Design Add, Overwrite, Undo, and Redo for Billing State
Last updated: September 4, 2026
Quick Overview
Design a system that allows users to add, overwrite, and manage billing states with undo and redo functionality.
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Design a system that allows users to add, overwrite, and manage billing states with undo and redo functionality.
This question is typically asked during a technical screen to assess a candidate's ability to design robust systems that handle state management effectively. Interviewers are looking for a clear understanding of system design principles, data management, and the ability to implement features that enhance user experience, such as undo and redo functionalities.
What the Interviewer Expects
- Clearly outline the requirements for each functionality (add, overwrite, undo, redo).
- Provide a high-level design including data structures and interactions.
- Discuss how to implement undo and redo mechanisms efficiently.
- Address potential trade-offs and scaling considerations.
- Explain how the design can handle concurrent modifications.
Key Topics to Cover
How to Approach This
- Apply SOLID principles. Single Responsibility makes code testable, Open/Closed makes it extensible.
- Choose data structures based on access patterns, not familiarity.
- Prefer immutable data and message passing over shared mutable state for concurrency.
- Design APIs with RESTful conventions, versioning, meaningful errors, and pagination from day one.
Possible Follow-up Questions
- How would you handle concurrent modifications to the billing state?
- What data structure would you choose for managing the history of changes?
- Can you explain how you would test the functionality of your design?
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Practice System Design ProblemsSample Answer
Requirements
The system should support the following functionalities:
- Add Amount: Users can add a specified amount to the billing state.
- Overwrite Amount: Users can overwrite the current amount with ...
High-Level Design
To implement these functionalities, we can use a stack data structure to manage the history of changes. Each operation (add or overwrite) will push the previous state onto the stack, allowing us to ea...
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