HTTP长连接、短连接使用及测试

我就是我 2022-06-16 01:39 224阅读 0赞

### 概念 ###

> HTTP短连接（非持久连接）是指，客户端和服务端进行一次HTTP请求/响应之后，就关闭连接。所以，下一次的HTTP请求/响应操作就需要重新建立连接。  
> HTTP长连接（持久连接）是指，客户端和服务端建立一次连接之后，可以在这条连接上进行多次请求/响应操作。持久连接可以设置过期时间，也可以不设置。

我为什么没有说HTTP/1.0 默认短连接，HTTP/1.1起，默认长连接呢？因为我第一次看这个说法的时候，以为自己懂了，其实并没有懂。长短连接操作上有什么区别，有的地方出现的持久连接又是怎么回事？

### 使用设置 ###

这里的设置，我们都以HTTP1.1协议为例子。

#### 设置HTTP短连接 ####

在首部字段中设置`Connection:close`，则在一次请求/响应之后，就会关闭连接。

#### 设置HTTP长连接，有过期时间 ####

在首部字段中设置`Connection:keep-alive` 和`Keep-Alive: timeout=60`，表明连接建立之后，空闲时间超过60秒之后，就会失效。如果在空闲第58秒时，再次使用此连接，则连接仍然有效，使用完之后，重新计数，空闲60秒之后过期。

#### 设置HTTP长连接，无过期时间 ####

在首部字段中只设置`Connection:keep-alive`，表明连接永久有效。

### 实现原理 ###

了解怎么设置之后，就开始用起来。然而，问题来了。在请求头中设置`Connection:keep-alive`，为什么连接空闲一段时间之后，还是断开了呢？这是因为connection字段只有服务端设置才有效。

HTTP操作是请求/响应成对出现的，即先有客户端发出请求，后有服务端处理请求。所以，一次HTTP操作的终点操作在服务端上，关闭也是由服务端发起的。  
接下来我们做做测试，以及show code。下面的测试都是使用Spring RestTemplate，封装apache http client进行的。为方便讲解代码，先说明长连接的情况，最后再对其他形式做测试总结。

#### 客户端连接失效时间大于服务端失效时间 ####

如下，为请求日志。客户端设置`Connection: Keep-Alive`和`Keep-Alive: timeout=60`， 服务端设置`Connection: Keep-Alive`和`Keep-Alive: timeout=5`。

## 客户端设置有效期为60s
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "POST /adx-api/api/creative/upload HTTP/1.1[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept: application/json, application/*+json, text/html, application/json, text/javascript[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Type: application/json;charset=UTF-8[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.16 Safari/537.36[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Encoding: gzip,deflate[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Language: zh-CN[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Length: 396[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Host: bizdomain[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "[\r][\n]"
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "request data"
    
    ##服务端设置有效期为5s
    [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "HTTP/1.1 200 OK[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Date: Wed, 26 Apr 2017 06:07:58 GMT[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Server: Apache-Coyote/1.1[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Content-Type: text/html;charset=utf-8[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Transfer-Encoding: chunked[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "63[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "response data"

客户端设置的有效期大于服务端的，那么实际连接的有效期呢？三分钟之后再次请求，从连接池中lease连接的时候，提示`Connection expired @ Wed Apr 26 14:08:05`，即在上一次请求之后的5s失效，说明是服务端的设置生效了。

[2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200]
    [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.CPool:?) - Connection [id:2][route:{}->http://bizdomain:80][state:null] expired @ Wed Apr 26 14:08:05 GMT+08:00 2017

#### 源码分析 ####

通过源代码了解一下连接失效时间的设置过程。

//org.apache.http.impl.execchain.MainClientExec#execute
    ......
    
    //从连接池中lease connection
    final HttpClientConnectionmanagedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS);
    
    ......
    
    //将conenction封装在ConnectionHolder中
    final ConnectionHolder connHolder = new ConnectionHolder(this.log, this.connManager, managedConn);
    
    ......
    
    // The connection is in or can be brought to a re-usable state.
    //如果返回值消息头中connection设置为close，则返回false
    if (reuseStrategy.keepAlive(response, context)) {
        // Set the idle duration of this connection
        //取出response消息头中，keep-alive的timeout值
        final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
        if (this.log.isDebugEnabled()) {
            final String s;
            if (duration > 0) {
                s = "for " + duration + " " + TimeUnit.MILLISECONDS;
            } else {
                s = "indefinitely";
            }
            this.log.debug("Connection can be kept alive " + s);
        }
        //设置失效时间
        connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
        connHolder.markReusable();
    } else {
        connHolder.markNonReusable();
    }

待读取响应之后，释放连接，即：`connHolder.releaseConnection()`。调用org.apache.http.impl.conn.PoolingHttpClientConnectionManager\#releaseConnection方法。

@Override
        public void releaseConnection(final HttpClientConnection managedConn, 
                final Object state,final long keepalive, final TimeUnit tunit) {
            Args.notNull(managedConn, "Managed connection");
            synchronized (managedConn) {
                final CPoolEntry entry = CPoolProxy.detach(managedConn);
                if (entry == null) {
                    return;
                }
                final ManagedHttpClientConnection conn = entry.getConnection();
                try {
                    if (conn.isOpen()) {
                        final TimeUnit effectiveUnit = tunit != null ? tunit : TimeUnit.MILLISECONDS;
                        entry.setState(state);
                        //设置失效时间
                        entry.updateExpiry(keepalive, effectiveUnit);
                    }
                } finally {
                。。。。。。
                    }
                }
            }
        }

然后再下一次HTTP操作，从连接池中获取连接时

//org.apache.http.impl.conn.PoolingHttpClientConnectionManager#requestConnection调用org.apache.http.pool.AbstractConnPool#lease，
    //调用getPoolEntryBlocking,调用org.apache.http.impl.conn.CPoolEntry#isExpired
    
    @Override
    public boolean isExpired(final long now) {
        final boolean expired = super.isExpired(now);
        if (expired && this.log.isDebugEnabled()) {
        //日志中看到的内容
            this.log.debug("Connection " + this + " expired @ " + new Date(getExpiry()));
        }
        return expired;
    }

综上，连接的实际有效时间，是根据response的设置来决定的。

#### 其他情况测试 ####

*  客户端设置`Connection: Close`

##connection:close请求，kept alive的连接为0
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200]
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connecting to bizdomain/127.0.0.195:80
    ## 建立新连接
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connection established 127.0.0.191:49239<->127.0.0.195:80
    ## 客户端设置短连接
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: Close[\r][\n]"
    ## 服务端返回的也是短连接
    [2017-04-26 13:57:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Connection: close[\r][\n]" ##请求完之后，关闭连接 [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultManagedHttpClientConnection:?) - http-outgoing-0: Close connection [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection discarded [2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]

如上，当服务端返回`Connection: Close`时，客户端接收完响应，便会关闭连接。

*  客户端设置60s超时，服务端设置5s超时

##Keep-Alive: timeout=60 第一次请求,与connection:close无差别
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200]
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80
    ## 客户端设置超时时间60s
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]"
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]"
    ## 服务端设置超时时间5s
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]" [2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]" ## 服务端设置生效，连接可以保持5s [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive for 5000 MILLISECONDS [2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection [id: 0][route: {}->http://bizdomain:80] can be kept alive for 5.0 seconds
    [2017-04-26 10:57:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200]
    
    ##Keep-Alive: timeout=60 非第一次请求
    [2017-04-26 14:11:00 DEBUG]  (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200]
    ## 连接在上一次请求结束后5s失效
    [2017-04-26 14:11:00 DEBUG]  (org.apache.http.impl.conn.CPool:?) - Connection [id:2][route:{}->http://bizdomain:80][state:null] expired @ Wed Apr 26 14:10:05 GMT+08:00 2017

*  客户端设置失效时间，服务端设置不失效

## 客户端设置30s超时
    [2017-04-26 17:45:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]"
    [2017-04-26 17:45:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=30[\r][\n]"
    ## 服务端设置永久连接
    [2017-04-26 17:45:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Connection: keep-alive[\r][\n]" ## 连接将一直保持 [2017-04-26 17:45:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive indefinitely

综上，http连接保持时间是由服务端的消息头connection字段和keep-alive字段定的。

在上面前两种情况，请求的是同一个服务端，那么为什么一个返回的是短连接，一个返回的是长连接呢？这里转一下 [这篇文章][Link 1]的解释：

> 不论request还是response的header中包含了值为close的connection，都表明当前正在使用的tcp链接在请求处理完毕后会被断掉。以后client再进行新的请求时就必须创建新的tcp链接了。 HTTP Connection的 close设置允许客户端或服务器中任何一方关闭底层的连接，双方都会要求在处理请求后关闭它们的TCP连接。

### 补充 ###

*  TCP长短连接  
    在网上搜资料的时候，看到很多“HTTP协议的长连接和短连接，实质上是TCP协议的长连接和短连接”。 HTTP和TCP是不同两层的东西，它们怎么会是一样的呢？HTTP是请求/响应模式的，就是说我们发一个请求一定要有一个回应。最直观的就是，浏览器上发请求，得不到响应就会一直转圈圈。 而TCP并不是一定要有响应。大家以前使用socket模拟一个IM聊天，A跟B打完招呼，完全可以不用等待B的回应，就自己关掉连接的。
 *  TCP keep-alive  
    另外还有HTTP协议的keep-alive和TCP的keep-alive含义是有差别的。HTTP的keep-alive是为了维持连接，以便复用连接。通过使用keep-alive机制，可以减少tcp连接建立次数，也意味着可以减少TIME\_WAIT状态连接，以此提高性能和提高httpd服务器的吞吐率(更少的tcp连接意味着更少的系统内核调用,socket的accept()和close()调用)。但是，长时间的tcp连接容易导致系统资源无效占用。配置不当的keep-alive，有时比重复利用连接带来的损失还更大。

而tcp keep-alive是TCP的一种检测TCP连接状况的机制，涉及到三个参数tcp\_keepalive\_time, tcp\_keepalive\_intvl, tcp\_keepalive\_probes。  
当网络两端建立了TCP连接之后，闲置（双方没有任何数据流往来）了tcp\_keepalive\_time后，服务器内核就会尝试向客户端发送侦测包，来判断TCP连接状况(有可能客户端崩溃、强制关闭了应用、主机不可达等等)。如果没有收到对方的回答(ack包)，则会在 tcp\_keepalive\_intvl后再次尝试发送侦测包，直到收到对方的ack。如果一直没有收到对方的ack,一共会尝试 tcp\_keepalive\_probes次。如果尝试tcp\_keepalive\_probes,依然没有收到对方的ack包，则会丢弃该TCP连接。TCP连接默认闲置时间是2小时，一般设置为30分钟足够了。

[Link 1]: http://blog.csdn.net/legend_x/article/details/39578681