AWS Lambda Function to Redirect HTTP Requests by Reading JSON Data in S3

lambda-cloudfront-s3-redirect.js

const REDIRECTS_DATA_REGION = 'us-east-1';
const REDIRECTS_DATA_BUCKET = 'pdfs';
const REDIRECTS_DATA_OBJECT = 'docs/pdf-redirects.json';

// https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/client/s3/command/GetObjectCommand/
import { S3Client, GetObjectCommand } from "@aws-sdk/client-s3";

const client = new S3Client({ 
    region: REDIRECTS_DATA_REGION
});

async function readFile(bucketName, filename) {
  const input = { Bucket: bucketName, Key: filename };
  const command = new GetObjectCommand(input);
  const response = await client.send(command);
  return response.Body.transformToString();
}

async function getRedirectsData() {
  const txt = await readFile(REDIRECTS_DATA_BUCKET, REDIRECTS_DATA_OBJECT);
  let data;
  
  try {
    data = JSON.parse(txt);
  } catch (e) {
    console.error('getRedirectsData parse failed', txt, e);
    data = {};
  }
  return data;
}

const supportedStatusCodes = {
  301: 'Moved Permanently',
  302: 'Found',
  307: 'Temporary Redirect',
};

export const handler = async (event, context) => {
    // Get the incoming request and the initial response from S3
    // https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-event-structure.html
    const { request, response } = event.Records[0].cf;

    // request.uri is just the URL path without hostname or querystring
    let path = request.uri;

    // Cut off trailing slash to normalize it
    if (path.slice(-1) === '/') {
        path = path.slice(0, -1);
    }

    const redirectsData = await getRedirectsData();
    const redirectData = redirectsData[path];

    // See if a redirect exists & sanity check that the redirect object is what we expect
    const shouldRedirect = !!(
        redirectData &&
        redirectData.to &&
        supportedStatusCodes[redirectData.statusCode]
    );

    if (shouldRedirect) {
        // Note: We can't completely create a new set of headers even for a redirect
        // because Cloudfront fails painfully if you set (or clear) a "read-only" header.
        // For Origin Response triggers, that includes leaving "Transfer-Encoding" alone.
        // https://github.com/awsdocs/amazon-cloudfront-developer-guide/blob/master/doc_source/lambda-requirements-limits.md#read-only-headers
        response.headers['location'] = [
        {
            key: 'Location',
            value: redirectData.to,
        },
        ];

        return {
        status: redirectData.statusCode,
        statusDescription: supportedStatusCodes[redirectData.statusCode],
        headers: response.headers,
        };
    }

    return response;
};