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Design Game Credit System with Add/Spend/Refund APIs
You are tasked with designing a backend service responsible for managing virtual game credits (in-game currency) for one or more video games.
The system must enable players to perform the following core operations:
- Add credits to their account (e.g., following a purchase or a bonus reward).
- Spend credits (e.g., to acquire in-game items), ensuring funds are sufficient before deduction.
- Refund a previously executed spend operation, restoring the corresponding amount to the player's balance.
Requirements
Functional Requirements
- Each player must have an account balance of credits.
- Support the three core operations (
add,spend,refund) via well-defined APIs. - The
spendoperation must only reduce the balance if sufficient funds exist in the player's account. - The
refundoperation must be linked to a specific, previousspendtransaction and restore its corresponding amount. - Maintain a comprehensive history of all credit-related operations for auditing and debugging purposes.
- Implement mechanisms to prevent double-spending and ensure consistent account balances, especially when multiple operations occur concurrently on the same account.
Design Deliverables
Your design should include:
- A database schema: Define tables/entities, key fields, and relationships necessary to store user/account balances and credit transactions (add, spend, refund), including references between related transactions.
- Three API endpoints (assuming REST over HTTP): One for
addcredits, one forspendcredits, and one forrefunda previous transaction.- For each API, specify:
- HTTP method and URL path.
- Request body format (fields, data types, example JSON).
- Response body format (including success and various error cases, example JSON).
- For each API, specify:
Additional Discussion Topics
Be prepared to discuss high-level design considerations, including:
- Identifying potential failure points in your design (e.g., database, network, service instances) and outlining the system's behavior in each failure scenario.
- Strategies to ensure consistency of balances and transactions, particularly under concurrent operations and partial system failures.
- Methods to prevent duplicate processing of the same request, especially when clients implement retry logic.
- How the proposed design could evolve to handle high traffic volumes (e.g., millions of players and numerous credit operations per second).
Assumptions
- Assume a typical modern backend stack, consisting of a stateless application layer interacting with a database.
- You are not required to write actual code, but clear data models, API definitions, and robust reasoning about reliability and consistency are expected.
AI interview prep
Core dilemma
Ensuring absolute transactional integrity (ACID properties) and strong consistency for financial-like operations under high concurrency and potential network/service failures, while maintaining auditability and scalability.
Mid-level signals3
Senior signals3
Red flags3
3 tradeoffs and 3 stress tests locked
Interview prep
Unlock the interviewer X-Ray rubric, tradeoff matrix, stress tests, clarifying questions, and key concepts.
Design Game Credit System with Add/Spend/Refund APIs
You are tasked with designing a backend service responsible for managing virtual game credits (in-game currency) for one or more video games.
The system must enable players to perform the following core operations:
- Add credits to their account (e.g., following a purchase or a bonus reward).
- Spend credits (e.g., to acquire in-game items), ensuring funds are sufficient before deduction.
- Refund a previously executed spend operation, restoring the corresponding amount to the player's balance.
Requirements
Functional Requirements
- Each player must have an account balance of credits.
- Support the three core operations (
add,spend,refund) via well-defined APIs. - The
spendoperation must only reduce the balance if sufficient funds exist in the player's account. - The
refundoperation must be linked to a specific, previousspendtransaction and restore its corresponding amount. - Maintain a comprehensive history of all credit-related operations for auditing and debugging purposes.
- Implement mechanisms to prevent double-spending and ensure consistent account balances, especially when multiple operations occur concurrently on the same account.
Design Deliverables
Your design should include:
- A database schema: Define tables/entities, key fields, and relationships necessary to store user/account balances and credit transactions (add, spend, refund), including references between related transactions.
- Three API endpoints (assuming REST over HTTP): One for
addcredits, one forspendcredits, and one forrefunda previous transaction.- For each API, specify:
- HTTP method and URL path.
- Request body format (fields, data types, example JSON).
- Response body format (including success and various error cases, example JSON).
- For each API, specify:
Additional Discussion Topics
Be prepared to discuss high-level design considerations, including:
- Identifying potential failure points in your design (e.g., database, network, service instances) and outlining the system's behavior in each failure scenario.
- Strategies to ensure consistency of balances and transactions, particularly under concurrent operations and partial system failures.
- Methods to prevent duplicate processing of the same request, especially when clients implement retry logic.
- How the proposed design could evolve to handle high traffic volumes (e.g., millions of players and numerous credit operations per second).
Assumptions
- Assume a typical modern backend stack, consisting of a stateless application layer interacting with a database.
- You are not required to write actual code, but clear data models, API definitions, and robust reasoning about reliability and consistency are expected.