Implement a rate limiter and a pub/sub API

Q: Implement a rate limiter and a pub/sub API

This question evaluates competency in concurrency control, synchronization, API and low-level system design, and messaging system concepts by combining a rate limiter implementation with a pub/sub API.

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Question

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You have two coding/design (low-level design) tasks.

1) Rate limiter

Design and implement a rate limiter with an API like:

boolean allow(String clientId) or boolean allow(String key)

It should enforce a policy such as:

N requests per time window (e.g., 100 requests per minute)

Discuss (and ideally implement) thread-safety and how you would adapt it for multiple application instances.

2) Pub/Sub

Design and implement a simple Pub/Sub component (Java-like interfaces are fine), supporting:

publish(topic, message)
subscribe(topic, subscriber)
unsubscribe(topic, subscriber)

Clarify delivery semantics (best-effort vs at-least-once), ordering per topic, and how you handle slow consumers.

Implement a rate limiter and a pub/sub API

Quick Overview

1) Rate limiter

2) Pub/Sub

Solution

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