How to Answer: "Describe a time you had to debug a nontrivial race condition."

“In our checkout service, under high concurrency, users were occasionally double-allocated inventory discounts. The issue was non-reproducible locally. I added high-resolution distributed tracing with transaction IDs and analyzed log timestamps. The root cause was a non-atomic read-modify-write operation across stateless container instances inspecting the database balance simultaneously. I fixed it by introducing Redis-based distributed locking with Redlock, alongside optimistic locking with version columns in PostgreSQL, and wrote a concurrent stress test using parallel threads to confirm zero duplicate allocations.”

“We encountered a race condition in our background job queue where two workers picked up the same task simultaneously. I used logging to verify that the status check and status update were not atomic. I resolved it by wrapping the task claim step in a database row-level pessimistic lock (`SELECT FOR UPDATE`), preventing duplicate processing.”

“I fixed a race condition in a React component where rapid button clicks triggered multiple API calls and overwrote state out of order. I solved it by canceling stale pending requests with an AbortController and disabling the submit button during active loading state.”