Using AWS lambda response streaming with Sveltekit

[H0R5E]

Hi, I am developing a SvelteKit adapter for AWS (which can be used with different IAC providers).

When I started building the adapter, AWS Lambda was not capable of generating a streamed response, so using SvelteKit's streaming features wasn't possible.

At the beginning of April, AWS announced AWS Lambda response streaming, with a basic example of a handler like so:

exports.handler = awslambda.streamifyResponse(
    async (event, responseStream, context) => {
        responseStream.setContentType(“text/plain”);
        responseStream.write(“Hello, world!”);
        responseStream.end();
    }
);

Alternatively, they suggest using a pipeline as so:

const pipeline = require("util").promisify(require("stream").pipeline);
const zlib = require('zlib');
const { Readable } = require('stream');

exports.gzip = awslambda.streamifyResponse(async (event, responseStream, _context) => {
    // As an example, convert event to a readable stream.
    const requestStream = Readable.from(Buffer.from(JSON.stringify(event)));
    
    await pipeline(requestStream, zlib.createGzip(), responseStream);
});

My adapter currently handles the request/response handling something like so:

import { Server } from '../index.js'
import { manifest } from '../manifest.js'

export async function handler(event, context) {
  const app = new Server(manifest)
  const {
    rawPath,
    headers,
    rawQueryString,
    body,
    requestContext,
    isBase64Encoded,
    cookies,
  } = event

  // Some manipulation here

  const request = new Request(rawURL, {
    method: requestContext.http.method,
    headers: new Headers(headers),
    body: rawBody,
  })

  const rendered = await app.respond(request, {
    platform: { context },
  })

  // Parse the response into lambda proxy response
  let response

  if (rendered) {
    response = {
      // some header handling here
      body: await rendered.text(),
      statusCode: rendered.status,
    }
  } else {
    response = {
      statusCode: 404,
      body: 'Not found.',
    }
  }

  return response
}

I was just wondering if anyone had any advice on modifying the adapter to use the AWS response stream features, before I need to take a deep dive?

Many thanks for your help,

Mat

[H0R5E]

Having had time for a look around, I think I can leverage the setResponse function here. So it might be as easy as:

import { setResponse } from '@sveltejs/kit/node';

exports.handler = awslambda.streamifyResponse(
    async (event, responseStream, context) => {
        const app = new Server(manifest)
        const {
          rawPath,
          headers,
          rawQueryString,
          body,
          requestContext,
          isBase64Encoded,
          cookies,
        } = event

        // Some manipulation here

         const request = new Request(rawURL, {
         method: requestContext.http.method,
          headers: new Headers(headers),
          body: rawBody,
        })

        const rendered = await app.respond(request, {
          platform: { context },
        })
        setResponse(responseStream, rendered)
    }
)

[GeoffreyBooth]

It would seem that @geoffrich’s article https://svelte.dev/blog/streaming-snapshots-sveltekit needs an update.

SvelteKit will only be able to stream responses if your app’s hosting platform supports it. In general, any platform built around AWS Lambda (e.g. serverless functions) will not support streaming, but any traditional Node.js server or edge-based runtime will.

This would appear to be no longer accurate, per https://docs.aws.amazon.com/lambda/latest/dg/configuration-response-streaming.html.

I also think that AWS lambda is a significant enough platform to justify a first-party SvelteKit adapter.