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Amazon CloudFront delivers content from the nearest AWS edge location, reducing latency and improving performance. By integrating CloudFront Functions and Lambda@Edge, you can inject custom logic into the request/response pipeline—right at the edge.
The image illustrates the flow of data through Amazon CloudFront, showing how CloudFront Functions and Lambda@Edge interact with various AWS services and edge locations to deliver content to a user.

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Supported Lifecycle Events

Edge functions trigger at specific points in CloudFront’s request/response cycle. Below is a quick reference:
Function TypeTriggers
CloudFront Functionsviewer-request, viewer-response
Lambda@Edgeviewer-request, origin-request, origin-response, viewer-response
The image is a comparison chart showing when functions run for CloudFront and Lambda@Edge, detailing specific request and response events.

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Detailed Request Flow

  1. Viewer Request
    At the viewer edge, both CloudFront Functions and Lambda@Edge can inspect or modify incoming HTTP requests.
  2. Cache Hit
    If the requested object is in cache, CloudFront returns it immediately. Use the viewer-response trigger to adjust headers or body content before it reaches the user.
  3. Cache Miss
    When an object isn’t cached, CloudFront makes an origin request:
    • Lambda@Edge can run custom code during this origin request.
    • After processing, the request is forwarded to your origin (e.g., Amazon S3, HTTP server).
  4. Origin Response
    On receiving data from the origin, use the origin-response Lambda@Edge trigger to transform or filter the response before caching.
  5. Viewer Response
    Finally, before dispatching to the viewer, both function types can adjust the response payload or headers.
The image is a diagram illustrating a CloudFront distribution with Lambda functions triggering on viewer requests and responses, showing the interaction with CloudFront cache.
The image is a diagram illustrating the flow of a cache miss in a CloudFront distribution, showing interactions between viewers, CloudFront cache, Lambda triggers, and an S3 bucket. It includes viewer request and response triggers, as well as origin request and response triggers.

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Common Use Cases

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CloudFront Functions

  • Cache Key Normalization
    Transform request attributes (like query strings or headers) to optimize cache keys.
  • Header Manipulation
    Add, modify, or strip HTTP headers in requests or responses.
  • URL Redirects / Rewrites
    Implement redirects or rewrite URLs at the edge without touching your origin.
  • Request Authorization
    Perform lightweight token validation (e.g., JWT) by inspecting authorization headers.
The image lists four use cases for CloudFront Functions: cache key normalization, header manipulation, URL redirects or rewrites, and request authorization.

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Lambda@Edge

  • Complex or Long-Running Logic
    Execute heavier workloads requiring more CPU, memory, or execution time.
  • Third-Party Libraries
    Package external dependencies with your function for richer functionality.
  • External Network Access
    Connect to APIs, databases, or other services outside the AWS network.
  • File System Operations
    Read/write temporary files in /tmp or process request bodies with custom logic.

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Feature Comparison

Use CloudFront Functions when you need sub-millisecond execution with minimal dependencies. Choose Lambda@Edge for advanced workloads or when you require external network and file system access.
The image is a comparison table between CloudFront Functions and Lambda@Edge, detailing aspects like programming languages, event sources, scale, function duration, and pricing.

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Choosing the Right Option

CriteriaCloudFront FunctionsLambda@Edge
Execution time< 1 msUp to 5 minutes
Supported languagesJavaScript (ECMAScript 6)Node.js, Python, etc.
Package size & librariesMust be very small, no external packagesSupports larger bundles & dependencies
Network & filesystemNoYes
Deployments with Lambda@Edge are replicated across all edge locations. Updates can take several minutes to propagate globally.

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Summary

Both CloudFront Functions and Lambda@Edge extend your CDN with programmable logic at AWS edge locations.
  • Choose CloudFront Functions for lightweight, high-scale tasks such as header manipulation, redirects, or cache key normalization.
  • Opt for Lambda@Edge when you need longer execution, external network access, or advanced libraries.
The image is a summary of CloudFront and Lambda@Edge functions, highlighting their use cases, operational details, and suitability for different tasks. It includes six key points about running code at edge locations, use cases, and function capabilities.

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References

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