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// Copyright (c) 2017 Uber Technologies, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package utils
import (
"sync"
"time"
)
// RateLimiter is a filter used to check if a message that is worth itemCost units is within the rate limits.
type RateLimiter interface {
CheckCredit(itemCost float64) bool
}
type rateLimiter struct {
sync.Mutex
creditsPerSecond float64
balance float64
maxBalance float64
lastTick time.Time
timeNow func() time.Time
}
// NewRateLimiter creates a new rate limiter based on leaky bucket algorithm, formulated in terms of a
// credits balance that is replenished every time CheckCredit() method is called (tick) by the amount proportional
// to the time elapsed since the last tick, up to max of creditsPerSecond. A call to CheckCredit() takes a cost
// of an item we want to pay with the balance. If the balance exceeds the cost of the item, the item is "purchased"
// and the balance reduced, indicated by returned value of true. Otherwise the balance is unchanged and return false.
//
// This can be used to limit a rate of messages emitted by a service by instantiating the Rate Limiter with the
// max number of messages a service is allowed to emit per second, and calling CheckCredit(1.0) for each message
// to determine if the message is within the rate limit.
//
// It can also be used to limit the rate of traffic in bytes, by setting creditsPerSecond to desired throughput
// as bytes/second, and calling CheckCredit() with the actual message size.
func NewRateLimiter(creditsPerSecond, maxBalance float64) RateLimiter {
return &rateLimiter{
creditsPerSecond: creditsPerSecond,
balance: maxBalance,
maxBalance: maxBalance,
lastTick: time.Now(),
timeNow: time.Now}
}
func (b *rateLimiter) CheckCredit(itemCost float64) bool {
b.Lock()
defer b.Unlock()
// calculate how much time passed since the last tick, and update current tick
currentTime := b.timeNow()
elapsedTime := currentTime.Sub(b.lastTick)
b.lastTick = currentTime
// calculate how much credit have we accumulated since the last tick
b.balance += elapsedTime.Seconds() * b.creditsPerSecond
if b.balance > b.maxBalance {
b.balance = b.maxBalance
}
// if we have enough credits to pay for current item, then reduce balance and allow
if b.balance >= itemCost {
b.balance -= itemCost
return true
}
return false
}
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