Request without Keep-Alive

As we all know that in browsers, multi HTTP requests can resue a TCP connection with the HTTP header "connection: keep-alive;". But how to do in Node.js?

For example, here is a simple code snippet of requesting in Node.js:



// server.js
const http = require('http')

http
  .createServer((req, res) => {
    res.writeHead(200)
    res.end('Hello World')
  })
  .listen(3000)

// client.js
const http = require('http')

function request() {
  http
    .request(
      {
        host: 'localhost',
        port: 3000,
        pathname: '/',
      },
      (res) => {
        let buffer = []
        res.on('data', (chunk) => {
          buffer.push(chunk)
        })
        res.on('end', () => {
          console.log(buffer.toString())
        })
      }
    )
    .end()
}

request()

setTimeout(() => {
  request()
}, 1000)

When using wireshark to debug, we can find that the client port of two requets is different and there are two "Three-way Handshake" processes:

Request with Keep-Alive using http.Agent

Now, let's request with keep-alive using http.Agent, we only need to add a short piece of code:



const agent = new http.Agent({
  keepAlive: true,
})

function request() {
  http
    .request(
      {
        agent,
        host: 'localhost',
        port: 3000,
        pathname: '/',
      },
      () => {
        // ...
      }
    )
    .end()
}

But, the result of wireshark won't change! Actually, we need to specify the maxSockets of the agent:



const agent = new http.Agent({
  keepAlive: true,
  maxSockets: 1,
})

Why? Because maxSockets indicats the max TCP connections that can be established per host. And the default value is Infinity. If we don't set its value, a new connection will always be established for every request.

Now, let's change our code a little:



setTimeout(() => {
  request()
}, 10000) // 1000 -> 10000

The wireshark will show:

The keep-alive didn't work again! And we can see that the server send a FIN packet in about 5s. So is there any parameter for server to control the timeout? Yes, that is keepAliveTimeout，let's set it to 10s:



const http = require('http')

const server = http
  .createServer((req, res) => {
    res.writeHead(200)
    res.end('Hello World')
  })
  .listen(3000)

server.keepAliveTimeout = 10000

And now the keep-alive works again.

To request with keep-alive using HTTP agent in Node.js

Request without Keep-Alive

Request with Keep-Alive using http.Agent