Maintainability and Legacy Code Questions

Covers strategies and principles for evolving codebases safely and keeping them easy to understand and change over time. Topics include design principles such as Single Responsibility, Open Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion, removing duplication, establishing appropriate abstraction boundaries, separation of concerns, identifying and remediating code smells, incremental refactoring approaches, regression risk mitigation via tests and feature toggles, backward compatibility and migration strategies, and prioritizing technical debt reduction. Interviewers assess the candidate ability to plan pragmatic refactors, minimize risk during change, and improve long term health of a codebase.

EasyTechnical

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Explain separation of concerns and give a concrete example of how you would refactor request handling code in a monitoring dashboard service to separate rendering, data fetching, and authorization. Include the expected benefits for maintainability and operational troubleshooting.

MediumTechnical

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You have legacy code that performs integration with a third-party database; you need to add unit tests without changing behavior. Explain techniques to isolate and test the logic: mocking, fakes, test doubles, and recording/replaying interactions. Describe when to use each technique and pitfalls to avoid.

MediumTechnical

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You want to introduce automated linting and coding standards across hundreds of repositories without breaking builds for many teams. Describe a gradual rollout plan, enforcement strategy, temporary exemptions, and how to measure adoption and developer friction.

HardTechnical

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A multithreaded C++ legacy process uses global mutable state for configuration and causes intermittent race conditions under load. Propose design changes to remove global state, explain thread-safety approaches (immutable config, thread-local storage, locks), and outline a plan to roll changes into production without causing instability.

HardSystem Design

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Design a minimally disruptive data migration strategy for a stateful service that must evolve its data model, supporting both old and new schema reads and writes during rolling upgrades. Cover versioned schemas, adapters, backfilling, consistency concerns and verification steps to ensure correctness with near-zero downtime.

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