Chunked response in Node.js on Heroku

Question

I have some very simple Node.js code that I want to emit as chunked responses. The code is:

const WAIT = 500;
const records = [...];

function getList(req, res, url) {
    var list = records.slice().map(rec => JSON.stringify(rec) + "\n");
    var len = list.reduce((size, cur) => { size += Buffer.byteLength(cur); return size; }, 0);

    function next() {
        var cur = list.shift();
        if(!cur) {
            res.end();
            return;
        }

        res.write(cur);

        setTimeout(next, WAIT);
    }

    res.writeHead(200, {
        "Access-Control-Allow-Origin": "*",
        "Content-Type": "application/x-ndjson",
        "Content-Length": len.toString(),
        "X-Accel-Buffering": "no"
    });

    setTimeout(next, WAIT);
}

const PORT = process.env.PORT || 8080;
require("http").createServer(getList).listen(PORT);

This should write out this data as ndjson chunks until the array is emptied.

Locally and in a generic VPS this works as expected. On Heroku it does not emit the chunks in a streaming fashion, but rather dumps out them all at the end.

Does anyone have an idea of what I might be doing wrong?

Answer 1

After the comments section - setting the Transfer-Encoding: Chunked header explicitly resolved this problem. Digital Ocean might be doing something smart with chunking while you need to specify this explicitly in Heroku.

Documentation regarding the header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Transfer-Encoding

Another potential problem why this might occur in your system: https://github.com/expressjs/compression/issues/56 (If you're using an older version of expressjs - response flushing contained a bug)