Lambda-Stream

This library adds types and local support for AWS Lambda Response Streaming. Locally, it'll buffer and return the entire response.

In AWS Lambda, it'll simply use the global awslambda.streamifyResponse.

This library exposes a ResponseStream class, the streamifyResponse method, and isInAWS method. Types are also included.

Why?

AWS added Lambda Response Streaming directly into the NodeJS runtime as a global method, instead of providing it with the AWS-SDK.

This makes functions leveraging Lambda Response Streaming difficult to test, run locally, or even write Lambda functions using a typical editor. Magic, runtime-only methods are a drag on developer experience!

So I wrote this library to fill that gap

End of Life

This library shouldn't exist.

If or when AWS decides to release and support a library which makes AWS Lambda Streaming Responses usable and testable locally, I'll happily archive this project.

Examples

Works like this:

import { APIGatewayProxyEventV2 } from 'aws-lambda'
import { streamifyResponse, ResponseStream } from 'lambda-stream'

export const handler = streamifyResponse(myHandler)

async function myHandler(
  event: APIGatewayProxyEventV2,
  responseStream: ResponseStream
): Promise<void> {
  console.log('Handler got event:', event)
  return new Promise((resolve, reject) => {
    responseStream.setContentType('text/plain')
    responseStream.write('Hello')
    setTimeout(() => {
        responseStream.write('World')
        responseStream.end()
        resolve()
    }, 1000)
  })
}

Or in commonjs:

'use strict'
const { streamifyResponse } = require('lambda-stream')

module.exports.hello = streamifyResponse(
  async (event, responseStream, context) => {
    responseStream.setContentType('text/plain')
    responseStream.write('Hello, world!')
    responseStream.end()
  }
)

Pipelining is also supported:

const pipeline = require('util').promisify(require('stream').pipeline)
const { Readable } = require('stream')
const { streamifyResponse } = require('lambda-stream')

const handler = async (event, responseStream, _context) => {
  // As an example, convert event to a readable stream.
  requestStream = Readable.from(Buffer.from(JSON.stringify({ hello: 'world' })))

  await pipeline(requestStream, zlib.createGzip(), responseStream)
}

module.exports.gzip = streamifyResponse(handler)

Headers, Status Code

AWS provides a helper function which allows you to specify additional headers, as well as craft the status code.

const { streamifyResponse } = require('lambda-stream')
const stream = require('stream')
module.exports.hello = streamifyResponse(
  async (event, responseStream, context) => {
    const metadata = {
      statusCode: 200,
      headers: {
        "Content-Type": "application/json",
        "CustomHeader": "outerspace"
      }
    };
    
    // Use global helper to pass metadata and status code
    responseStream = awslambda.HttpResponseStream.from(responseStream, metadata);

    responseStream.write("Streaming with Helper \n");
    await new Promise(r => setTimeout(r, 1000));
    responseStream.write("Hello 0 \n");
    await new Promise(r => setTimeout(r, 1000));
    responseStream.write("Hello 1 \n");
    await new Promise(r => setTimeout(r, 1000));
    responseStream.write("Hello 3 \n");
    responseStream.end();
    // Suggested in Documentation: https://docs.aws.amazon.com/lambda/latest/dg/response-streaming-tutorial.html
    // But NOT defined in node 16 or node 18
    // Only defined in node 14
    await responseStream.finished();
  }
);

We are considering shimming our own helper to support types as well.

Contributors

AJ Stuyvenberg

Luka Matic

Paulo Ricardo Xavier Giusti