Design game credit system with add/spend/refund APIs

Q: Design game credit system with add/spend/refund APIs

This question evaluates a candidate's ability to design transactional backend services, covering API design, database schema for account balances and transaction history, concurrency control, idempotency, and fault tolerance; it is in the System Design category and targets the backend/database/API domain.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Technical Screen rounds at Ixl.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Ixl during technical interviews.

Question

You are designing a backend service to manage virtual game credits (in-game currency) for one or more video games.

The system should allow players to:

Add credits to their account (e.g., after a purchase or a bonus).
Spend credits (e.g., to buy in-game items).
Refund a previous spend (e.g., when a purchase is cancelled or failed).

Requirements

Functional requirements
- Each player has an account balance of credits.
- Support three core operations via APIs:
  - add credits to a player's balance.
  - spend credits, reducing the balance only if sufficient funds exist.
  - refund a previous spend, restoring the corresponding amount.
- Track a history of credit-related operations for auditing and debugging.
- Prevent double-spending and inconsistent balances when multiple operations happen concurrently on the same account.
What you need to design
- A database schema (tables/entities, key fields, relationships) to store:
  - User/account balances.
  - Credit transactions (add, spend, refund), including references between them.
- Three API endpoints (you can assume REST over HTTP):
  - add credits
  - spend credits
  - refund a previous transaction
- For each API, specify:
  - HTTP method and URL.
  - Request body format (fields, types, example JSON).
  - Response body format (success and error cases, example JSON).
Additional discussion topics Be prepared to answer high-level design questions , including:
- Which parts of your design could fail (e.g., database, network, service instances), and what happens in each failure case?
- How you ensure consistency of balances and transactions, especially under concurrency and partial failures.
- How you would prevent duplicate processing of the same request (e.g., when clients retry requests).
- How the design might evolve to handle high traffic (many players and many credit operations per second).

Assume a typical modern backend stack (e.g., a stateless application layer and a database). You do not need to write actual code, but you should provide clear data models, API definitions, and reasoning about reliability and consistency.

Design game credit system with add/spend/refund APIs

Quick Overview

Requirements

Solution

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