问题

在最近一次上线中，检查日志发现，存在很多的HTTP请求异常，具体报错内容为：

 dial tcp Host:Port: connect: cannot assign requested address

再查看监控，发现CPU、服务器连接均较高，CPU负载100%、连接数达到7w左右，基本可以判断为连接数过多，本机的端口基本被占满，无法发起新的HTTP请求导致，因为连接会占用CPU资源，故而CPU也被占满。

问题解决

其后经过服务器升级、代码检查、持久化连接等，终于找到问题原因，主要是一个不用的数据被使用时导致大量Go协程并发，解决后，服务器基本正常。

问题思考

虽然问题已经解决，但是如果真的服务器的连接数爆满，那么该如何解决呢？本文基于此问题进行展开阐述。

应用基本场景

在项目开发过程中，总会遇到使用HTTP方式访问其他服务，不管是外部的三方服务还是自己公司内部的其他服务，基于HTTP的请求大都是短链接，即访问完成后链接就会被释放。但是在高并发场景下由于对方服务的情况，无法在短时间内获得请求结果，就会导致客户端积压大量的链接，甚至占满客户端的端口，无法发起新的请求。

HTTP的连接

1. 串行连接

每次发起一次HTTP请求都需要一次链接，使用TCP的三次握手完成一次链接，发起请求获取数据。

必须等待上一次连接结束后在发起新的HTTP请求，建立新的TCP连接。

2. 持久连接

HTTP/1.1 允许客户端在发起请求结束后仍然保持在打开状态的TCP连接，便于后续请求继续使用。

减少TCP连接建立握手的时间延迟
减少了打开连接的潜在数量

3. 并行连接

通过多条TCP连接发起并发的HTTP请求。

每个连接之间有较小的时间延迟
每次发起的HTTP请求也是一个独立的TCP连接
并行连接数不能太多，占用本地CPU、内存、端口等各类资源，目前浏览器基本也都支持并行连接，一般限制连接数的值，比如4个

4. 管道化连接

通过共享的TCP连接发起并发的HTTP请求。

建立在持久化连接的基础上，将多条请求放入队列，一同发往服务端
降低网络的环回时间，提高性能
必须确保服务端支持持久化连接
做好连接会在任意时间关闭的准备，准备好重复所有未完成的管道化请求

Golang中持久化连接

在Golang中使用持久化连接发起HTTP请求，主要依赖Transport，官方封装的net库中已经支持。

Transport实现了RoundTripper接口，该接口只有一个方法RoundTrip()，故Transport的入口函数就是RoundTrip()。
Transport的主要功能：

缓存了长连接，用于大量http请求场景下的连接复用
对连接做一些限制，连接超时时间，每个host的最大连接数

在实际应用中，需要在初始化HTTP的client时传入transport，以进行保持连接，Transport的主要结构为：

 type Transport struct {
    // DialContext specifies the dial function for creating unencrypted TCP connections.
    // If DialContext is nil (and the deprecated Dial below is also nil),
    // then the transport dials using package net.
    //
    // DialContext runs concurrently with calls to RoundTrip.
    // A RoundTrip call that initiates a dial may end up using
    // a connection dialed previously when the earlier connection
    // becomes idle before the later DialContext completes.
    DialContext func(ctx context.Context, network, addr string) (net.Conn, error)

    // MaxIdleConns controls the maximum number of idle (keep-alive)
    // connections across all hosts. Zero means no limit.
    MaxIdleConns int

    // MaxIdleConnsPerHost, if non-zero, controls the maximum idle
    // (keep-alive) connections to keep per-host. If zero,
    // DefaultMaxIdleConnsPerHost is used.
    MaxIdleConnsPerHost int

    // MaxConnsPerHost optionally limits the total number of
    // connections per host, including connections in the dialing,
    // active, and idle states. On limit violation, dials will block.
    //
    // Zero means no limit.
    MaxConnsPerHost int

    // IdleConnTimeout is the maximum amount of time an idle
    // (keep-alive) connection will remain idle before closing
    // itself.
    // Zero means no limit.
    IdleConnTimeout time.Duration
}

具体使用Demo如下

 package main

import (
    "fmt"
    "io/ioutil"
    "net"
    "net/http"
    "time"
)

var HTTPTransport = &http.Transport{
    DialContext: (&net.Dialer{
        Timeout:   30 * time.Second, // 连接超时时间
        KeepAlive: 60 * time.Second, // 保持长连接的时间
    }).DialContext, // 设置连接的参数
    MaxIdleConns:          500, // 最大空闲连接
    IdleConnTimeout:       60 * time.Second, // 空闲连接的超时时间
    ExpectContinueTimeout: 30 * time.Second, // 等待服务第一个响应的超时时间
    MaxIdleConnsPerHost:   100, // 每个host保持的空闲连接数
}

func main() {
    times := 50
    uri := "#34;
    // uri := "#34;
    

    // 短连接的情况
    start := time.Now()
    client := http.Client{} // 初始化http的client
    for i := 0; i < times; i++ {
        req, err := http.NewRequest(http.MethodGet, uri, nil)
        if err != nil {
            panic("Http Req Failed " + err.Error())
        }
        resp, err := client.Do(req) // 发起请求
        if err != nil {
            panic("Http Request Failed " + err.Error())
        }
        defer resp.Body.Close()
        ioutil.ReadAll(resp.Body)
    }
    fmt.Println("Orig GoNet Short Link", time.Since(start))
    

    // 长连接的情况
    start2 := time.Now()
    client2 := http.Client{Transport: HTTPTransport} // 初始化一个带有transport的http的client
    for i := 0; i < times; i++ {
        req, err := http.NewRequest(http.MethodGet, uri, nil)
        if err != nil {
            panic("Http Req Failed " + err.Error())
        }
        resp, err := client2.Do(req)
        if err != nil {
            panic("Http Request Failed " + err.Error())
        }
        defer resp.Body.Close()
        ioutil.ReadAll(resp.Body) // 如果不及时从请求中获取结果，此连接会占用，其他请求服务复用连接
    }
    fmt.Println("Orig GoNet Long Link", time.Since(start2))
}

经过本地测试，使用transport确实能控制客户端的连接数，使得本地资源使用得到大幅度的降低。通过netstat可以查看具体的连接情况：

长连接下连接数

在测试时也发现一个有意思的情况，如果发起请求后，而不获取请求的结果，即缺少如下代码

 ioutil.ReadAll(resp.Body)

则客户端会重新建立连接发起新的请求，也即没有利用到持久化连接的优势，通过netstat可以查看到连接在持续增加：

未能及时获取请求返回数据时连接数在增加

所以大家在发起HTTP请求的时候，一定记得接收返回值，哪怕返回值数据不需要进行处理。

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智云一二三科技

Golang开发中如何使用长连接

问题

问题解决

问题思考

应用基本场景

HTTP的连接

1. 串行连接

2. 持久连接

3. 并行连接

4. 管道化连接

Golang中持久化连接

关于作者: 智云科技

32条评论

问题

问题解决

问题思考

应用基本场景

HTTP的连接

1. 串行连接

2. 持久连接

3. 并行连接

4. 管道化连接

Golang中持久化连接

给这篇文章的作者打赏

关于作者: 智云科技

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