Stripe SWE Internship Per-Key Rate-Limiter Screen

Role: Software Engineering Intern
Result: Not disclosed
Timing: The report's structured season conflicts with its publication date. October 2025 follows the report month.

The 45-minute screen used a per-key rate limiter with two operations:

• hit(key, timestamp) records an access.
• allowed(key, timestamp) checks whether the recent-window count is within the limit.

The interviewer allowed either a sliding window or a fixed window. The design had to support many keys and high concurrency.

Discussion covered deques or ring buffers, cleanup of expired entries, burst traffic, absent keys, out-of-order timestamps, long idle periods, and time and space complexity. Exact boundary behavior was not stated.

Practice: Per-Key Rate Limiter.