CloudFront Functions vs. Lambda@Edge: What You Need to Know

Deciding between CloudFront Functions and Lambda@Edge often comes down to what you’re trying to accomplish at the edge of the AWS network. Both enable logic execution close to users, but each is optimized for different use cases. Understanding how they differ will help you choose the right tool for your workload.

What Are CloudFront Functions?

CloudFront Functions are lightweight JavaScript functions executed at CloudFront edge locations. They support only viewer request and viewer response events, making them suitable for tasks that require fast execution and low latency.

Key characteristics:

Written in ECMAScript 5.1
Maximum execution time: 1 ms
Max function size: 10 KB
No external network calls
Runs in over 400 edge locations

Best for:

URL rewrites and redirects
Header manipulation
Access control based on cookies or IP
Basic A/B testing

These functions are built for speed. They run on a custom-built runtime that prioritizes low latency. If you need logic executed before the origin is even touched, and it doesn’t require additional libraries or long execution time, this is the tool.

What Is Lambda@Edge?

Lambda@Edge uses AWS Lambda functions replicated to edge locations. It supports viewer request, viewer response, origin request, and origin response events. You can write code in Node.js or Python and use AWS SDKs.

Key characteristics:

Supports external API calls
Max execution time: 5 seconds (viewer events), 30 seconds (origin events)
Function size: up to 50 MB (compressed)
Can access environment variables
Suitable for complex business logic

Best for:

Authentication and authorization workflows
Dynamic content generation
Integration with external APIs
Real-time image or document transformation
Setting signed cookies or tokens

While more powerful, Lambda@Edge also adds more cold start time and cost. It’s ideal when your function needs to call a backend, transform data, or interact with other AWS services.

Core Differences

Here’s how the two compare side-by-side:

Feature	CloudFront Functions	Lambda@Edge
Languages	JavaScript (ECMAScript 5.1)	Node.js, Python
Execution Time	Max 1 ms	5–30 seconds
Supported Triggers	Viewer request/response	All four request/response stages
Package Size	10 KB	Up to 50 MB
Environment Variables	Not supported	Supported
External Calls	Not allowed	Allowed
Use Cases	Lightweight tasks	Heavy-duty edge computing

Which Should You Use?

Choose CloudFront Functions if:

You care about speed over complexity
Your use case is limited to viewer-level events
Your function doesn’t need to fetch external data

Choose Lambda@Edge if:

You need access to origin-level events
Your logic requires external service calls or complex computation
You’re working with larger payloads or custom SDKs

Common Scenarios

URL Rewriting

Use CloudFront Functions. They’re fast and don’t need origin interaction.

Country-Based Redirects

CloudFront Functions handle this quickly using headers like CloudFront-Viewer-Country.

JWT Authentication

Requires decoding tokens and perhaps checking them with an auth server. Use Lambda@Edge.

Image Resizing

CloudFront Functions can’t process binaries. Use Lambda@Edge with libraries like Sharp.

AB Testing with Cookies

Both can do this, but CloudFront Functions are faster for simple percentage splits.

Pricing Considerations

CloudFront Functions are billed per 1M invocations and are cheaper. Lambda@Edge charges for invocations and duration. For high-frequency, simple tasks, the difference in cost can be significant.

Example:

1M invocations per month
- CloudFront Functions: $0.10
- Lambda@Edge: ~$0.60 (viewer request, assuming 10ms average)

Deployment and Management

CloudFront Functions are managed within CloudFront directly. No IAM roles required.
Lambda@Edge functions must be created in the US East (N. Virginia) region and then associated with a CloudFront distribution.

Final Recommendation

Start with CloudFront Functions for speed-sensitive, simple logic. Use Lambda@Edge when your edge logic involves complexity or integration beyond headers and URLs. Each has its place. Let the use case drive the choice.

Cloud & Infrastructure