// 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 (
"fmt"
"math"
"sync"
"time"
"golang.org/x/net/context"
)
// 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 https://en.wikipedia.org/wiki/Token_bucket 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 {
//maximum token, token num per second
limit Limit
//burst field, max token num
burst int
mu sync.Mutex
//tokens num, change
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 {
lim.mu.Lock()
defer lim.mu.Unlock()
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
}
// 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 is shorthand for AllowN(time.Now(), 1).func (lim *Limiter) Allow() bool {
return lim.AllowN(time.Now(), 1)
}
// AllowN reports whether n events may happen at time now.// Use this method if you intend to drop / skip events that exceed the rate limit.// Otherwise use Reserve or Wait.func (lim *Limiter) AllowN(now time.Time, n int) bool {
return lim.reserveN(now, 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
//This is the time to action
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(1<<63 - 1)
// 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(now time.Time) time.Duration {
if !r.ok {
return InfDuration
}
delay := r.timeToAct.Sub(now)
if delay < 0 {
return 0
}
return delay
}
// Cancel is shorthand for CancelAt(time.Now()).func (r *Reservation) Cancel() {
r.CancelAt(time.Now())
return
}
// 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(now time.Time) {
if !r.ok {
return
}
r.lim.mu.Lock()
defer r.lim.mu.Unlock()
if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) {
return
}
// 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 {
return
}
// advance time to now
now, _, tokens := r.lim.advance(now)
// calculate new number of tokens
tokens += restoreTokens
if burst := float64(r.lim.burst); tokens > burst {
tokens = burst
}
// update state
r.lim.last = now
r.lim.tokens = tokens
if r.timeToAct == r.lim.lastEvent {
prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))
if !prevEvent.Before(now) {
r.lim.lastEvent = prevEvent
}
}
return
}
// 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.// ReserveN returns false if n exceeds the Limiter's burst size.// Usage example:// r, ok := lim.ReserveN(time.Now(), 1)// if !ok {// // Not allowed to act! Did you remember to set lim.burst to be > 0 ?// }// 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(now time.Time, n int) *Reservation {
r := lim.reserveN(now, 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.func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
if n > lim.burst {
return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, lim.burst)
}
// Check if ctx is already cancelled
select {
case <-ctx.Done():
return ctx.Err()
default:
}
// Determine wait limit
now := time.Now()
waitLimit := InfDuration
if deadline, ok := ctx.Deadline(); ok {
waitLimit = deadline.Sub(now)
}
// Reserve
r := lim.reserveN(now, n, waitLimit)
if !r.ok {
return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
}
// Wait
t := time.NewTimer(r.DelayFrom(now))
defer t.Stop()
select {
case <-t.C:
// 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.
r.Cancel()
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(now time.Time, newLimit Limit) {
lim.mu.Lock()
defer lim.mu.Unlock()
now, _, tokens := lim.advance(now)
lim.last = now
lim.tokens = tokens
lim.limit = newLimit
}
// 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(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
lim.mu.Lock()
defer lim.mu.Unlock()
if lim.limit == Inf {
return Reservation{
ok: true,
lim: lim,
tokens: n,
timeToAct: now,
}
}
now, last, tokens := lim.advance(now)
// 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 = now.Add(waitDuration)
}
// Update state
if ok {
lim.last = now
lim.tokens = tokens
lim.lastEvent = r.timeToAct
} else {
lim.last = last
}
return r
}
// advance calculates and returns an updated state for lim resulting from the passage of time.// lim is not changed.func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
last := lim.last
if now.Before(last) {
last = now
}
// Avoid making delta overflow below when last is very old.
maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
elapsed := now.Sub(last)
if elapsed > maxElapsed {
elapsed = maxElapsed
}
// Calculate the new number of tokens, due to time that passed.
delta := lim.limit.tokensFromDuration(elapsed)
tokens := lim.tokens + delta
if burst := float64(lim.burst); tokens > burst {
tokens = burst
}
return now, last, 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 {
seconds := tokens / float64(limit)
return time.Nanosecond * time.Duration(1e9*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 {
return d.Seconds() * float64(limit)
}
