速率限制是控制资源利用和维持服务质量的重要机制。通过goroutines，channel和ticker都可以优雅地支持速率限制。
首先我们来看一下基本速率限制。假设想限制对传入请求的处理。我们会在相同名称的通道上放送这些要求。
这个限制器通道将每200毫秒接收一个值。这是速率限制方案中的调节器。
通过在服务每个请求之前阻塞来自限制器信道的接收，我们限制自己每200毫秒接收1个请求。
我们可能希望在速率限制方案中允许短脉冲串请求，同时保持总体速率限制。可以通过缓冲的限制器通道来实现。这个burstylimiter通道将允许最多3个事件的突发。
填充通道以表示允许突发。
每200毫秒，将尝试向burstylimiter添加一个新值，最大限制为3。现在模拟5个更多的传入请求。这些传入请求中的前3个未超过burstylimiter值。
运行程序后，就会看到第一批请求每〜200毫秒处理一次。
对于第二批请求，程序会立即服务前3个，因为突发速率限制，然后剩余2服务都具有〜200ms延迟。
所有的示例代码，都放在
f:\worksp\golang目录下。安装go编程环境请参考：http://www.h3.com/go/go_environment.html
rate-limiting.go的完整代码如下所示 -
package main
import "time"
import "fmt"
func main() {
// first we'll look at basic rate limiting. suppose
// we want to limit our handling of incoming requests.
// we'll serve these requests off a channel of the
// same name.
requests := make(chan int, 5)
for i := 1; i <= 5; i++ {
requests <- i
}
close(requests)
// this `limiter` channel will receive a value
// every 200 milliseconds. this is the regulator in
// our rate limiting scheme.
limiter := time.tick(time.millisecond * 200)
// by blocking on a receive from the `limiter` channel
// before serving each request, we limit ourselves to
// 1 request every 200 milliseconds.
for req := range requests {
<-limiter
fmt.println("request", req, time.now())
}
// we may want to allow short bursts of requests in
// our rate limiting scheme while preserving the
// overall rate limit. we can accomplish this by
// buffering our limiter channel. this `burstylimiter`
// channel will allow bursts of up to 3 events.
burstylimiter := make(chan time.time, 3)
// fill up the channel to represent allowed bursting.
for i := 0; i < 3; i++ {
burstylimiter <- time.now()
}
// every 200 milliseconds we'll try to add a new
// value to `burstylimiter`, up to its limit of 3.
go func() {
for t := range time.tick(time.millisecond * 200) {
burstylimiter <- t
}
}()
// now simulate 5 more incoming requests. the first
// 3 of these will benefit from the burst capability
// of `burstylimiter`.
burstyrequests := make(chan int, 5)
for i := 1; i <= 5; i++ {
burstyrequests <- i
}
close(burstyrequests)
for req := range burstyrequests {
<-burstylimiter
fmt.println("request", req, time.now())
}
}
执行上面代码，将得到以下输出结果 -
f:\worksp\golang>go run rate-limiting.go
request 1 2017-01-21 14:43:39.1445218 +0800 cst
request 2 2017-01-21 14:43:39.345767 +0800 cst
request 3 2017-01-21 14:43:39.5460635 +0800 cst
request 4 2017-01-21 14:43:39.7441739 +0800 cst
request 5 2017-01-21 14:43:39.9444929 +0800 cst
request 1 2017-01-21 14:43:39.9464898 +0800 cst
request 2 2017-01-21 14:43:39.9504928 +0800 cst
request 3 2017-01-21 14:43:39.9544955 +0800 cst
request 4 2017-01-21 14:43:40.1467214 +0800 cst
request 5 2017-01-21 14:43:40.3469624 +0800 cst
