Design a configurable monthly API rate limiter

Q: Design a configurable monthly API rate limiter

This is a System Design interview question from Salesforce for Software Engineer roles. View the full question and solution on PracHub.

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Question

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Design a rate-limiting system for an expensive API.

Requirements

Each user (or API key) has a monthly quota (e.g., 10,000 calls/month).
Users can manually configure (update) their monthly quota via a UI/API.
If a user exceeds the quota, further requests must be rejected (e.g., HTTP 429) until the next monthly reset.

Scale & reliability

Assume high QPS, multiple stateless API servers, and the limiter must work in a distributed environment.
Discuss correctness goals (strict vs eventually consistent) and trade-offs.

Deliverables

High-level architecture and request flow
Data model/storage choices for counters and quota configuration
How to handle monthly reset, time zones, retries, and idempotency
Operational concerns: monitoring, failure modes, and abuse cases

Design a configurable monthly API rate limiter

Requirements

Scale & reliability

Deliverables

Solution

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