LambdaEdge

Lambda@Edge is a feature of AWS Lambda that enables running code at CloudFront edge locations to customize content as it is delivered to users. It lets developers implement logic close to viewers, reducing latency and allowing dynamic behavior without changing the origin server or application code.

How it works. A Lambda function is created in the US East (N. Virginia) region, published as a version, and attached to a CloudFront distribution. CloudFront replicates the function to edge locations. The function is invoked in response to four event types: viewer-request, origin-request, origin-response, and viewer-response. Inside the function, you can inspect and modify incoming requests or outgoing responses, generate a direct response, and add or adjust headers, cookies, and redirects. The function runs in the edge and does not have access to resources inside a VPC.

Limitations and lifecycle. Updates require publishing a new version, which is then propagated to edge locations. Lambda@Edge functions adhere to Lambda-style execution constraints, and practical considerations include cold starts and execution time limits. Functions run in isolated environments and cannot access VPC resources, which can influence design choices for backend connectivity and data access.

Use cases. Common scenarios include URL redirects and rewrites, header manipulation for security or caching, geolocation- or device-based personalization, authentication checks, and A/B testing of content or behavior at the edge.

Relation to CloudFront Functions. CloudFront Functions is a newer, lighter-weight option for simple, high-performance tasks, while Lambda@Edge supports more complex logic and multiple runtimes. Both integrate with CloudFront to customize content delivery at the edge.