Design card payments with tips and settlement
PremiumBackend systemHard
- Tip adjustment
- Incremental authorization
- Settlement batch
Quick answer
This backend system interview question asks you to design card payments with tips and settlement. 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 card payments with tips and settlement for restaurants: authorise the bill, add the tip later, capture the final amount and settle it in a nightly batch with the card network. The hard parts are changing an amount after authorisation, never charging twice, and matching every cent to the network report.
At the table, the server taps or dips the card for the bill. The guest adds a tip on the terminal or the paper receipt, sometimes an hour later. The restaurant closes out at night, and the platform sends one clearing file per acquiring bank to the card network.
What must the system do?
- Authorise and tip: Authorise the bill, then add or change the tip until capture, using tip tolerance or an incremental authorisation.
- No double charges: Retried requests from terminals or the platform never authorise, capture or refund twice.
How well must it do it?
- Correctness: No payment is captured twice, left out of a batch, or settled for a different amount than captured. Amounts are integer minor units.
- Availability: Authorisation stays at 99.99% through dinner peaks; the batch may be late but never wrong.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Tip adjustment
- Incremental authorization
- Settlement batch
- Idempotency keys
- Reconciliation
- Auth and capture
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 card payments with tips and settlement 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