Does CloudFront / Lambda / AWS support http 103 early hints?

Question

Http status code 103, early hints, allows servers or edge nodes to send link headers early, so that clients can load resources in parallel to server processing. In practice, clients will first receive a 103 response with resources to preload, followed by a 200 containing the actual document.

CloudFlare supports http 103 out of the box: https://blog.cloudflare.com/early-hints/

Does anyone know if CloudFront / AWS can support this design, perhaps via Lambdas?

Answer 1

There are 4 phases between your end users and original servers as the following:

1. After CloudFront receives a request from a viewer (viewer request)
2. Before CloudFront forwards the request to the origin (origin request)
3. After CloudFront receives the response from the origin (origin response)
4. Before CloudFront forwards the response to the viewer (viewer response)

There are 2 ways so far for you to manipulate content that CloudFront delivers, which are CloudFront Functions and Lambda@Edge.

Take Lambda@Edge as example, you could consider implementing a Lambda function at the 'Origin Request' phase where early hints would be sent to allow clients to start preloading resources. It is feasible to use the Lambda function to generate an HTTP response that CloudFront returns directly to the viewer without forwarding the response to the origin. The following is a simple example:

import json

def lambda_handler(event, context):
    # Get the response object from the event
    response = event['Records'][0]['cf']['response']

    # Define preload resources
    preload_resources = [
        '</styles/main.css>; rel=preload; as=style',
        '</scripts/main.js>; rel=preload; as=script'
    ]

    # Join the preload resources with commas and add them to the "Link" header
    link_header_value = ', '.join(preload_resources)
    response['headers']['link'] = [{'key': 'Link', 'value': link_header_value}]

    # Return the modified response
    return response

A brief about the difference between Lambda@Edge and CloudFront Functions. For more details, please refer to here.

	CloudFront Functions	Lambda@Edge
Programming languages	JavaScript (ECMAScript 5.1 compliant)	Node.js and Python
Event sources	Viewer request Viewer response	Viewer request Viewer response Viewer response Origin request Origin response
Scale	10,000,000 requests per second or more	Up to 10,000 requests per second per Region
Function duration	Submillisecond	Up to 5 seconds (viewer request and viewer response) Up to 30 seconds (origin request and origin response)
Maximum memory	2 MB	128 – 3,008 MB
Maximum size of the function code and included libraries	10 KB	1 MB (viewer request and viewer response) 50 MB (origin request and origin response)
Network access	No	Yes
File system access	No	Yes
Access to the request body	No	Yes
Access to geolocation and device data	Yes	No (viewer request) Yes (origin request, origin response, and viewer response)
Can build and test entirely within CloudFront	Yes	No
Function logging and metrics	Yes	Yes
Pricing	Free tier available; charged per request	No free tier; charged per request and function duration

Here are some examples how you implement CloudFront Functions and Lambda@Edge with CDK.

1. https://github.com/aws-samples/amazon-cloudfront-functions
2. https://github.com/aws-samples/ab-testing-at-edge