Go Academy · Lesson

golang.org/x/time/rate

Use the rate limiter.

Lesson 2 of 413 steps

golang.org/x/time/rate is a free Go Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, and your progress syncs across the web and the CoddyKit app. The Go Academy course includes 4 lessons in total.

The rate Package

golang.org/x/time/rate provides a production-ready token bucket Limiter. It is the standard choice for rate limiting in Go.

Battle-tested implementation
Allow, Wait, and Reserve methods
Context-aware

Creating a Limiter

rate.NewLimiter takes a rate (tokens per second) and a burst size. rate.Limit is a float64 of events per second.

limiter := rate.NewLimiter(rate.Limit(10), 5)

rate.Every

If you think in intervals rather than per-second rates, rate.Every converts a duration into a rate. One event every 200ms equals five per second.

limiter := rate.NewLimiter(rate.Every(200*time.Millisecond), 1)

Allow

Allow reports whether a token is available right now. It returns true and consumes a token, or returns false immediately. Use it to reject excess requests.

if !limiter.Allow() {
    fmt.Println("rejected: too many requests")
    return
}

Wait

Wait blocks until a token is available or the context is cancelled. It is ideal when you want to pace work rather than drop it.

if err := limiter.Wait(ctx); err != nil {
    return err
}
doWork()

Reserve

Reserve returns a Reservation describing how long to wait. Call Delay() to learn the wait, and Cancel() if you decide not to proceed.

r := limiter.Reserve()
if !r.OK() {
    return
}
time.Sleep(r.Delay())

Note: External Package

The rate package is not in the standard library, so it requires go get golang.org/x/time/rate and a module. Snippets that import it are illustrative and not run inline here.

A Runnable Standard-Library Analog

Since the rate package is external, here is a standard-library pacing loop that mimics Wait by ticking every 200ms.

package main

import (
    "fmt"
    "time"
)

func main() {
    tick := time.NewTicker(200 * time.Millisecond)
    defer tick.Stop()
    for i := 0; i < 4; i++ {
        <-tick.C
        fmt.Println("processed event", i)
    }
}

Tuning at Runtime

You can change limits on the fly with SetLimit and SetBurst. This is handy when a config reload or an upstream 429 response tells you to slow down.

limiter.SetLimit(rate.Limit(20))
limiter.SetBurst(10)

Per-Client Limiters

Keep a map from client key to *rate.Limiter, guarded by a mutex. Create a limiter on first request for that key, then reuse it for subsequent requests.

func getLimiter(m map[string]*rate.Limiter, key string) *rate.Limiter {
    if l, ok := m[key]; ok {
        return l
    }
    l := rate.NewLimiter(rate.Limit(5), 10)
    m[key] = l
    return l
}

HTTP Middleware

Wrap handlers so each request calls Allow() first. On false, respond with HTTP 429 Too Many Requests. This is the canonical server-side rate limit.

Quick Check

Test your rate package knowledge.

Answer it in the interactive lesson

Recap

You learned golang.org/x/time/rate:

NewLimiter(rate, burst) creates the bucket
rate.Every converts an interval to a rate
Allow rejects, Wait paces, Reserve reports delay
SetLimit and SetBurst tune at runtime
Per-client map plus HTTP 429 for servers

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Use the rate limiter. You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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