Design UPI Payment System | System Design Interview

Design a scalable UPI-style payment system with idempotency, ledger correctness, webhooks, Kafka, reconciliation, and failure handling.

The core challenge in payment architecture is not just initiating a transaction. The real challenge is guaranteeing that no successful payment is lost, no user is charged twice, and every state transition is strictly controlled even under retries, timeouts, duplicated callbacks, and worker crashes.

In this video, we cover:

* UPI-style payment system requirements
* Payment initiation flow from mobile app to API Gateway
* Risk and fraud validation
* Redis-based idempotency lock using userId + requestId
* Transaction DB as the mutable payment state machine
* Payment states: INITIATED, PENDING, SUCCESS, FAILED
* Provider handoff to UPI/bank network
* Handling pending response and asynchronous polling
* Bank/provider webhook callback handling
* Deduplication of provider callbacks
* Immutable ledger DB for financial correctness
* Kafka event bus for async side effects
* Notification worker, analytics worker, merchant dashboard worker
* Transactional outbox pattern for zero message loss
* Sync vs async architectural decoupling
* Strict schema-level constraints
* Failure handling for timeouts, retries, worker crashes, and ledger mismatch
* Reconciliation worker for bank settlement verification
* Security and observability metrics for payment systems

This video is useful for:

* Senior Software Engineer system design interviews
* Staff Engineer HLD interviews
* Backend architecture preparation
* Payment system design
* UPI architecture understanding
* Wallet and ledger system design
* FinTech distributed systems
* Reliability and idempotency design

Common interview questions answered:

* How do you design a UPI payment system?
* How do you prevent duplicate payments?
* How do you handle payment retries and double taps?
* How do you design an immutable ledger?
* Why do payment systems need reconciliation?
* How do you process bank webhooks safely?
* How do you avoid losing Kafka events after DB commit?
* What is the transactional outbox pattern?
* How do you separate synchronous payment path from async side effects?
* How do you guarantee no successful payment is dropped?

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