Rate Limiter / Token Bucket

Problem Overview

xAI asks this in two distinct flavors across rounds. Coding variant (Round 1): given a pre-built template with DistributedCache and TokenBucket classes, implement (1) refillTokenBucket with lazy-refresh logic — compute tokens to add from elapsed time since last refill, cap at max capacity, update the refill timestamp — and (2)…

• 3 candidate-reported follow-ups — 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