Design a payment system at scale — requirements, capacity, and trade-offs.
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hardSystem Design
How would you design a payment system?
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01
Understand the problem
system-designcase-study
02
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Sketch your approach before reading the solution — that's what interviews test.
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03
Study the solution
Design a Payment System
Payments require exactly-once effect despite at-least-once networks.
| Concern | Pattern |
|---|---|
| Idempotency | idempotency-key UNIQUE on payments table; retry returns same result |
| State | PENDING → AUTHORIZED → CAPTURED → SETTLED Saga + outbox |
| Double spend | `op |
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Dossier 98 of 99 decoded in the System Design track. One more won't hurt.