Rate Limiter / Token Bucket

System DesignxAILast reported January 2026Low Frequency
Reported
2× across candidate reports
First seen
September 2025
Last reported
January 2026
Reported outcome
mixed

Problem Overview

Design a rate limiter, in xAI's onsite design round. Two reports. One records the twist: each user id can use a different amount of tokens -- one user is allowed 10,000 tokens per second while a regular user gets 100. The other report is a single line: the loop had…

  • 1 candidate-reported follow-up — the exact probes interviewers asked, with the trigger for each
  • The rest of the problem statement — full requirements, constraints, and edge cases
  • Approach and trade-offs — what passing candidates did, and the mistakes that sink people
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Verified Source
Every question is reconstructed from multiple independent candidate reports. Follow-ups candidates reported, not invented ones.
Codex Fact-Checked
Technical claims, formulas, and scale numbers are reviewed against primary sources.
Interviewer Follow-ups
The exact follow-ups reported by candidates, with the trigger that prompts each one — plus the mistakes that sink people.
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