Design a rate limiter for Affirm's merchant API
Last updated: August 6, 2025
Quick Overview
Design a distributed rate limiting system that protects Affirm's payment APIs from abuse while providing fair access to thousands of merchants with different tier-based rate limits.
Affirm
August 6, 202510
8
3,710 solved
Design a distributed rate limiting system that protects Affirm's payment APIs from abuse while providing fair access to thousands of merchants with different tier-based rate limits.
Affirm's merchant-facing APIs must handle high traffic during shopping events while protecting backend services from overload. This is a common system design question that Affirm tailors to their fintech context.
What the Interviewer Expects
- Compare rate limiting algorithms (token bucket, sliding window, fixed window)
- Design per-merchant rate limits with tiered access levels
- Handle distributed rate limiting across multiple API gateway instances
- Discuss graceful degradation when rate limits are hit
- Consider how rate limiting interacts with idempotent payment retries
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 rate limiting during Black Friday traffic spikes?
- Should payment capture requests have different limits than status check requests?
- How do you communicate rate limit status to merchants via API headers?
- How would you prevent a single merchant's traffic from affecting others?
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Practice System Design ProblemsSample Answer
Requirements
Functional Requirements:
- Merchant API Protection: Implement rate limiting for Affirm's payment APIs to prevent abuse by merchants.
- Tier-based Rate Limits: Support multiple tiers of r...
Capacity Estimation
Assuming Affirm serves approximately 10,000 merchant API requests per second during peak hours:
- Request Distribution: If we assume 60% of these requests come from the top 1,000 merchants (high v...
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