← Capital One Interview Insights
This took me way longer than it should have to even understand the authorization flow.
Start by reading the implementation and test cases to understand expected behavior, then reproduce each failure by tracing the code path. For each bug, identify the root cause (e.g., missing expiration check, incorrect merchant comparison, lack of atomicity, race condition) and apply a minimal fix. Finally, write regression tests that cover each failure scenario, including concurrency, to prevent future regressions.
Pro tip: When dealing with concurrency bugs, don't just patch the symptom—use proper synchronization primitives (e.g., locks, atomic operations) and consider idempotency keys to prevent double-spend. Also, write tests that simulate concurrent requests to verify the fix.
Read the code and test cases to grasp the intended behavior. Run the failing tests to observe the actual vs. expected outcomes for each scenario.
For each failing test, trace the execution path to pinpoint where the logic deviates. Identify the specific bug: missing expiration validation, incorrect merchant ID comparison, non-atomic balance deduction, or race condition in concurrent authorization.
Implement fixes that address the root cause without introducing side effects. For concurrency, use locks or atomic operations; for double-spend, ensure single-use cards are marked used atomically.
Create tests that reproduce each original failure and verify the fix. Include edge cases and a concurrency test that spawns multiple threads to attempt double-spend.
Run all tests to ensure they pass. Discuss potential trade-offs (e.g., performance impact of locking) and how you would monitor or prevent similar issues in production.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.