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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package rate provides a rate limiter.
package rate
import (
// Limit defines the maximum frequency of some events.
// Limit is represented as number of events per second.
// A zero Limit allows no events.
type Limit float64
// Inf is the infinite rate limit; it allows all events (even if burst is zero).
const Inf = Limit(math.MaxFloat64)
// Every converts a minimum time interval between events to a Limit.
func Every(interval time.Duration) Limit {
if interval <= 0 {
return Inf
return 1 / Limit(interval.Seconds())
// A Limiter controls how frequently events are allowed to happen.
// It implements a "token bucket" of size b, initially full and refilled
// at rate r tokens per second.
// Informally, in any large enough time interval, the Limiter limits the
// rate to r tokens per second, with a maximum burst size of b events.
// As a special case, if r == Inf (the infinite rate), b is ignored.
// See for more about token buckets.
// The zero value is a valid Limiter, but it will reject all events.
// Use NewLimiter to create non-zero Limiters.
// Limiter has three main methods, Allow, Reserve, and Wait.
// Most callers should use Wait.
// Each of the three methods consumes a single token.
// They differ in their behavior when no token is available.
// If no token is available, Allow returns false.
// If no token is available, Reserve returns a reservation for a future token
// and the amount of time the caller must wait before using it.
// If no token is available, Wait blocks until one can be obtained
// or its associated context.Context is canceled.
// The methods AllowN, ReserveN, and WaitN consume n tokens.
type Limiter struct {
mu sync.Mutex
limit Limit
burst int
tokens float64
// last is the last time the limiter's tokens field was updated
last time.Time
// lastEvent is the latest time of a rate-limited event (past or future)
lastEvent time.Time
// Limit returns the maximum overall event rate.
func (lim *Limiter) Limit() Limit {
return lim.limit
// Burst returns the maximum burst size. Burst is the maximum number of tokens
// that can be consumed in a single call to Allow, Reserve, or Wait, so higher
// Burst values allow more events to happen at once.
// A zero Burst allows no events, unless limit == Inf.
func (lim *Limiter) Burst() int {
return lim.burst
// TokensAt returns the number of tokens available at time t.
func (lim *Limiter) TokensAt(t time.Time) float64 {
_, tokens := lim.advance(t) // does not mutate lim
return tokens
// Tokens returns the number of tokens available now.
func (lim *Limiter) Tokens() float64 {
return lim.TokensAt(time.Now())
// NewLimiter returns a new Limiter that allows events up to rate r and permits
// bursts of at most b tokens.
func NewLimiter(r Limit, b int) *Limiter {
return &Limiter{
limit: r,
burst: b,
// Allow reports whether an event may happen now.
func (lim *Limiter) Allow() bool {
return lim.AllowN(time.Now(), 1)
// AllowN reports whether n events may happen at time t.
// Use this method if you intend to drop / skip events that exceed the rate limit.
// Otherwise use Reserve or Wait.
func (lim *Limiter) AllowN(t time.Time, n int) bool {
return lim.reserveN(t, n, 0).ok
// A Reservation holds information about events that are permitted by a Limiter to happen after a delay.
// A Reservation may be canceled, which may enable the Limiter to permit additional events.
type Reservation struct {
ok bool
lim *Limiter
tokens int
timeToAct time.Time
// This is the Limit at reservation time, it can change later.
limit Limit
// OK returns whether the limiter can provide the requested number of tokens
// within the maximum wait time. If OK is false, Delay returns InfDuration, and
// Cancel does nothing.
func (r *Reservation) OK() bool {
return r.ok
// Delay is shorthand for DelayFrom(time.Now()).
func (r *Reservation) Delay() time.Duration {
return r.DelayFrom(time.Now())
// InfDuration is the duration returned by Delay when a Reservation is not OK.
const InfDuration = time.Duration(math.MaxInt64)
// DelayFrom returns the duration for which the reservation holder must wait
// before taking the reserved action. Zero duration means act immediately.
// InfDuration means the limiter cannot grant the tokens requested in this
// Reservation within the maximum wait time.
func (r *Reservation) DelayFrom(t time.Time) time.Duration {
if !r.ok {
return InfDuration
delay := r.timeToAct.Sub(t)
if delay < 0 {
return 0
return delay
// Cancel is shorthand for CancelAt(time.Now()).
func (r *Reservation) Cancel() {
// CancelAt indicates that the reservation holder will not perform the reserved action
// and reverses the effects of this Reservation on the rate limit as much as possible,
// considering that other reservations may have already been made.
func (r *Reservation) CancelAt(t time.Time) {
if !r.ok {
if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(t) {
// calculate tokens to restore
// The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved
// after r was obtained. These tokens should not be restored.
restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct))
if restoreTokens <= 0 {
// advance time to now
t, tokens := r.lim.advance(t)
// calculate new number of tokens
tokens += restoreTokens
if burst := float64(r.lim.burst); tokens > burst {
tokens = burst
// update state
r.lim.last = t
r.lim.tokens = tokens
if r.timeToAct == r.lim.lastEvent {
prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))
if !prevEvent.Before(t) {
r.lim.lastEvent = prevEvent
// Reserve is shorthand for ReserveN(time.Now(), 1).
func (lim *Limiter) Reserve() *Reservation {
return lim.ReserveN(time.Now(), 1)
// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.
// The Limiter takes this Reservation into account when allowing future events.
// The returned Reservation’s OK() method returns false if n exceeds the Limiter's burst size.
// Usage example:
// r := lim.ReserveN(time.Now(), 1)
// if !r.OK() {
// // Not allowed to act! Did you remember to set lim.burst to be > 0 ?
// return
// }
// time.Sleep(r.Delay())
// Act()
// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events.
// If you need to respect a deadline or cancel the delay, use Wait instead.
// To drop or skip events exceeding rate limit, use Allow instead.
func (lim *Limiter) ReserveN(t time.Time, n int) *Reservation {
r := lim.reserveN(t, n, InfDuration)
return &r
// Wait is shorthand for WaitN(ctx, 1).
func (lim *Limiter) Wait(ctx context.Context) (err error) {
return lim.WaitN(ctx, 1)
// WaitN blocks until lim permits n events to happen.
// It returns an error if n exceeds the Limiter's burst size, the Context is
// canceled, or the expected wait time exceeds the Context's Deadline.
// The burst limit is ignored if the rate limit is Inf.
func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
// The test code calls lim.wait with a fake timer generator.
// This is the real timer generator.
newTimer := func(d time.Duration) (<-chan time.Time, func() bool, func()) {
timer := time.NewTimer(d)
return timer.C, timer.Stop, func() {}
return lim.wait(ctx, n, time.Now(), newTimer)
// wait is the internal implementation of WaitN.
func (lim *Limiter) wait(ctx context.Context, n int, t time.Time, newTimer func(d time.Duration) (<-chan time.Time, func() bool, func())) error {
burst := lim.burst
limit := lim.limit
if n > burst && limit != Inf {
return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, burst)
// Check if ctx is already cancelled
select {
case <-ctx.Done():
return ctx.Err()
// Determine wait limit
waitLimit := InfDuration
if deadline, ok := ctx.Deadline(); ok {
waitLimit = deadline.Sub(t)
// Reserve
r := lim.reserveN(t, n, waitLimit)
if !r.ok {
return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
// Wait if necessary
delay := r.DelayFrom(t)
if delay == 0 {
return nil
ch, stop, advance := newTimer(delay)
defer stop()
advance() // only has an effect when testing
select {
case <-ch:
// We can proceed.
return nil
case <-ctx.Done():
// Context was canceled before we could proceed. Cancel the
// reservation, which may permit other events to proceed sooner.
return ctx.Err()
// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit).
func (lim *Limiter) SetLimit(newLimit Limit) {
lim.SetLimitAt(time.Now(), newLimit)
// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated
// or underutilized by those which reserved (using Reserve or Wait) but did not yet act
// before SetLimitAt was called.
func (lim *Limiter) SetLimitAt(t time.Time, newLimit Limit) {
t, tokens := lim.advance(t)
lim.last = t
lim.tokens = tokens
lim.limit = newLimit
// SetBurst is shorthand for SetBurstAt(time.Now(), newBurst).
func (lim *Limiter) SetBurst(newBurst int) {
lim.SetBurstAt(time.Now(), newBurst)
// SetBurstAt sets a new burst size for the limiter.
func (lim *Limiter) SetBurstAt(t time.Time, newBurst int) {
t, tokens := lim.advance(t)
lim.last = t
lim.tokens = tokens
lim.burst = newBurst
// reserveN is a helper method for AllowN, ReserveN, and WaitN.
// maxFutureReserve specifies the maximum reservation wait duration allowed.
// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN.
func (lim *Limiter) reserveN(t time.Time, n int, maxFutureReserve time.Duration) Reservation {
if lim.limit == Inf {
return Reservation{
ok: true,
lim: lim,
tokens: n,
timeToAct: t,
} else if lim.limit == 0 {
var ok bool
if lim.burst >= n {
ok = true
lim.burst -= n
return Reservation{
ok: ok,
lim: lim,
tokens: lim.burst,
timeToAct: t,
t, tokens := lim.advance(t)
// Calculate the remaining number of tokens resulting from the request.
tokens -= float64(n)
// Calculate the wait duration
var waitDuration time.Duration
if tokens < 0 {
waitDuration = lim.limit.durationFromTokens(-tokens)
// Decide result
ok := n <= lim.burst && waitDuration <= maxFutureReserve
// Prepare reservation
r := Reservation{
ok: ok,
lim: lim,
limit: lim.limit,
if ok {
r.tokens = n
r.timeToAct = t.Add(waitDuration)
// Update state
lim.last = t
lim.tokens = tokens
lim.lastEvent = r.timeToAct
return r
// advance calculates and returns an updated state for lim resulting from the passage of time.
// lim is not changed.
// advance requires that is held.
func (lim *Limiter) advance(t time.Time) (newT time.Time, newTokens float64) {
last := lim.last
if t.Before(last) {
last = t
// Calculate the new number of tokens, due to time that passed.
elapsed := t.Sub(last)
delta := lim.limit.tokensFromDuration(elapsed)
tokens := lim.tokens + delta
if burst := float64(lim.burst); tokens > burst {
tokens = burst
return t, tokens
// durationFromTokens is a unit conversion function from the number of tokens to the duration
// of time it takes to accumulate them at a rate of limit tokens per second.
func (limit Limit) durationFromTokens(tokens float64) time.Duration {
if limit <= 0 {
return InfDuration
seconds := tokens / float64(limit)
return time.Duration(float64(time.Second) * seconds)
// tokensFromDuration is a unit conversion function from a time duration to the number of tokens
// which could be accumulated during that duration at a rate of limit tokens per second.
func (limit Limit) tokensFromDuration(d time.Duration) float64 {
if limit <= 0 {
return 0
return d.Seconds() * float64(limit)