附图，个人对http.Agent的理解：
image.png
一个 Agent是在client端用来管理链接的持久性和重用。对于一个host+port维持着一个请求队列，这些请求重复使用一个socket，直到这个队列空，这时，这个socket会被destroy或者放到pool里，在pool里时这个socket将会被再次重用(这两个行为取决于keepAlive的配置）
keepAlive
keepAliveMsecs
maxSockets
maxFreeSockets
在pool中的链接已经开启了tcp的Keep-Alive，然而在server端会有如下行为影响pool中的链接：
测试代码准备：
client.js
const http = require('http');
const agent = new http.Agent({
keepAlive: true,
keepAliveMsecs: 1000,
maxSockets: 4,
maxFreeSockets: 2
});
const test = () => {
return new Promise((resolve, reject) => {
const option = {
protocol: 'http:',
host: 'localhost',
port: 9990,
path: `/`,
agent: agent,
// agent: agent,
headers: {"Connection": "keep-alive"},
method: 'GET'
};
const req = http.request(option, function(res) {
res.setEncoding('utf8');
let body = '';
res.on('data', (chunk) => {
body += chunk;
});
res.on('end', () => {
resolve(body)
});
});
req.on('error', (e) => {
console.error(`problem with request: ${e.message}`);
console.log(e.stack)
});
req.end();
})
};
const sendReq = (count) => {
let arr = [];
for (let i=0;i<count;i++) arr.push(test())
Promise.all(arr).then(function(){
console.log('======end======')
})
}
server.js
const http = require('http');
let server = http.createServer(function(req, res) {
console.log(req.connection.remotePort);
res.end('200');
}).listen(9990);
server.keepAliveTimeout = 5000; // 这个值默认就是5s,可以直接赋值修改
- server端主动关闭空闲链接：client收到通知后，当前socket会从pool中移除，下一次请求时会创建一个新的socket
Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port.
client.js补充
sendReq(1); // 先发送一个req
setTimeout(() => {sendReq(1)}, 10 * 1000); //隔10s后再次发送一次req
server.js输出如下：
// console.log(req.connection.remotePort);
53957 // 发送第一个请求的socket port
54011 // 隔10s后发送第二个请求的socket port。port不同，说明第一个socket已经被关闭
wireshark抓包如下：
image.png
可以看到每隔1s发送向server端发送了一次TCP Keep-Alive探测。由于server端设置的keepAliveTimeout为5s(默认就是5s)，所以在5s后关闭了这个tcp链接，相应的，client端收到关闭的信号就会close到当前的socket，并从pool中移除这个socket
_http_agent.js
Agent.prototype.removeSocket = function removeSocket(s, options) {
var name = this.getName(options);
debug('removeSocket', name, 'writable:', s.writable);
var sets = [this.sockets];
// If the socket was destroyed, remove it from the free buffers too.
if (!s.writable)
sets.push(this.freeSockets);
for (var sk = 0; sk < sets.length; sk++) {
var sockets = sets[sk];
if (sockets[name]) {
var index = sockets[name].indexOf(s);
if (index !== -1) {
sockets[name].splice(index, 1);
// Don't leak
if (sockets[name].length === 0)
delete sockets[name];
}
}
}
// 省略其他代码
};
- server端拒绝多个请求共用一个tcp链接，在这种情况下，在每次请求时链接都会建立并且不能被pool。agent仍然会处理请求的发送，只是每个请求都会建立在一个新的tcp链接上
Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.
client.js不变
server.js添加如下代码
res.shouldKeepAlive = false; // 禁用shouldkeepAlive
res.end('200');
wireshark抓包如下：
image.png
可以看到，请求结束后，server就会关闭socket
When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).
当想要保持一个http请求很长时间并不在pool中，可以调用“agentRemove”（这个时间取决于server端socket close的时间）
Sockets are removed from an agent when the socket emits either a 'close' event or an 'agentRemove' event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:
client.js
// new
req.on('socket', (socket) => {
socket.emit('agentRemove');
});
server.js
server.keepAliveTimeout = 20000; // 为了清楚，服务端设置20s后再关闭
wireshark抓包如下：
image.png
可以看到，触发“agentRemove”后，当前socket并没有发送探测包，并且知道server端通知关闭才关闭。
当agent参数设置为false时，client将会为每一个http请求都创建一个链接。
node keep-alive还是很有必要开启的，尤其时作为中间层代理，当有如下模式时：高并发下优势更明显
browser浏览器 -> nginx -> node -> nginx -> java
当nginx和node都开启keep-alive时，性能测试如下：
image.png
参考资料mark：
http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html
https://stackoverflow.com/questions/19043355/how-to-use-request-js-node-js-module-pools
https://github.com/nodejs/node/issues/10774
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