Lambda response streaming tutorial with NodeJS

Enable Response Stream

• Expand the Advanced settings by clicking on it.
• Enable function URL by clicking the checkbox.
• Select the Auth type to NONE.
• Choose RESPONSE_STREAM in the Invoke mode at the bottom.

Once you’re done, simply click on Create Function button at the bottom of the page. You’ll be greeted with your new Lambda and a successful message.

Simple example which writes from 0 to 100

We can now use a for loop which will write to the response stream after waiting for 20 milliseconds.

Your Lambda will timeout if you try to make it wait for longer than the timeout which is set on the Lambda by default.

export const handler = awslambda.streamifyResponse(
  async (event, responseStream, _context) => {
    // Metadata is a JSON serializable JS object. Its shape is not defined here.
    const metadata = {
      statusCode: 200,
      headers: {
        "Content-Type":	"text/plain"
      }
    };

    // Assign to the responseStream parameter to prevent accidental reuse of the non-wrapped stream.
    responseStream = awslambda.HttpResponseStream.from(responseStream, metadata);
    for (var i = 0; i <= 100; i++) {
      await new Promise(r => setTimeout(r, 20));
      responseStream.write(`Hello ${i} \n`);
    }
    // finish the 
    responseStream.end();
    await responseStream.finished();
  }
);

Test response streaming with curl

• Copy your Lambda function URL which looks like xxx.us-east-1.on.aws from the Lambda console.

Browsers might not show you the text until it’s finished loading, so we use curl to see the real magic of response streaming.

Open your terminal and make a GET request with curl:

curl `xxx.us-east-1.on.aws`

Here is the screen recording of my terminal which will blow your mind:

Let’s see how we can send an image from Lambda

Cloud9 IDE doesn’t give us the ability to upload files directly so we will have to zip the image with the code.

So your zip should look like:

stream-demo:
- image.jpeg
- index.mjs

Code for Sending Image in the stream

It’s just 3 lines of code which will do the magic for us. Here is what’s changed from the previous code:

• Import fs from node:fs and pipeline from node:stream/promises
• Specify the "Content-Type": "image/jpeg" in the metadata.
• Create file stream with fs.createReadStream("./persian-cat.jpg")
• Put them in the pipeline await pipeline(imgStream, responseStream);

import fs from "node:fs";
import { pipeline } from "node:stream/promises";

export const handler = awslambda.streamifyResponse(
  async (event, responseStream, _context) => {
    // Metadata is a JSON serializable JS object. Its shape is not defined here.
    const metadata = {
      statusCode: 200,
      headers: {
        "Content-Type": "image/jpeg",
      },
    };

    // Assign to the responseStream parameter to prevent accidental reuse of the non-wrapped stream.
    responseStream = awslambda.HttpResponseStream.from(
      responseStream,
      metadata
    );

    const imgStream = fs.createReadStream("./persian-cat.jpg");
    await pipeline(imgStream, responseStream);

    responseStream.end();
    await responseStream.finished();
  }
);

Now go to your Lambda and upload the zip file

• In the Code tab go to top right corner where it shows Upload from button and click on .zip file
• Click on Upload button and select the your zipped Lambda.
• Click on Save

Here are the screenshots to help you out:

Test the Image stream

Open the function URL in your browser and you’ll see the image loading slowly on the page. We can throttle the network speed to 3G to see it load.

Even the favicon got updated with Persian cat.

Here is a video demo of showing image being loaded in Slow 3G:

Limitations of AWS Lambda with response streaming

• Only supports NodeJS runtime
• Can only be invoked via Function URL or AWS API
• Can’t be used with CDN to cache response
• No TypeScript typings

I hope you learnt something new, if you did make sure to comment your ideas in the comments.