Support for response streaming

[serg06]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

AWS just released the ability to stream a Lambda function's response: https://aws.amazon.com/blogs/compute/introducing-aws-lambda-response-streaming/

It would be great if this framework supported it.

It requires wrapping the handler in streamifyResponse().

Motivation

This is an extremely useful feature which greatly decreases TTFB.

Example

// API which returns chunked data.
// Locally TTFB is 0, but on Lambda TTFB is 5s.
// Presumably because the handler is not wrapped in streamifyResponse.
fastify_streamtest.get('/chunked', (request, reply) => {
  // Create a buffer to hold the response chunks
  var buffer = new stream.Readable();
  buffer._read = () => {};

  // Generate 5 chunks with 1 second interval
  var count = 5;
  var emit = () => {
    var data = `Hello World ${count}`;
    console.log(`sending "${data}"`);
    buffer.push(data);

    count--;
    if (count > 0) {
      setTimeout(emit, 1000);
    } else {
      console.log('end sending.');
      buffer.push(null);
    }
  };

  emit();
  void reply.type('text/html').send(buffer);
});

[adrai]

[serg06]

It should still work with direct Lambda URLs, no?

[adrai]

Might work with Lambda function URLs, but...

beside that I don't know if the fastify inject function is able to handle chunked responses in a way to pass them to the awslambda.streamifyResponse 🤷‍♂️
Maybe @mcollina can confirm?

[mcollina]

Given the current limitations I don't see this as such a big deal. It will definitely help Vercel.

I think it's possible to make a few modifications to inject to support this. However I don't have time to work on them right now.

[adrai]

thank you for the info

[Deanfei]

Does this library support the lambda streaming response yet? That's the only main blocker to use it.

[adrai]

The Fastify inject function (and maybe also light-my-request), needs to be modified first.

[Deanfei]

Any native support for the streaming now?

[loukmane-issa]

If it helps, I was able to get this working like this:

import awsLambdaFastify from "@fastify/aws-lambda";
import { initServer } from "./app";

const server = initServer();
const proxy = awsLambdaFastify(server);
export default proxy;


export const handle = awslambda.streamifyResponse(async (event, responseStream, context) => { 
  // Return response to client
  const response = await proxy(event, context);
  responseStream.setContentType('application/json');
  responseStream.write(response);
  responseStream.end();
  console.log("Before sleep");
  await new Promise((resolve) => setTimeout(resolve, 5000));
  console.log("After sleep");
});

In typescript, you need to setup the global types, it's explained here: https://betterdev.blog/lambda-response-streaming-dynamic-content/

I needed this because I want to send my requests to CloudWatch logs and metrics after the response is sent to client for performance reason.

[adrai]

Seems light-my-request got an nice update that introduced payloadAsStream => fastify/light-my-request#308

I've created a new version (v5.1.0) which is able to pass that option to the inject function.

Since I personally don't use aws Function Urls, please try it and let me know... I'll then put some infos in the readme.

import awsLambdaFastify from '@fastify/aws-lambda'
import { promisify } from 'util'
import stream from 'stream'
import app from './app.js'

const pipeline = promisify(stream.pipeline)
const proxy = awsLambdaFastify(app, { payloadAsStream: true })
export const handler = awslambda.streamifyResponse(async (event, responseStream, context) => {
  const { meta, stream } = await proxy(event, context)
  responseStream = awslambda.HttpResponseStream.from(responseStream, meta)
  await pipeline(stream, responseStream)
})
await app.ready() // https://github.com/fastify/aws-lambda-fastify/issues/89

[mcollina]

@adrai this is exaclty one of the usecase I built that for!

I think we can close this now.

[spaceemotion]

@adrai thanks for the example! is the awslambda.streamifyResponse call something every lambda includes? Typescript is giving me a warning about it being unknown.