System designHardsd-g391

Subject PaymentsLevel Senior–Staff~45 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design a payment-routing layer that, on a soft decline from one PSP, cascades the same charge to a second (and third) PSP to recover the sale — 'decline recovery / payment cascading' — while guaranteeing the customer is charged at most once even though the same payment is attempted across multiple processors. ~5,000 charges/sec, each potentially hitting 1–3 PSPs. The danger: PSP A times out (you don't know if it charged), you cascade to PSP B which succeeds, and later PSP A's charge also lands — double charge. How do you cascade safely?

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

Loading whiteboard…

Run or narrate your approach, then ask the coach.