372 lines
11 KiB
Go
372 lines
11 KiB
Go
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package rate provides a rate limiter.
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package rate
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import (
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"fmt"
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"math"
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"sync"
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"time"
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"golang.org/x/net/context"
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)
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// Limit defines the maximum frequency of some events.
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// Limit is represented as number of events per second.
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// A zero Limit allows no events.
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type Limit float64
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// Inf is the infinite rate limit; it allows all events (even if burst is zero).
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const Inf = Limit(math.MaxFloat64)
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// Every converts a minimum time interval between events to a Limit.
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func Every(interval time.Duration) Limit {
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if interval <= 0 {
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return Inf
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}
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return 1 / Limit(interval.Seconds())
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}
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// A Limiter controls how frequently events are allowed to happen.
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// It implements a "token bucket" of size b, initially full and refilled
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// at rate r tokens per second.
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// Informally, in any large enough time interval, the Limiter limits the
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// rate to r tokens per second, with a maximum burst size of b events.
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// As a special case, if r == Inf (the infinite rate), b is ignored.
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// See https://en.wikipedia.org/wiki/Token_bucket for more about token buckets.
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//
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// The zero value is a valid Limiter, but it will reject all events.
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// Use NewLimiter to create non-zero Limiters.
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//
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// Limiter has three main methods, Allow, Reserve, and Wait.
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// Most callers should use Wait.
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//
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// Each of the three methods consumes a single token.
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// They differ in their behavior when no token is available.
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// If no token is available, Allow returns false.
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// If no token is available, Reserve returns a reservation for a future token
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// and the amount of time the caller must wait before using it.
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// If no token is available, Wait blocks until one can be obtained
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// or its associated context.Context is canceled.
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//
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// The methods AllowN, ReserveN, and WaitN consume n tokens.
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type Limiter struct {
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limit Limit
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burst int
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mu sync.Mutex
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tokens float64
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// last is the last time the limiter's tokens field was updated
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last time.Time
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// lastEvent is the latest time of a rate-limited event (past or future)
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lastEvent time.Time
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}
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// Limit returns the maximum overall event rate.
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func (lim *Limiter) Limit() Limit {
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lim.mu.Lock()
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defer lim.mu.Unlock()
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return lim.limit
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}
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// Burst returns the maximum burst size. Burst is the maximum number of tokens
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// that can be consumed in a single call to Allow, Reserve, or Wait, so higher
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// Burst values allow more events to happen at once.
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// A zero Burst allows no events, unless limit == Inf.
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func (lim *Limiter) Burst() int {
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return lim.burst
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}
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// NewLimiter returns a new Limiter that allows events up to rate r and permits
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// bursts of at most b tokens.
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func NewLimiter(r Limit, b int) *Limiter {
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return &Limiter{
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limit: r,
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burst: b,
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}
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}
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// Allow is shorthand for AllowN(time.Now(), 1).
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func (lim *Limiter) Allow() bool {
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return lim.AllowN(time.Now(), 1)
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}
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// AllowN reports whether n events may happen at time now.
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// Use this method if you intend to drop / skip events that exceed the rate limit.
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// Otherwise use Reserve or Wait.
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func (lim *Limiter) AllowN(now time.Time, n int) bool {
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return lim.reserveN(now, n, 0).ok
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}
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// A Reservation holds information about events that are permitted by a Limiter to happen after a delay.
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// A Reservation may be canceled, which may enable the Limiter to permit additional events.
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type Reservation struct {
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ok bool
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lim *Limiter
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tokens int
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timeToAct time.Time
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// This is the Limit at reservation time, it can change later.
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limit Limit
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}
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// OK returns whether the limiter can provide the requested number of tokens
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// within the maximum wait time. If OK is false, Delay returns InfDuration, and
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// Cancel does nothing.
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func (r *Reservation) OK() bool {
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return r.ok
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}
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// Delay is shorthand for DelayFrom(time.Now()).
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func (r *Reservation) Delay() time.Duration {
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return r.DelayFrom(time.Now())
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}
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// InfDuration is the duration returned by Delay when a Reservation is not OK.
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const InfDuration = time.Duration(1<<63 - 1)
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// DelayFrom returns the duration for which the reservation holder must wait
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// before taking the reserved action. Zero duration means act immediately.
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// InfDuration means the limiter cannot grant the tokens requested in this
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// Reservation within the maximum wait time.
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func (r *Reservation) DelayFrom(now time.Time) time.Duration {
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if !r.ok {
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return InfDuration
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}
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delay := r.timeToAct.Sub(now)
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if delay < 0 {
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return 0
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}
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return delay
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}
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// Cancel is shorthand for CancelAt(time.Now()).
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func (r *Reservation) Cancel() {
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r.CancelAt(time.Now())
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return
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}
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// CancelAt indicates that the reservation holder will not perform the reserved action
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// and reverses the effects of this Reservation on the rate limit as much as possible,
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// considering that other reservations may have already been made.
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func (r *Reservation) CancelAt(now time.Time) {
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if !r.ok {
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return
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}
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r.lim.mu.Lock()
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defer r.lim.mu.Unlock()
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if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) {
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return
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}
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// calculate tokens to restore
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// The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved
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// after r was obtained. These tokens should not be restored.
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restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct))
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if restoreTokens <= 0 {
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return
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}
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// advance time to now
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now, _, tokens := r.lim.advance(now)
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// calculate new number of tokens
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tokens += restoreTokens
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if burst := float64(r.lim.burst); tokens > burst {
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tokens = burst
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}
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// update state
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r.lim.last = now
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r.lim.tokens = tokens
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if r.timeToAct == r.lim.lastEvent {
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prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))
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if !prevEvent.Before(now) {
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r.lim.lastEvent = prevEvent
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}
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}
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return
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}
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// Reserve is shorthand for ReserveN(time.Now(), 1).
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func (lim *Limiter) Reserve() *Reservation {
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return lim.ReserveN(time.Now(), 1)
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}
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// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.
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// The Limiter takes this Reservation into account when allowing future events.
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// ReserveN returns false if n exceeds the Limiter's burst size.
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// Usage example:
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// r := lim.ReserveN(time.Now(), 1)
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// if !r.OK() {
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// // Not allowed to act! Did you remember to set lim.burst to be > 0 ?
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// return
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// }
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// time.Sleep(r.Delay())
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// Act()
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// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events.
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// If you need to respect a deadline or cancel the delay, use Wait instead.
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// To drop or skip events exceeding rate limit, use Allow instead.
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func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation {
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r := lim.reserveN(now, n, InfDuration)
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return &r
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}
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// Wait is shorthand for WaitN(ctx, 1).
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func (lim *Limiter) Wait(ctx context.Context) (err error) {
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return lim.WaitN(ctx, 1)
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}
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// WaitN blocks until lim permits n events to happen.
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// It returns an error if n exceeds the Limiter's burst size, the Context is
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// canceled, or the expected wait time exceeds the Context's Deadline.
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// The burst limit is ignored if the rate limit is Inf.
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func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
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if n > lim.burst && lim.limit != Inf {
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return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, lim.burst)
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}
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// Check if ctx is already cancelled
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select {
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case <-ctx.Done():
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return ctx.Err()
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default:
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}
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// Determine wait limit
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now := time.Now()
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waitLimit := InfDuration
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if deadline, ok := ctx.Deadline(); ok {
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waitLimit = deadline.Sub(now)
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}
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// Reserve
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r := lim.reserveN(now, n, waitLimit)
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if !r.ok {
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return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
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}
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// Wait
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t := time.NewTimer(r.DelayFrom(now))
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defer t.Stop()
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select {
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case <-t.C:
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// We can proceed.
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return nil
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case <-ctx.Done():
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// Context was canceled before we could proceed. Cancel the
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// reservation, which may permit other events to proceed sooner.
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r.Cancel()
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return ctx.Err()
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}
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}
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// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit).
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func (lim *Limiter) SetLimit(newLimit Limit) {
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lim.SetLimitAt(time.Now(), newLimit)
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}
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// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated
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// or underutilized by those which reserved (using Reserve or Wait) but did not yet act
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// before SetLimitAt was called.
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func (lim *Limiter) SetLimitAt(now time.Time, newLimit Limit) {
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lim.mu.Lock()
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defer lim.mu.Unlock()
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now, _, tokens := lim.advance(now)
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lim.last = now
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lim.tokens = tokens
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lim.limit = newLimit
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}
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// reserveN is a helper method for AllowN, ReserveN, and WaitN.
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// maxFutureReserve specifies the maximum reservation wait duration allowed.
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// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN.
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func (lim *Limiter) reserveN(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
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lim.mu.Lock()
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if lim.limit == Inf {
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lim.mu.Unlock()
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return Reservation{
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ok: true,
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lim: lim,
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tokens: n,
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timeToAct: now,
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}
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}
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now, last, tokens := lim.advance(now)
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// Calculate the remaining number of tokens resulting from the request.
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tokens -= float64(n)
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// Calculate the wait duration
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var waitDuration time.Duration
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if tokens < 0 {
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waitDuration = lim.limit.durationFromTokens(-tokens)
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}
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// Decide result
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ok := n <= lim.burst && waitDuration <= maxFutureReserve
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// Prepare reservation
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r := Reservation{
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ok: ok,
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lim: lim,
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limit: lim.limit,
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}
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if ok {
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r.tokens = n
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r.timeToAct = now.Add(waitDuration)
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}
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// Update state
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if ok {
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lim.last = now
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lim.tokens = tokens
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lim.lastEvent = r.timeToAct
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} else {
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lim.last = last
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}
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lim.mu.Unlock()
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return r
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}
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// advance calculates and returns an updated state for lim resulting from the passage of time.
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// lim is not changed.
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func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
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last := lim.last
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if now.Before(last) {
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last = now
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}
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// Avoid making delta overflow below when last is very old.
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maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
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elapsed := now.Sub(last)
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if elapsed > maxElapsed {
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elapsed = maxElapsed
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}
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// Calculate the new number of tokens, due to time that passed.
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delta := lim.limit.tokensFromDuration(elapsed)
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tokens := lim.tokens + delta
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if burst := float64(lim.burst); tokens > burst {
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tokens = burst
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}
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return now, last, tokens
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}
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// durationFromTokens is a unit conversion function from the number of tokens to the duration
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// of time it takes to accumulate them at a rate of limit tokens per second.
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func (limit Limit) durationFromTokens(tokens float64) time.Duration {
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seconds := tokens / float64(limit)
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return time.Nanosecond * time.Duration(1e9*seconds)
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}
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// tokensFromDuration is a unit conversion function from a time duration to the number of tokens
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// which could be accumulated during that duration at a rate of limit tokens per second.
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func (limit Limit) tokensFromDuration(d time.Duration) float64 {
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return d.Seconds() * float64(limit)
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}
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