客戶端禁用Keep-alive， 服務端開啟Keep-alive，會怎麼樣？

部落格猿馬甲哥發表於2022-02-08
客戶端Keep-Alive服務端

最近部署的web程式，在伺服器上出現不少time_wait的連線狀態，會佔用tcp埠，費了幾天時間排查。

之前我有結論：HTTP keep-alive 是在應用層對TCP連線的滑動續約複用，如果客戶端、伺服器穩定續約，就成了名副其實的長連線。

目前所有的HTTP網路庫(不論是客戶端、服務端)都預設開啟了HTTP Keep-Alive，通過Request/Response的Connection標頭來協商複用連線。

非常規做法導致的短連線

我手上有個專案，由於歷史原因，客戶端禁用了Keep-Alive，服務端預設開啟了Keep-Alive，如此一來協商複用連線失敗， 客戶端每次請求會使用新的TCP連線， 也就是回退為短連線。

客戶端強制禁用Keep-Alive

package main

import (
	"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服務端預設開啟Keep-Alive

package main

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

// 根據RemoteAddr 知道客戶端使用的持久連線
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 預設開啟持久連線
	if err := http.ListenAndServe(":8081", http.HandlerFunc(IndexHandler)); err != nil {
		log.Fatal(err)
	}
}

從服務端的日誌看，確實是短連線。

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]]

誰是主動斷開方？

我想當然的以為 客戶端是主動斷開方，被現實啪啪打臉。

某一天伺服器上超過300的time_wait報警告訴我這tmd是伺服器主動終斷連線。

常規的TCP4次揮手， 主動斷開方會進入time_wait狀態，等待2MSL後釋放佔用的SOCKET

以下是從伺服器上tcpdump抓取的tcp連線資訊。

紅框2,3部分明確提示是從 Server端發起TCP的FIN訊息, 之後Client回應ACK確認收到Server的關閉通知； 之後Client再發FIN訊息，告知現在可以關閉了， Server端最終發ACK確認收到，並進入Time_WAIT狀態，等待2MSL的時間關閉Socket。

特意指出，紅框1表示TCP雙端同時關閉，此時會在Client,Server同時留下time_wait痕跡，發生概率較小。

沒有原始碼說個串串

此種情況是服務端主動關閉，我們往回翻一翻golang httpServer的原始碼

  • http.ListenAndServe(":8081")
  • server.ListenAndServe()
  • srv.Serve(ln)
  • go c.serve(connCtx) 使用go協程來處理每個請求

伺服器連線處理請求的簡略原始碼如下：

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)
		......
		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{})
	}
}

我們需要關注

① for迴圈，表示嘗試複用該conn，用於處理迎面而來的請求

② w.shouldReuseConnection() = false， 表明讀取到ClientConnection：Close標頭，設定closeAfterReply=true，跳出dor迴圈，協程即將結束，結束之前執行defer函式，defer函式內close該連線

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

③ 如果 w.shouldReuseConnection() = true，則將該連線狀態置為idle， 並繼續走for迴圈，處理後續請求。

我的收穫

  1. tcp 4次揮手的八股文
  2. 短連線在伺服器上的效應，time_wait，佔用可用的SOCKET， 根據實際業務看是否需要切換為長連線
  3. golang http keep-alive複用tcp連線的原始碼級分析
  4. tcpdump抓包的姿勢

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