# Keep-Alive

## 作用

* 重複利用已建立連線，減少每次建立連線交握時間
* 不正常結束連線時，port 無法及時釋放，導致占用
* [windows](https://learn.microsoft.com/zh-tw/biztalk/technical-guides/settings-that-can-be-modified-to-improve-network-performance)

## Keep-Alive max 沒有作用

透過 __HttpURLConnection__ 發送請求時，發現 __KeepAliveCache__ 最多只保存 5 個連線

* __sun.net.www.http.KeepAliveCache__

用來緩存 __HttpClient__ 的容器

* 設定上限(預設 __5__ )

        -Dhttp.maxConnections=N

* __getMaxConnections()__

        static int getMaxConnections() {
            if (result == -1) {
                result = AccessController.doPrivileged(
                    new GetIntegerAction("http.maxConnections", MAX_CONNECTIONS))
                    .intValue();
                if (result <= 0) {
                    result = MAX_CONNECTIONS;
                }
            }
            return result;
        }

* __put__

        synchronized void put(HttpClient h) {
            if (size() >= KeepAliveCache.getMaxConnections()) {
                h.closeServer(); // otherwise the connection remains in limbo
            } else {
                push(new KeepAliveEntry(h, System.currentTimeMillis()));
            }
        }

* __sun.net.www.http.HttpClient__

透過解析 __Response Header__ 決定緩存連線使用次數，但是 __keepAliveConnections__ 每次都會被更新為 __5__ 怪怪的。

* __parseHTTPHeader__

        HeaderParser p = new HeaderParser(responses.findValue("Keep-Alive"));
        /* default should be larger in case of proxy */
        keepAliveConnections = p.findInt("max", usingProxy?50:5);
        keepAliveTimeout = p.findInt("timeout", usingProxy?60:5);

* __finished__

        public void finished() {
            if (reuse) /* will be reused */
                return;
            keepAliveConnections--;
            poster = null;
            if (keepAliveConnections > 0 && isKeepingAlive() &&
                   !(serverOutput.checkError())) {
                /* This connection is keepingAlive && still valid.
                 * Return it to the cache.
                 */
                putInKeepAliveCache();
            } else {
                closeServer();
            }
        }

## Spring Boot & Tomcat

* 配置 __yml__

    server:
      tomcat:
        uri-encoding: UTF-8
        max-keep-alive-requests: 500 // 沒有作用
        keep-alive-timeout: 99s

* __org.apache.coyote.http11.Http11Processor__

雖然配置了 `max-keep-alive-requests`，但是只用來檢查是否關閉 __Keep Alive__ ，沒有如預期回覆 `Keep-Alive: max=500; timeout=99`，但是 __max__ 已被棄用。

[RFC](https://datatracker.ietf.org/doc/html/draft-thomson-hybi-http-timeout-03#section-2.2.1)

* __service__

        int maxKeepAliveRequests = protocol.getMaxKeepAliveRequests();
        if (maxKeepAliveRequests == 1) {
            keepAlive = false;
        } else if (maxKeepAliveRequests > 0 &&
                socketWrapper.decrementKeepAlive() <= 0) {
            keepAlive = false;
        }

* __prepareResponse__

        if (connectionKeepAlivePresent) {
            int keepAliveTimeout = protocol.getKeepAliveTimeout();

            if (keepAliveTimeout > 0) {
                String value = "timeout=" + keepAliveTimeout / 1000L;
                headers.setValue(Constants.KEEP_ALIVE_HEADER_NAME).setString(value);

                if (http11) {
                    // Append if there is already a Connection header,
                    // else create the header
                    MessageBytes connectionHeaderValue = headers.getValue(Constants.CONNECTION);
                    if (connectionHeaderValue == null) {
                        headers.addValue(Constants.CONNECTION).setString(Constants.KEEP_ALIVE_HEADER_VALUE_TOKEN);
                    } else {
                        connectionHeaderValue.setString(
                                connectionHeaderValue.getString() + ", " + Constants.KEEP_ALIVE_HEADER_VALUE_TOKEN);
                    }
                }
            }
        }

## 總結

__Client__ 需要設置 `http.maxConnections` 才不會發生以下錯誤。

java.net.BindException: Address already in use