客户端禁用Keep-alive，服务端开启Keep-alive，会怎么样？

最近部署的web程序，在服务器上出现不少time_wait的连接状态，会占用tcp端口，费了几天时间排查。

之前我有结论：HTTP keep-alive 是在应用层对TCP连接的滑动续约复用，如果客户端、服务器稳定续约，就成了名副其实的长连接。

目前所有的HTTP网络库(不论是客户端、服务端)都默认开启了HTTP Keep-Alive，通过Request/Response的Connection标头来协商复用连接。

非常规做法导致的短连接

我手上有个项目，由于历史原因，客户端禁用了Keep-Alive，服务端默认开启了Keep-Alive，如此一来协商复用连接失败， 客户端每次请求会使用新的TCP连接， 也就是回退为短连接。

客户端强制禁用Keep-Alive

package mainimport (    "fmt"    "io/ioutil"    "log"    "net/http"    "time")func main() {    tr := http.Transport{        DisableKeepAlives: true,    }    client := &http.Client{        Timeout:   10 * time.Second,        Transport: &tr,    }    for {        requestWithClose(client)        time.Sleep(time.Second * 1)    }}func requestWithClose(client *http.Client) {    resp, err := client.Get("http://10.100.219.9:8081")    if err != nil {        fmt.Printf("error occurred while fetching page, error: %s", err.Error())        return    }    defer resp.Body.Close()    c, err := ioutil.ReadAll(resp.Body)    if err != nil {        log.Fatalf("Couldn't parse response body. %+v", err)    }    fmt.Println(string(c))}

web服务端默认开启Keep-Alive

package mainimport (    "fmt"    "log"    "net/http")// 根据RemoteAddr 知道客户端使用的持久连接func IndexHandler(w http.ResponseWriter, r *http.Request) {    fmt.Println("receive a request from:", r.RemoteAddr, r.Header)    w.Write([]byte("ok"))}func main() {    fmt.Printf("Starting server at port 8081\n")    // net/http 默认开启持久连接    if err := http.ListenAndServe(":8081", http.HandlerFunc(IndexHandler)); err != nil {        log.Fatal(err)    }}

从服务端的日志看，确实是短连接。

receive a request from: 10.22.38.48:54722 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54724 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54726 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54728 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54731 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54733 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54734 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54738 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54740 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54741 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54743 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54744 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.38.48:54746 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

谁是主动断开方？

我想当然的以为 客户端是主动断开方，被现实啪啪打脸。

某一天服务器上超过300的time_wait报警告诉我这tmd是服务器主动终断连接。

常规的TCP4次挥手， 主动断开方会进入time_wait状态，等待2MSL后释放占用的SOCKET

以下是从服务器上tcpdump抓取的tcp连接信息。

红框2,3部分明确提示是从 Server端发起TCP的FIN消息, 之后Client回应ACK确认收到Server的关闭通知； 之后Client再发FIN消息，告知现在可以关闭了， Server端最终发ACK确认收到，并进入Time_WAIT状态，等待2MSL的时间关闭Socket。

特意指出，红框1表示TCP双端同时关闭，此时会在Client,Server同时留下time_wait痕迹，发生概率较小。

没有源码说个串串

此种情况是服务端主动关闭，我们往回翻一翻golang httpServer的源码

• http.ListenAndServe(":8081")
• server.ListenAndServe()
• srv.Serve(ln)
• go c.serve(connCtx) 使用go协程来处理每个请求

服务器连接处理请求的简略源码如下：

func (c *conn) serve(ctx context.Context) {    c.remoteAddr = c.rwc.RemoteAddr().String()    ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())    defer func() {    if !c.hijacked() {            c.close()            c.setState(c.rwc, StateClosed, runHooks)        }    }()  ......    // HTTP/1.x from here on.    ctx, cancelCtx := context.WithCancel(ctx)    c.cancelCtx = cancelCtx    defer cancelCtx()    c.r = &connReader{conn: c}    c.bufr = newBufioReader(c.r)    c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)    for {        w, err := c.readRequest(ctx)        switch {            case err == errTooLarge:                const publicErr = "431 Request Header Fields Too Large"                fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)                c.closeWriteAndWait()                return            case isUnsupportedTEError(err):                code := StatusNotImplemented                fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s%sUnsupported transfer encoding", code, StatusText(code), errorHeaders)                return            case isCommonNetReadError(err):                return // don't reply            default:                if v, ok := err.(statusError); ok {                    fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s: %s%s%d %s: %s", v.code, StatusText(v.code), v.text, errorHeaders, v.code, StatusText(v.code), v.text)                    return                }                publicErr := "400 Bad Request"                fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)                return            }        }            serverHandler{c.server}.ServeHTTP(w, w.req)        w.cancelCtx()        if c.hijacked() {            return        }        w.finishRequest()        if !w.shouldReuseConnection() {            if w.requestBodyLimitHit || w.closedRequestBodyEarly() {                c.closeWriteAndWait()            }            return        }        c.setState(c.rwc, StateIdle, runHooks)        c.curReq.Store((*response)(nil))        if !w.conn.server.doKeepAlives() {            // We're in shutdown mode. We might've replied            // to the user without "Connection: close" and            // they might think they can send another            // request, but such is life with HTTP/1.1.            return        }        if d := c.server.idleTimeout(); d != 0 {            c.rwc.SetReadDeadline(time.Now().Add(d))            if _, err := c.bufr.Peek(4); err != nil {                return            }        }        c.rwc.SetReadDeadline(time.Time{})    }}

我们需要关注

① for循环，表示尝试复用该conn，用于处理迎面而来的请求

② w.shouldReuseConnection() = false， 表明读取到ClientConnection：Close标头，设置closeAfterReply=true，跳出dor循环，协程即将结束，结束之前执行defer函数，defer函数内close该连接

c.close()......// Close the connection.func (c *conn) close() {    c.finalFlush()    c.rwc.Close()}

③ 如果 w.shouldReuseConnection() = true，则将该连接状态置为idle， 并继续走for循环，处理后续请求。

我的收获

1. tcp 4次挥手的八股文
2. 短连接在服务器上的效应，time_wait，占用可用的SOCKET， 根据实际业务看是否需要切换为长连接
3. golang http keep-alive复用tcp连接的源码级分析
4. tcpdump抓包的姿势