Design a buy now pay later system
PremiumMachine codingMedium
- BNPL
- Credit limit
- Atomic checkout
Quick answer
This machine coding interview question asks you to design a buy now pay later system. 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 buy now pay later system in memory, like the pay-later option at an online shop checkout. Users buy items now and pay later within a credit limit. The hard parts are keeping stock and credit correct when two buyers act at once, and tracking dues that become late.
The system is a library with a small driver. Time comes from a clock the caller can replace, so tests can jump 30 days ahead without waiting.
What must a buy now pay later system do?
- Buy: buy(userId, {itemId: qty}, mode) with mode PREPAID or BNPL; BNPL succeeds only if the order total fits the user's available credit.
- Clear dues: clearDues(userId, amount) pays dues oldest first; a payment can cover part of a due, and paid money frees credit again.
How well must it do it?
- Thread safety: Two buyers racing for the last unit must never both succeed, and two BNPL orders must never push one user past the limit.
- Correct money: Amounts are whole rupees as integers; credit, dues and stock always add up after every call.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- BNPL
- Credit limit
- Atomic checkout
- Due tracking
- Injectable clock
- Lock ordering
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 buy now pay later 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 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