Res Stream Backpressure

Stage 2 (Layer A backpressure): a streamed `res` uses non-blocking writes. When res.write fills the socket's send buffer it hits EAGAIN and the handler's fiber PARKS on socket writability — the reactor keeps running and resumes the fiber when the client drains, instead of a synchronous write blocking the whole server on one slow consumer. Here the handler streams ~2 MB in 1 KB chunks; the client consumes it through fetch's streaming body and the full byte count comes back intact across all the park/resume cycles.

import http from 'http';
import type { AddressInfo } from 'net';

const CHUNK = "x".repeat(1024);

async function consume(port: number): Promise<void> {
  const res = await fetch("http://127.0.0.1:" + port + "/");
  let total = 0;
  for await (const piece of res.body!) {
    total = total + piece.length;
  }
  console.log("streamed-bytes:", total);
  process.exit(0);
}

const server = http.createServer((req: http.IncomingMessage, res: http.ServerResponse) => {
  res.writeHead(200, { "Content-Type": "application/octet-stream" });
  let i = 0;
  while (i < 2048) { // ~2 MB, well past the socket send buffer
    res.write(CHUNK);
    i = i + 1;
  }
  res.end();
});

server.listen(0, () => {
  const port: number = (server.address() as AddressInfo).port;
  console.log("listening:", port > 0);
  setTimeout(() => { consume(port); }, 50);
});

View source on GitHubexamples/http/res_stream_backpressure.ts

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