KoaJS: how to send partial responses?

Question

KoaJS: how to send partial responses?

I'm learning KOA and Node.JS streams.

What I'm trying to do is sending a partial HTTP response, and after a short while send the rest of the response.

app.get("/", function*(next) {

  // TEST:
  function delay(ms) {
    return function(callback) {
      setTimeout(callback, ms);
    }
  }

  this.type = "text/plain";

  var Readable = require("stream").Readable;
  var stream = this.body = new Readable();

  stream._read = function () {};

  stream.push('First line.\n');

  yield delay(2000);

  stream.push('Last line.\n');

  stream.push(null);

  console.log("done");

});

I'd expect that while loading the page in the browser, "First line." is immediately displayed and after 2 seconds also "Second line". Instead, it seems that the response is sent as a whole.

What am I missing here?

In the end I wand to stream internally generated logging data in a long-living connecton to the browser.

Node 0.11.3 with Koa 0.10.0

node.js
stream
koa

Answer 1

Stream is piped later, you don't have a chance to flush data inside the function.
Check the code:

var koa = require('koa')
var Readable = require('stream').Readable
var app = koa()

app.use(function*(next){

    this.type = 'text/plain'

    var stream = this.body = new Readable()
    stream._read = function () {}

    // isn't piped
    console.log(stream._readableState.pipes != null)

    yield function (callback) { setTimeout(callback, 2000) }

    // not yet
    console.log(stream._readableState.pipes != null)

    var i = 0
    setTimeout(function f(){
        if (i == 0)
            // now it is
            console.log(stream._readableState.pipes != null)
        stream.push(Date() + '\n');
        if (++i < 200)
            setTimeout(f, 50)
        else
            stream.push(null)
    }, 100)

})

app.listen(80)

Now, I'm not too familiar with koa, so another solution may exist.