination and enforces usage limits. The hard parts are stacking rules, exact limits under load and releasing holds when an order fails.
Some offers apply automatically; others need a code. The cart total, user facts and payment method are inputs; the engine does not price the cart.
What must a coupon and offers engine do?
- Define offers: createOffer(spec) with an optional code, a flat or percent discount (percent has a cap), a validity window, a minimum cart value, a stacking group and eligibility rules.
- Best combination: best(ctx, codes) returns the largest saving. At most one offer per stacking group; an exclusive offer cannot combine with any other. Savings are computed on the given cart total.
How well must it do it?
- Exact limits: 1,000 concurrent reserves for the last 5 uses give exactly 5 successes, and budgets are never overspent. Held uses count toward limits.
- Fast selection: best answers within 10 ms with 50 eligible offers.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Eligibility rules
- Usage limits
- Stacking groups
- Best offer selection
- Reserve commit
- Concurrency
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- 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 machine coding 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 machine coding question. You design a coupon and offers engine 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