# System Design Interview: Payment Settlement Batch Processing

Page: https://stenobird.com/podcast/software-engineer-interview-prep-podcast-7756520/system-design-interview-payment-settlement-batch-processing
Text version: https://stenobird.com/podcast/software-engineer-interview-prep-podcast-7756520/system-design-interview-payment-settlement-batch-processing.md
Podcast: [Software Engineer Interview Prep Podcast](https://stenobird.com/podcast/software-engineer-interview-prep-podcast-7756520)
Published: 2026-05-17T11:45:50+00:00
Episode link: https://podcasters.spotify.com/pod/show/prabuddha-ganegoda/episodes/System-Design-Interview-Payment-Settlement-Batch-Processing-e3jg0dr
Audio file: https://anchor.fm/s/10f2c0f8c/podcast/play/120110971/https%3A%2F%2Fd3ctxlq1ktw2nl.cloudfront.net%2Fstaging%2F2026-4-17%2F424314545-44100-2-580e05915292d.mp3
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JSON: https://stenobird.com/v1/public/podcasts/software-engineer-interview-prep-podcast-7756520/episodes/system-design-interview-payment-settlement-batch-processing
Duration seconds: 571

## Resource

Design a batch processing system for end-of-day payment settlement at a payments company that processes 50 million transactions per day . The system must net merchant positions, calculate fees, and initiate fund transfers to merchant bank accounts within a strict bank cutoff window. Walk me through your design, covering reliability, scalability, and how you'd handle failures. Key Takeaways The outbox pattern is the canonical solution to the dual-write problem. Know it cold. Partition keys define ordering and parallelism — choose with intention, usually around the natural aggregation boundary (here, merchant ID). Throttling must be distributed when you have multiple workers — Redis-backed buckets or a sidecar. Idempotency is non-negotiable in payments. Design for at-least-once and dedupe. Retries are tiered : in-process for transient, delay-queue for slower-resolving, DLQ for terminal. Backpressure beats dropping — use Kafka lag as your buffer when downstream is slow. Reconciliation closes the loop — you don't know it worked until ground truth confirms. Corrections are new events — never rewrite history in a financial system.

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