feat(url-shortener): use a Lambda function URL

[jogold]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @jogold, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant architectural improvement to the URL shortener by migrating its redirect functionality from a Lambda@Edge implementation to a more modern and simplified AWS Lambda Function URL. This change reduces operational overhead and complexity by removing the need for global Lambda@Edge deployments and their associated custom resources, while still leveraging CloudFront for content delivery and custom domain support. The new setup integrates the Lambda Function URL directly as a CloudFront origin, and also adds an HTTPS record to Route53 for enhanced security and standard web practices.

Highlights

• Architectural Shift: The URL shortener's redirect mechanism has been refactored to utilize AWS Lambda Function URLs instead of Lambda@Edge, simplifying the deployment and management of the redirect logic.
• Lambda Function URL Integration: The CloudFront distribution now uses the Lambda Function URL as its origin, replacing the previous S3 bucket origin combined with Lambda@Edge for request handling.
• Resource Simplification: Removed complex cross-region custom resources and associated IAM policies previously required for Lambda@Edge deployments, streamlining the CloudFormation template.
• Route53 HTTPS Record: A new HTTPS record has been added to Route53, ensuring that the URL shortener's custom domain properly handles HTTPS traffic.

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[gemini-code-assist]

Code Review

This pull request refactors the URL shortener to use a Lambda Function URL as the CloudFront origin, replacing the previous implementation that used an S3 origin with a Lambda@Edge function. This is a great architectural simplification that removes the complexity of Lambda@Edge. The changes to the CDK stack and supporting configuration files are well-aligned with this goal.

I've found one critical issue in the new redirect.lambda.ts due to a typo, which would prevent the redirects from working. I've also included a couple of suggestions to improve error handling and add validation for more robust behavior. Overall, this is a solid improvement.

[gemini-code-assist]

After parsing the JSON from the S3 object, there's no validation to ensure that the redirect.url property exists. If the S3 object is malformed or doesn't contain a url property, this will result in a redirect response with an empty or missing Location header, which is not ideal. It's good practice to add a check for redirect.url and throw an error if it's missing. The catch block will then handle it, returning an appropriate error status code to the client.

    const redirect = JSON.parse(await data.Body.transformToString());

    if (!redirect.url) {
      throw new Error('Redirect object in S3 is missing a `url` property.');
    }

    return {
      statusCode: 301,
      headers: {
        Location: redirect.url,
      },
    };

[gemini-code-assist]

The current error handling returns a 404 Not Found for any error that occurs within the try block. This can make debugging difficult, as it hides the underlying cause of the error (e.g., permissions issues, throttling, or other S3 errors). It would be better to distinguish between a 'Not Found' error (NoSuchKey) and other server-side errors. For unexpected errors, returning a 500 Internal Server Error would be more appropriate and provide better observability.

    console.log(err);
    // The AWS SDK v3 for JS surfaces `NoSuchKey` as an error with name 'NoSuchKey'.
    if (err instanceof Error && err.name === 'NoSuchKey') {
      return { statusCode: 404 };
    }
    return { statusCode: 500 };