Project 5: Rate Limiter Library

A production-grade rate limiting library implementing multiple algorithms (token bucket, sliding window, leaky bucket) with a clean API, suitable for use in HTTP middleware.

Quick Reference

Attribute Value
Primary Language Go
Alternative Languages Rust, Java, C++
Difficulty Level 3: Advanced
Time Estimate 1-2 weeks
Knowledge Area Algorithms, Time Management, API Design
Tooling None (from scratch)
Prerequisites Completed Projects 1-4. Strong understanding of concurrency. Familiarity with rate limiting concepts.

What You Will Build

A production-grade rate limiting library implementing multiple algorithms (token bucket, sliding window, leaky bucket) with a clean API, suitable for use in HTTP middleware.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Implementing rate limiting algorithms → maps to time package and algorithm design
  • Making it thread-safe → maps to sync.Mutex and atomic operations
  • Designing a clean API → maps to interface design and Go idioms
  • Testing time-dependent code → maps to test design and mocking

Key Concepts

  • Token bucket algorithm: System Design resources
  • Time handling: “The Go Programming Language” Ch. 6 - Donovan & Kernighan
  • sync/atomic: “Concurrency in Go” Ch. 3 - Katherine Cox-Buday
  • Interface design: “100 Go Mistakes” Ch. 2 - Teiva Harsanyi

Real-World Outcome

// Create a rate limiter: 100 requests per second, burst of 10
limiter := ratelimit.NewTokenBucket(100, 10)

// Use in HTTP middleware
func RateLimitMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        clientIP := r.RemoteAddr

        if !limiter.Allow(clientIP) {
            http.Error(w, "Rate limit exceeded", http.StatusTooManyRequests)
            return
        }

        next.ServeHTTP(w, r)
    })
}

// Different algorithms available
tokenBucket := ratelimit.NewTokenBucket(rate, burst)
slidingWindow := ratelimit.NewSlidingWindow(rate, windowSize)
leakyBucket := ratelimit.NewLeakyBucket(rate)

// Check current state
info := limiter.Status("client-123")
fmt.Printf("Remaining: %d, Reset in: %v\n", info.Remaining, info.ResetIn)

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: LEARN_GO_DEEP_DIVE.md
  • “100 Go Mistakes and How to Avoid Them” by Teiva Harsanyi