通过实例理解Go标准库http包是如何处理keep-alive连接的

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HTTP是如今互联网的基础协议，承载了互联网上的绝大部分应用层流量，并且从目前趋势来看，在未来10年，http仍然会是互联网应用的主要协议。Go语言自带“电池”，基于Go标准库我们可以轻松建立起一个http server处理客户端http请求，或创建一个http client向服务端发送http请求。

最初早期的http 1.0协议只支持短连接，即客户端每发送一个请求，就要和服务器端建立一个新TCP连接，请求处理完毕后，该连接将被拆除。显然每次tcp连接握手和拆除都将带来较大损耗，为了能充分利用已建立的连接，后来的http 1.0更新版和http 1.1支持在http请求头中加入Connection: keep-alive来告诉对方这个请求响应完成后不要关闭链接，下一次还要复用这个连接以继续传输后续请求和响应。后HTTP协议规范明确规定了HTTP/1.0版本如果想要保持长连接，需要在请求头中加上Connection: keep-alive，而HTTP/1.1版本将支持keep-alive长连接作为默认选项，有没有这个请求头都可以。

本文我们就来一起看看Go标准库中net/http包的http.Server和http.Client对keep-alive长连接的处理以及如何在Server和Client侧关闭keep-alive机制。

1. http包默认启用keep-alive

按照HTTP/1.1的规范，Go http包的http server和client的实现默认将所有连接视为长连接，无论这些连接上的初始请求是否带有Connection: keep-alive。

下面分别是使用go http包的默认机制实现的一个http client和一个http server：

默认开启keep-alive的http client实现：

//github.com/bigwhite/experiments/http-keep-alive/client-keepalive-on/client.go
package main

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

func main() {
    c := &http.Client{}
    req, err := http.NewRequest("Get", "http://localhost:8080", nil)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%#v\n", *req)

    for i := 0; i < 5; i++ {
        resp, err := c.Do(req)
        if err != nil {
            fmt.Println("http get error:", err)
            return
        }
        defer resp.Body.Close()

        b, err := ioutil.ReadAll(resp.Body)
        if err != nil {
            fmt.Println("read body error:", err)
            return
        }
        fmt.Println("response body:", string(b))
    }
}

默认开启keep-alive的http server实现：

//github.com/bigwhite/experiments/http-keep-alive/server-keepalive-on/server.go
package main

import (
    "fmt"
    "net/http"
)

func Index(w http.ResponseWriter, r *http.Request) {
    fmt.Println("receive a request from:", r.RemoteAddr, r.Header)
    w.Write([]byte("ok"))
}

func main() {
    var s = http.Server{
        Addr:    ":8080",
        Handler: http.HandlerFunc(Index),
    }
    s.ListenAndServe()
}

现在我们启动上面的http server：

// server-keepalive-on目录下
$go run server.go

我们使用上面的client向该server发起5次http请求：

// client-keepalive-on目录下
$go run client.go
http.Request{Method:"Get", URL:(*url.URL)(0xc00016a000), Proto:"HTTP/1.1", ProtoMajor:1, ProtoMinor:1, Header:http.Header{}, Body:io.ReadCloser(nil), GetBody:(func() (io.ReadCloser, error))(nil), ContentLength:0, TransferEncoding:[]string(nil), Close:false, Host:"localhost:8080", Form:url.Values(nil), PostForm:url.Values(nil), MultipartForm:(*multipart.Form)(nil), Trailer:http.Header(nil), RemoteAddr:"", RequestURI:"", TLS:(*tls.ConnectionState)(nil), Cancel:(<-chan struct {})(nil), Response:(*http.Response)(nil), ctx:(*context.emptyCtx)(0xc00012c008)}
response body: ok
response body: ok
response body: ok
response body: ok
response body: ok

这期间server端输出的日志如下：

receive a request from: [::1]:55238 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
receive a request from: [::1]:55238 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
receive a request from: [::1]:55238 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
receive a request from: [::1]:55238 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
receive a request from: [::1]:55238 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

我们简单分析一下两端的输出结果：

从server端打印的请求的头部字段来看，客户端发来的请求header中并没有显式包含Connection: keep-alive，而仅有Accept-Encoding和User-Agent两个header字段；
server端处理的5个请求均来自同一个连接“[::1]:55238”，Server端默认保持了该连接，而不是在处理完一个请求后将连接关闭，说明两端均复用了第一个请求创建的http连接。

即便我们的client端每间隔5秒发送一次请求，server端默认也不会关闭连接(我们将fmt包缓冲log包，输出带有时间戳的日志)：

// client-keepalive-on目录下
$go run client-with-delay.go
http.Request{Method:"Get", URL:(*url.URL)(0xc00016a000), Proto:"HTTP/1.1", ProtoMajor:1, ProtoMinor:1, Header:http.Header{}, Body:io.ReadCloser(nil), GetBody:(func() (io.ReadCloser, error))(nil), ContentLength:0, TransferEncoding:[]string(nil), Close:false, Host:"localhost:8080", Form:url.Values(nil), PostForm:url.Values(nil), MultipartForm:(*multipart.Form)(nil), Trailer:http.Header(nil), RemoteAddr:"", RequestURI:"", TLS:(*tls.ConnectionState)(nil), Cancel:(<-chan struct {})(nil), Response:(*http.Response)(nil), ctx:(*context.emptyCtx)(0xc00012c008)}
2021/01/03 12:25:21 response body: ok

2021/01/03 12:25:26 response body: ok
2021/01/03 12:25:31 response body: ok
2021/01/03 12:25:36 response body: ok
2021/01/03 12:25:41 response body: ok

// server-keepalive-on目录下
$go run server.go
2021/01/03 12:25:21 receive a request from: [::1]:58419 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 12:25:26 receive a request from: [::1]:58419 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 12:25:31 receive a request from: [::1]:58419 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 12:25:36 receive a request from: [::1]:58419 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 12:25:41 receive a request from: [::1]:58419 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

2. http client端基于非keep-alive连接发送请求

有时候http client在一条连接上的数据请求密度并不高，因此client端并不想长期保持这条连接(占用端口资源)，那么client端如何协调Server端在处理完请求返回应答后就关闭这条连接呢？我们看看在Go中如何实现这一场景需求：

//github.com/bigwhite/experiments/http-keep-alive/client-keepalive-off/client.go
... ...
func main() {
    tr := &http.Transport{
        DisableKeepAlives: true,
    }
    c := &http.Client{
        Transport: tr,
    }
    req, err := http.NewRequest("Get", "http://localhost:8080", nil)
    if err != nil {
        panic(err)
    }

    for i := 0; i < 5; i++ {
        resp, err := c.Do(req)
        if err != nil {
            fmt.Println("http get error:", err)
            return
        }
        defer resp.Body.Close()

        b, err := ioutil.ReadAll(resp.Body)
        if err != nil {
            fmt.Println("read body error:", err)
            return
        }
        log.Println("response body:", string(b))
        time.Sleep(5 * time.Second)
    }

}

http.Client底层的数据连接建立和维护是由http.Transport实现的，http.Transport结构有一个DisableKeepAlives字段，其默认值为false，即启动keep-alive。这里我们将其置为true，即关闭keep-alive，然后将该Transport实例作为初值，赋值给http Client实例的Transport字段。

接下来，我们使用这个client向上面那个http server发送五个请求，请求间间隔5秒(模拟连接空闲的状态)，我们得到如下结果(从server端打印信息观察)：

// 在client-keepalive-off下面
$go run client.go
2021/01/03 12:42:38 response body: ok
2021/01/03 12:42:43 response body: ok
2021/01/03 12:42:48 response body: ok
2021/01/03 12:42:53 response body: ok
2021/01/03 12:42:58 response body: ok

// 在server-keepalive-on下面

$go run server.go
2021/01/03 12:42:38 receive a request from: [::1]:62287 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

2021/01/03 12:42:43 receive a request from: [::1]:62301 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

2021/01/03 12:42:48 receive a request from: [::1]:62314 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

2021/01/03 12:42:53 receive a request from: [::1]:62328 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

2021/01/03 12:42:58 receive a request from: [::1]:62342 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

从Server的输出结果来看，来自客户端的请求中增加了Connection:[close]的头字段，当收到这样的请求后，Server端便不再保持这一连接了。我们也看到上面日志中，每个请求都是通过不同的客户端端口发送出来的，显然这是五条不同的连接。

3. 建立一个不支持keep-alive连接的http server

假设我们有这样的一个需求，server端完全不支持keep-alive的连接，无论client端发送的请求header中是否显式带有Connection: keep-alive，server端都会在返回应答后关闭连接。那么在Go中，我们如何来实现这一需求呢？我们来看下面代码：

//github.com/bigwhite/experiments/http-keep-alive/server-keepalive-off/server.go

package main

import (
    "log"
    "net/http"
)

func Index(w http.ResponseWriter, r *http.Request) {
    log.Println("receive a request from:", r.RemoteAddr, r.Header)
    w.Write([]byte("ok"))
}

func main() {
    var s = http.Server{
        Addr:    ":8080",
        Handler: http.HandlerFunc(Index),
    }
    s.SetKeepAlivesEnabled(false)
    s.ListenAndServe()
}

我们看到在ListenAndServe前，我们调用了http.Server的SetKeepAlivesEnabled方法，并传入false参数，这样我们就在全局层面关闭了该Server对keep-alive连接的支持，我们用前面client-keepalive-on下面的client向该Server发送五个请求：

// 在client-keepalive-on下面
$go run client.go
http.Request{Method:"Get", URL:(*url.URL)(0xc000174000), Proto:"HTTP/1.1", ProtoMajor:1, ProtoMinor:1, Header:http.Header{}, Body:io.ReadCloser(nil), GetBody:(func() (io.ReadCloser, error))(nil), ContentLength:0, TransferEncoding:[]string(nil), Close:false, Host:"localhost:8080", Form:url.Values(nil), PostForm:url.Values(nil), MultipartForm:(*multipart.Form)(nil), Trailer:http.Header(nil), RemoteAddr:"", RequestURI:"", TLS:(*tls.ConnectionState)(nil), Cancel:(<-chan struct {})(nil), Response:(*http.Response)(nil), ctx:(*context.emptyCtx)(0xc00013a008)}
2021/01/03 13:30:08 response body: ok
2021/01/03 13:30:08 response body: ok
2021/01/03 13:30:08 response body: ok
2021/01/03 13:30:08 response body: ok
2021/01/03 13:30:08 response body: ok

// 在server-keepalive-off下面
$go run server.go
2021/01/03 13:30:08 receive a request from: [::1]:53005 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 13:30:08 receive a request from: [::1]:53006 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 13:30:08 receive a request from: [::1]:53007 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 13:30:08 receive a request from: [::1]:53008 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 13:30:08 receive a request from: [::1]:53009 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

我们看到该Server在处理完每个请求后就关闭了传输该请求的连接，这导致client测不得不为每个请求建立一个新连接(从server输出的客户端地址和端口看出)。

4. 支持长连接闲置超时关闭的http server

显然上面的server处理方式“太过霸道”，对于想要复用连接，提高请求和应答传输效率的client而言，上面的“一刀切”机制并不合理。那么是否有一种机制可以让http server即可以对高密度传输数据的连接保持keep-alive，又可以及时清理掉那些长时间没有数据传输的idle连接，释放占用的系统资源呢？我们来看下面这个go实现的server：

//github.com/bigwhite/experiments/http-keep-alive/server-keepalive-with-idletimeout/server.go

package main

import (
    "log"
    "net/http"
    "time"
)

func Index(w http.ResponseWriter, r *http.Request) {
    log.Println("receive a request from:", r.RemoteAddr, r.Header)
    w.Write([]byte("ok"))
}

func main() {
    var s = http.Server{
        Addr:        ":8080",
        Handler:     http.HandlerFunc(Index),
        IdleTimeout: 5 * time.Second,
    }
    s.ListenAndServe()
}

从代码中我们看到，我们仅在创建http.Server实例时显式为其字段IdleTimeout做了一次显式赋值，设置idle连接的超时时间为5s。下面是Go标准库中关于http.Server的字段IdleTimeout的注释：

// $GOROOT/src/net/server.go

// IdleTimeout是当启用keep-alive时等待下一个请求的最大时间。
// 如果IdleTimeout为零，则使用ReadTimeout的值。如果两者都是
// 零，则没有超时。
IdleTimeout time.Duration

我们来看看效果如何，是否是我们期望那样的。为了测试效果，我们改造了client端，放在client-keepalive-on-with-idle下面：

//github.com/bigwhite/experiments/http-keep-alive/client-keepalive-on-with-idle/client.go
... ...
func main() {
    c := &http.Client{}
    req, err := http.NewRequest("Get", "http://localhost:8080", nil)
    if err != nil {
        panic(err)
    }

    for i := 0; i < 5; i++ {
        log.Printf("round %d begin:\n", i+1)
        for j := 0; j < i+1; j++ {
            resp, err := c.Do(req)
            if err != nil {
                fmt.Println("http get error:", err)
                return
            }
            defer resp.Body.Close()

            b, err := ioutil.ReadAll(resp.Body)
            if err != nil {
                fmt.Println("read body error:", err)
                return
            }
            log.Println("response body:", string(b))
        }
        log.Printf("round %d end\n", i+1)
        time.Sleep(7 * time.Second)
    }
}

client端请求分为5轮，轮与轮之间间隔7秒，下面是通信过程与结果：

// 在client-keepalive-on-with-idle下
$go run client.go
2021/01/03 14:17:05 round 1 begin:
2021/01/03 14:17:05 response body: ok
2021/01/03 14:17:05 round 1 end
2021/01/03 14:17:12 round 2 begin:
2021/01/03 14:17:12 response body: ok
2021/01/03 14:17:12 response body: ok
2021/01/03 14:17:12 round 2 end
2021/01/03 14:17:19 round 3 begin:
2021/01/03 14:17:19 response body: ok
2021/01/03 14:17:19 response body: ok
2021/01/03 14:17:19 response body: ok
2021/01/03 14:17:19 round 3 end
2021/01/03 14:17:26 round 4 begin:
2021/01/03 14:17:26 response body: ok
2021/01/03 14:17:26 response body: ok
2021/01/03 14:17:26 response body: ok
2021/01/03 14:17:26 response body: ok
2021/01/03 14:17:26 round 4 end
2021/01/03 14:17:33 round 5 begin:
2021/01/03 14:17:33 response body: ok
2021/01/03 14:17:33 response body: ok
2021/01/03 14:17:33 response body: ok
2021/01/03 14:17:33 response body: ok
2021/01/03 14:17:33 response body: ok
2021/01/03 14:17:33 round 5 end

// 在server-keepalive-with-idletimeout下
$go run server.go

2021/01/03 14:17:05 receive a request from: [::1]:64071 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

2021/01/03 14:17:12 receive a request from: [::1]:64145 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:12 receive a request from: [::1]:64145 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

2021/01/03 14:17:19 receive a request from: [::1]:64189 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:19 receive a request from: [::1]:64189 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:19 receive a request from: [::1]:64189 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

2021/01/03 14:17:26 receive a request from: [::1]:64250 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:26 receive a request from: [::1]:64250 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:26 receive a request from: [::1]:64250 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:26 receive a request from: [::1]:64250 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

2021/01/03 14:17:33 receive a request from: [::1]:64304 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:33 receive a request from: [::1]:64304 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:33 receive a request from: [::1]:64304 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:33 receive a request from: [::1]:64304 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:17:33 receive a request from: [::1]:64304 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

我们看到：
- 在每轮内，client端的所有请求都是复用已建立的连接；
- 但每轮之间，由于Sleep了7秒，超出了server端idletimeout的时长，上一轮的连接被拆除，新一轮只能重建连接。

我们期望的效果实现了！

5. 一个http client可管理到多个server的连接

Go标准库的http.Client与一个server可不是一对一的关系，它可以实现一对多的http通信，也就是说一个http client可管理到多个server的连接，并优先复用到同一server的连接(keep-alive)，而不是建立新连接，就像我们上面看到的那样。我们来创建一个向多个server发送请求的client：

//github.com/bigwhite/experiments/http-keep-alive/client-keepalive-on-to-multiple-servers/client.go
... ...
func main() {
    c := &http.Client{}
    req1, err := http.NewRequest("Get", "http://localhost:8080", nil)
    if err != nil {
        panic(err)
    }
    req2, err := http.NewRequest("Get", "http://localhost:8081", nil)
    if err != nil {
        panic(err)
    }

    for i := 0; i < 5; i++ {
        resp1, err := c.Do(req1)
        if err != nil {
            fmt.Println("http get error:", err)
            return
        }
        defer resp1.Body.Close()

        b1, err := ioutil.ReadAll(resp1.Body)
        if err != nil {
            fmt.Println("read body error:", err)
            return
        }
        log.Println("response1 body:", string(b1))

        resp2, err := c.Do(req2)
        if err != nil {
            fmt.Println("http get error:", err)
            return
        }
        defer resp2.Body.Close()

        b2, err := ioutil.ReadAll(resp2.Body)
        if err != nil {
            fmt.Println("read body error:", err)
            return
        }
        log.Println("response2 body:", string(b2))

        time.Sleep(5 * time.Second)
    }

}

我们建立两个默认的http server，分别监听8080和8081，运行上面client：

$go run client.go
2021/01/03 14:52:20 response1 body: ok
2021/01/03 14:52:20 response2 body: ok
2021/01/03 14:52:25 response1 body: ok
2021/01/03 14:52:25 response2 body: ok
2021/01/03 14:52:30 response1 body: ok
2021/01/03 14:52:30 response2 body: ok
2021/01/03 14:52:35 response1 body: ok
2021/01/03 14:52:35 response2 body: ok
2021/01/03 14:52:40 response1 body: ok
2021/01/03 14:52:40 response2 body: ok

server端的输出结果如下：

// server1(8080):
2021/01/03 14:52:20 receive a request from: [::1]:63871 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:25 receive a request from: [::1]:63871 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:30 receive a request from: [::1]:63871 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:35 receive a request from: [::1]:63871 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:40 receive a request from: [::1]:63871 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

// server2(8081):
2021/01/03 14:52:20 receive a request from: [::1]:63872 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:25 receive a request from: [::1]:63872 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:30 receive a request from: [::1]:63872 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:35 receive a request from: [::1]:63872 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2021/01/03 14:52:40 receive a request from: [::1]:63872 map[Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]

我们看到client同时支持与多个server进行通信，并针对每个server可以使用keep-alive的连接进行高效率通信。

本文涉及源代码可以在这里(https://github.com/bigwhite/experiments/tree/master/http-keep-alive)下载。

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Nemo

2021/01/09

1. 第一段代码里会 defer resp.Body.Close() 会导致循环结束后才真正关闭连接
2. 第二段代码里s设置了Handler之后，http.HandleFunc(“/”, Index) 就没有必要写了

回复
- bigwhite
  
  2021/01/13
  
  问题1：有意而为之，毕竟是demo。可能不是好的示范；
  问题2：改成自建server后，忘记删除第一行了。:)
  
  感谢提醒。
  
  回复
Sirius

2022/09/30

目前来看在http server层，我们可以通过 SetKeepAlivesEnabled() 方法控制是否开启KeepAlive；另外可以通过配置 IdleTimeout参数来设置超时时间，但目前并不能控制KeepAlive的心跳间隔时间。 tcp层有个SetKeepAlivePeriod()方法，但是貌似tcp和http的KeepAlive概念并不完全一致。

回复
- ching
  
  2023/02/01
  
  我理解心跳检测由client负责维持，在http client层，可以通过 Transport 下 KeepAlive参数来控制心跳检测间隔。
  
  回复
Z

2023/07/06

文章里：

http.Client底层的数据连接建立和维护是由http.Transport实现的，http.Transport结构有一个DisableKeepAlives字段，其默认值为false，即启动keep-alive。这里我们将其置为false，即关闭keep-alive，然后将该Transport实例作为初值，赋值给http Client实例的Transport字段。

—-

应该是将其设为true，关闭keep-alive吧。

回复
- bigwhite
  
  2023/07/06
  
  笔误，感谢指出！已经修改了。
  
  回复