Payment Events and Duplicate Processing

A Real Interview Deep Dive into Idempotency, Retries, and Distributed Correctness

Payment systems are unforgiving.

If a notification is sent twice, users get annoyed.
If a payment is processed twice, users lose money.

That’s why this scenario discussion is very important. It tests whether you understand a brutal truth of distributed systems:

Retries are guaranteed. Duplicates are inevitable. Correctness is your responsibility.

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Previously, we covered User Tracking Service. Parent story where all things are at one place — link

Interview Conversation

Interviewer (Jai):
You’re working on a payment system. Producers retry on failures. Consumers run in parallel. How do you ensure a payment is never processed twice?

Candidate (Sara):
I don’t try to prevent duplicate delivery. I design the system so duplicate processing is harmless.