Design a payment processing system

Q: Design a payment processing system

This is a System Design interview question from OpenAI for Software Engineer roles. View the full question and solution on PracHub.

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Question

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Scenario

Design a backend payment system for an online product that supports:

One-time charges and refunds.
Idempotent payment APIs (clients may retry).
Asynchronous interaction with external payment processors (e.g., card networks via a payment gateway).
Accurate accounting/ledger and reporting.

What to cover

Core APIs and data model.
Handling retries, duplicates, and failures.
Ensuring correctness (no double charge; consistent ledger).
Observability and reconciliation.

You can make reasonable assumptions and state them explicitly.

Design a payment processing system

Scenario

What to cover

Solution

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