How do you implement rate limiting in a Go HTTP server?

Rate limiting protects services from overload. Go provides the token bucket algorithm via golang.org/x/time/rate. Production implementations typically rate-limit per client IP or API key, not globally.

import "golang.org/x/time/rate"

// Global limiter (simple, not per-client)
var globalLimiter = rate.NewLimiter(rate.Limit(100), 10)
// 100 requests/second with burst of 10

func globalRateLimitMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if !globalLimiter.Allow() {
            http.Error(w, "too many requests", http.StatusTooManyRequests)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Per-client rate limiter (production pattern)
type IPRateLimiter struct {
    mu       sync.Mutex
    limiters map[string]*rate.Limiter
    r        rate.Limit
    b        int
}

func NewIPRateLimiter(r rate.Limit, b int) *IPRateLimiter {
    return &IPRateLimiter{limiters: make(map[string]*rate.Limiter), r: r, b: b}
}

func (l *IPRateLimiter) getLimiter(ip string) *rate.Limiter {
    l.mu.Lock()
    defer l.mu.Unlock()
    if lim, ok := l.limiters[ip]; ok {
        return lim
    }
    lim := rate.NewLimiter(l.r, l.b)
    l.limiters[ip] = lim
    return lim
}

func (l *IPRateLimiter) Middleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        ip, _, _ := net.SplitHostPort(r.RemoteAddr)
        if !l.getLimiter(ip).Allow() {
            w.Header().Set("Retry-After", "1")
            http.Error(w, "rate limit exceeded", http.StatusTooManyRequests)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Blocking wait: hold request until token available (or ctx expires)
if err := limiter.Wait(ctx); err != nil {
    http.Error(w, "request cancelled", http.StatusServiceUnavailable)
}

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