我们知道HTTP协议采用“请求-应答”模式,当使用普通模式,即非KeepAlive模式时,每个请求/应答客户和服务器都要新建一个连接,完成 之后立即断开连接(HTTP协议为无连接的协议);当使用Keep-Alive模式(又称持久连接、连接重用)时,Keep-Alive功能使客户端到服 务器端的连接持续有效,当出现对服务器的后继请求时,Keep-Alive功能避免了建立或者重新建立连接。

http 1.0中默认是关闭的,需要在http头加入"Connection: Keep-Alive",才能启用Keep-Alive;http 1.1中默认启用Keep-Alive,如果加入"Connection: close ",才关闭。目前大部分浏览器都是用http1.1协议,也就是说默认都会发起Keep-Alive的连接请求了,所以是否能完成一个完整的Keep- Alive连接就看服务器设置情况。




启用Keep-Alive模式肯定更高效,性能更高。因为避免了建立/释放连接的开销。下面是RFC2616上的总结: -

- By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways, tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
- HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
- Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to determine the congestion state of the network.
- Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.
- HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using     future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old   semantics after an error is reported.

RFC2616还指出:单用户客户端与任何服务器或代理之间的连接数不应该超过2个。一个代理与其它服务器或代码之间应该使用超过2 * N的活跃并发连接。这是为了提高HTTP响应时间,避免拥塞(冗余的连接并不能代码执行性能的提升)


  1. HTTP长连接不可能一直保持 例如 Keep-Alive: timeout=5, max=100,表示这个TCP通道可以保持5秒,max=100,表示这个长连接最多接收100次请求就断开。
  2. HTTP是一个无状态协议,这意味着每个请求都是独立的,Keep-Alive没能改变这个结果
  3. Keep-Alive也不能保证客户端和服务器之间的连接一定是活跃的 唯一能保证的就是当连接被关闭时你能得到一个通知,所以不应该让程序依赖于Keep-Alive的保持连接特性,否则会有意想不到的后果。
  4. 使用长连接之后,客户端、服务端怎么知道本次传输结束呢 两部分:1:判断传输数据是否达到了Content-Length 指示的大小;2. 动态生成的文件没有 Content-Length ,它是分块传输(chunked),这时候就要根据 chunked 编码来判断,chunked 编码的数据在最后有一个空 chunked 块,表明本次传输数据结束