Rate-Limited PubSub

Implementing a rate-limited publish-subscribe system

This medium-difficulty coding problem tests your ability to design and implement a distributed messaging system with rate-limiting constraints—a core concern at quantitative trading firms where market-data feeds can overwhelm downstream consumers. The challenge lies in balancing correct subscription management, efficient message delivery, and accurate rate-limit tracking across a sliding time window.

A working solution requires careful bookkeeping: maintaining subscriber-to-topic mappings, tracking message counts per subscriber within the active time window, and enforcing a global (not per-topic) rate limit. You'll need to handle window expiry correctly as time advances, manage subscriptions and unsubscriptions, and silently drop messages when a subscriber hits their quota—without affecting other subscribers to the same topic.

• Sliding-window rate limiting and timestamp-based expiry
• Efficient data structures for topic subscriptions and message counts
• State management under sequential time progression
• Edge cases: duplicate subscriptions, unsubscribe during window, window boundary conditions