Streaming and SSE

All three of Nest’s streaming mechanisms work unchanged because the response object implements the node:stream Writable interface (Bun’s native implementation of it) and turns into a Web ReadableStream the moment something writes to it.

Server-Sent Events

@Sse('events')
events(): Observable<MessageEvent> {
  return interval(1000).pipe(map((i) => ({ data: { tick: i } })));
}

curl -N http://localhost:3000/events

Nest commits the SSE headers (Content-Type: text/event-stream, Cache-Control: no-cache) and writes each message. When the client disconnects, the adapter aborts the request-scoped signal and Nest unsubscribes from the observable, so producers stop.

StreamableFile

@Get('report')
report() {
  return new StreamableFile(createReadStream('report.pdf'), {
    type: 'application/pdf',
    disposition: 'attachment; filename="report.pdf"',
  });
}

Nest pipes the Node stream into the response; the adapter forwards each chunk to the client as it arrives.

Manual streaming with @Res()

@Get('log')
log(@Res() res: BunResponse) {
  res.setHeader('content-type', 'text/plain');
  res.write('starting\n');
  setTimeout(() => res.end('done\n'), 1000);
}

The first write() (or writeHead() / flushHeaders()) commits the status and headers. After that, headers cannot change.

Sending a file with zero copies

For whole files, skip the stream and hand Bun the file:

res.sendBunFile(Bun.file('./big.mp4'));

Content-Type and Content-Length are set from the file and Bun uses sendfile under the hood. res.sendFile(absolutePath) is the Express-compatible spelling.

Idle connections

Long-lived streams that send nothing for longer than serve.idleTimeout (default 120 seconds) are closed by Bun. Send a heartbeat comment from SSE handlers that can go quiet, or raise the timeout up to Bun’s maximum of 255 seconds.