Golang倒腾一款简配的具有请求排队功能的并发受限服务器

golang官方指南^[1]给了一些代码片段，层层递进演示了信道的能力:

1>. 信号量
2>. 限流能力

var sem = make(chan int, MaxOutstanding) 
 
func Serve(queue chan *Request) {
    for req := range queue {
        req:= req
        sem <- 1   
        go func() {   // 只会开启MaxOutstanding个并发协程
            process(req)
            <-sem
        }()
    }
}

上面出现了两个信道：

① sem 提供了限制服务端并发处理请求的信号量
② queue 提供了一个客户端请求队列，起媒介/解耦的作用

进一步指南给出了信道的另一个用法： 3>. 解多路复用

多路复用是网络编程中一个耳熟能详的概念，nginx redis等高性能web、内存kv都用到了这个技术。

这个解多路复用是怎么理解呢？

离散/独立/并发的客户端请求被服务端Serve收敛之后， Serve就起到了多路复用的概念，在Request定义resultChan信道，就给每个客户端请求提供了独立获取请求结果的能力，这便是一种解多路复用。

从实际效果看这就是常见的互联网web服务器：一款具备请求排队功能的并发限流服务器。

官方指南并没有完整实现客户端和服务器端工程。

下面是我的工程化实现，记录实践中遇到的问题。

并发受限服务器

信道queue接收客户端请求,解耦客户端和服务器，天然具备排队能力
信号量信道sem提供了并发受限的能力
服务器处理完，向解多路复用信道req.resultChan写入响应结果。

/* 实现一个有请求队列功能的并发请求受限服务器*/

package main

import (
 "fmt"
 "sync"
 "time"
)

var sem = make(chan int, Maxoutstanding)

var wg2 sync.WaitGroup

func server(queue chan *Request) {
 fmt.Printf("Server is already, listen req \n")

 for req := range queue {
 req := req
 sem <- 1

 wg2.Add(1)
 go func() {
 defer wg2.Done()
 process(req)
 <-sem
 }()
 }
}

func process(req *Request) {
 s := sum(req.args)
 req.resultChan <- s
}
func sum(a []int) (s int) {
 for i := 1; i <= a[0]; i++ {
 s += i
 }
 time.Sleep(time.Millisecond * 20)
 return s
}

time.Sleep模拟服务器处理请求单次耗时20ms，输出数字的累加，
eg： input： 100；
output: (1+100)/2*100 =5050

wg2 sync.WaitGroup是一个动态活跃的Goroutine计数器，注意用法和位置，wg2的作用是：等待所有请求处理完成。

并发客户端请求

for循环开启并发客户端请求，

每个请求入驻一个独立的Goroutine，独立向信道queue投递请求和接收响应

package main

import (
 "fmt"
 "sync"
)

type Request struct {
 args       []int
 resultChan chan int
}

var wg1 sync.WaitGroup

func clients() {
 fmt.Printf("start %d concurrency client request\n ", concurrencyClients)
 for i := 1; i <= concurrencyClients; i++ {
 r := &Request{
 args:       []int{i},
 resultChan: make(chan int),
 }
 wg1.Add(1)
 go ClientReq(r)
 }
 wg1.Wait() 

}

func ClientReq(r *Request) {
 defer wg1.Done()
 queue <- r
 go func() {
 res := <-r.resultChan
 fmt.Printf("current args is %d, the result is %d \n", r.args[0], res)
 }()
}

wg1 WaitGroup的目的是确保所有的客户端请求都已经发出，之后客户端任务结束，所以此处我们新开Goroutine处理响应结果（这里又有闭包的参与）。

工程化

工程化代码的先后顺序，决定了代码是否死锁。
server需要处于监听状态，故先启动。

本处clients在主协程整体上是同步发送，如果放在clients()的后面，clients内的wg1可能会有部分请求Goroutine阻塞在信道queue, 且没法唤醒, 运行时会检测出报死锁。

package main

import (
 "fmt"
 "time"
)

var concurrencyClients = 1000
var queueLength = 100
var queue = make(chan *Request, queueLength) // 请求队列长度
var Maxoutstanding int = 10                  // 服务器并发受限10

func main() {

 go server(queue)
 var start = time.Now()

 clients() // 确保所有的请求都已经发出去

 wg2.Wait() // 确保服务器处理完所有的请求
 fmt.Printf("客户端并发%d请求，服务器请求队列长度%d，服务器限流%d，总共耗时%d ms \n", concurrencyClients, queueLength, Maxoutstanding, time.Since(start).Milliseconds())
}

上面出现了3个配置变量
1>. 客户端并发请求数量concurrencyClients=100
2>. 服务器排队队列长度queueLength=100，会作用到信道queue
3>. 服务器并发受限阈值Maxoutstanding=10

start 1000 concurrency client request
 Server is already, listen req 
current args is 14, the result is 105 
current args is 2, the result is 3 
current args is 3, the result is 6 
current args is 1, the result is 1 
current args is 4, the result is 10 
current args is 8, the result is 36 
current args is 6, the result is 21 
current args is 12, the result is 78 
current args is 5, the result is 15 
current args is 7, the result is 28 
current args is 18, the result is 171 
current args is 16, the result is 136 
current args is 15, the result is 120 
current args is 20, the result is 210 
current args is 19, the result is 190 
current args is 13, the result is 91 
current args is 21, the result is 231 
current args is 10, the result is 55 
current args is 17, the result is 153 
current args is 9, the result is 45 
current args is 22, the result is 253 
current args is 28, the result is 406 
current args is 27, the result is 378 
current args is 11, the result is 66 
current args is 26, the result is 351 
current args is 30, the result is 465 
current args is 23, the result is 276 
current args is 25, the result is 325 
current args is 29, the result is 435 
current args is 24, the result is 300 
current args is 31, the result is 496 
current args is 34, the result is 595 
current args is 38, the result is 741 
current args is 36, the result is 666 
current args is 41, the result is 861 
current args is 32, the result is 528 
current args is 35, the result is 630 
current args is 33, the result is 561 
current args is 37, the result is 703 
current args is 39, the result is 780 
current args is 52, the result is 1378 
current args is 46, the result is 1081 
current args is 47, the result is 1128 
current args is 49, the result is 1225 
current args is 45, the result is 1035 
current args is 43, the result is 946 
current args is 48, the result is 1176 
current args is 40, the result is 820 
current args is 42, the result is 903 
current args is 44, the result is 990 
current args is 59, the result is 1770 
current args is 55, the result is 1540 
current args is 53, the result is 1431 
current args is 57, the result is 1653 
current args is 51, the result is 1326 
current args is 54, the result is 1485 
current args is 50, the result is 1275 
current args is 56, the result is 1596 
current args is 58, the result is 1711 
current args is 60, the result is 1830 
current args is 66, the result is 2211 
current args is 63, the result is 2016 
current args is 70, the result is 2485 
current args is 62, the result is 1953 
current args is 61, the result is 1891 
current args is 65, the result is 2145 
current args is 67, the result is 2278 
current args is 64, the result is 2080 
current args is 68, the result is 2346 
current args is 69, the result is 2415 
current args is 76, the result is 2926 
current args is 77, the result is 3003 
current args is 71, the result is 2556 
current args is 80, the result is 3240 
current args is 75, the result is 2850 
current args is 74, the result is 2775 
current args is 73, the result is 2701 
current args is 72, the result is 2628 
current args is 78, the result is 3081 
current args is 81, the result is 3321 
current args is 89, the result is 4005 
current args is 83, the result is 3486 
current args is 88, the result is 3916 
current args is 82, the result is 3403 
current args is 79, the result is 3160 
current args is 86, the result is 3741 
current args is 84, the result is 3570 
current args is 90, the result is 4095 
current args is 85, the result is 3655 
current args is 87, the result is 3828 
current args is 101, the result is 5151 
current args is 92, the result is 4278 
current args is 94, the result is 4465 
current args is 93, the result is 4371 
current args is 98, the result is 4851 
current args is 91, the result is 4186 
current args is 99, the result is 4950 
current args is 100, the result is 5050 
current args is 95, the result is 4560 
current args is 96, the result is 4656 
current args is 109, the result is 5995 
current args is 107, the result is 5778 
current args is 108, the result is 5886 
current args is 102, the result is 5253 
current args is 103, the result is 5356 
current args is 106, the result is 5671 
current args is 105, the result is 5565 
current args is 104, the result is 5460 
current args is 111, the result is 6216 
current args is 97, the result is 4753 
current args is 120, the result is 7260 
current args is 112, the result is 6328 
current args is 113, the result is 6441 
current args is 114, the result is 6555 
current args is 110, the result is 6105 
current args is 119, the result is 7140 
current args is 115, the result is 6670 
current args is 117, the result is 6903 
current args is 116, the result is 6786 
current args is 118, the result is 7021 
current args is 123, the result is 7626 
current args is 122, the result is 7503 
current args is 130, the result is 8515 
current args is 121, the result is 7381 
current args is 126, the result is 8001 
current args is 129, the result is 8385 
......

current args is 988, the result is 488566 
current args is 992, the result is 492528 
current args is 976, the result is 476776 
current args is 984, the result is 484620 
current args is 995, the result is 495510 
current args is 999, the result is 499500 
current args is 1000, the result is 500500 
current args is 990, the result is 490545 
客户端并发1000请求，服务器请求队列长度100，服务器限流10，总共耗时2099 ms

读者可以随意调整3个参数的大小，来感受服务器调参的魅力。