Describe a challenging bug you fixed

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Lesson 3: Describe a challenging bug you fixed

1. Can you describe a challenging bug you fixed recently?
Sure, we had an issue where orders were occasionally being processed twice.
It only happened under high traffic, so it was hard to reproduce at first.
It took me almost three days to track down the root cause.

2. How did you find the root cause?
I started by adding more detailed logging around the payment service.
After a while, I noticed the bug only appeared when two requests arrived within the same second.
That's when I realized we were missing a proper idempotency check.

3. What did you do to fix it?
I added an idempotency key to every order request.
This way, the server can recognize duplicate requests and safely ignore them.
We also added a test case to make sure this doesn't happen again.

4. Did you face any pressure while fixing it?
Yes, customers were being charged twice, so the pressure was real.
I had to update my manager every few hours on the progress.
That pressure actually helped me stay focused on the problem.

5. What did you learn from that experience?
I learned that logging is often more valuable than guessing.
I also learned to think more carefully about idempotency early in the design process.
Now I try to ask about duplicate requests whenever I design a new API.

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