Design a checkout integration SDK for merchant onboarding
Last updated: August 6, 2025
Quick Overview
Design a merchant-facing SDK and backend that allows e-commerce platforms to integrate Affirm BNPL at checkout with minimal engineering effort. Cover the JavaScript widget, server-side APIs, and webhook notifications.
Affirm
August 6, 202514
7
3,899 solved
Design a merchant-facing SDK and backend that allows e-commerce platforms to integrate Affirm BNPL at checkout with minimal engineering effort. Cover the JavaScript widget, server-side APIs, and webhook notifications.
Merchant integration is how Affirm grows its network. This question assesses your ability to design developer-facing products, which is important at Affirm since the merchant experience directly impacts adoption and revenue.
What the Interviewer Expects
- Design a clean API for merchant checkout integration (REST endpoints for creating charges, capturing, refunding)
- Include a JavaScript SDK for embedding Affirm's checkout widget on merchant sites
- Discuss webhook design for notifying merchants about payment events
- Handle versioning and backward compatibility for the SDK
- Address security: authentication, data validation, and PCI implications
Key Topics to Cover
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- How would you handle a merchant who sends duplicate capture requests?
- How do you ensure webhook delivery reliability?
- How would you support merchant-specific customization of the checkout widget?
- What analytics would you provide to merchants?
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Practice System Design ProblemsSample Answer
Requirements
Functional Requirements:
- JavaScript SDK: A lightweight, customizable JavaScript widget for embedding Affirm's BNPL option on merchant checkout pages.
- REST API Endpoints:
- **Crea...
Capacity Estimation
Assuming Affirm partners with 1,000 e-commerce platforms, with each platform processing an average of 500 transactions per day:
- Total Daily Transactions: 1,000 platforms * 500 transactions = 50...
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