AWS Lambda to translate Snowplow CloudFront logs to usable JSON - Using S3 create events as the trigger

index.js

console.log('Loading function');

const aws = require('aws-sdk');
const zlib = require('zlib');
const CloudFrontParser = require('cloudfront-log-parser');

const s3 = new aws.S3({ apiVersion: '2006-03-01' });

let getS3FileContents = async(bucketName, objectKey) => {
    const params = {
        Bucket: bucketName,
        Key: objectKey,
    };
    try {
        const { Body } = await s3.getObject(params).promise();
        var buffer = new Buffer(Body);
        var decodedBody = zlib.gunzipSync(buffer).toString('utf8')      
        return decodedBody;
    } catch (err) {
        console.log(err);
        const message = `Error getting object ${objectKey} from bucket ${bucketName} or decoding object. Make sure they exist and your bucket is in the same region as this function.`;
        console.log(message);
        throw new Error(message);
    }
};

let putJsonObjAsGzipToS3 = async(bucketName, objectKey, jsonString) => {
    var bufferObject = new Buffer.from(jsonString);
    var encodedBody = zlib.gzipSync(bufferObject)
    const params = {
        Bucket: bucketName,
        Key: objectKey,
        Body: encodedBody,
        ContentType: "application/json",
        ContentEncoding: 'gzip'
    };
    try {
        await s3.putObject(params).promise();
    } catch (err) {
        console.log(err);
        const message = `Error saving object ${objectKey} to bucket ${bucketName}. Make sure they exist and your bucket is in the same region as this function.`;
        console.log(message);
        throw new Error(message);
    }
};


let mapCloudFrontObjectToSnowPlowObject = (access) =>{
    // SEE - https://github.com/snowplow/snowplow/wiki/snowplow-tracker-protocol#21-web-specific-parameters
    var queryString = access['cs-uri-query'];
    if(queryString == '-') return access;
    var queryStringParams = decodeURI(decodeURI(queryString)).split("&");
    let dict = {}
    queryStringParams.forEach(element => {
        let kvp = element.split("=");
        dict[kvp[0]] = kvp[1]
    });
    if(dict['cx']){
        dict['context'] = Buffer.from(dict['cx'], 'base64').toString('utf8');
    }   
    if(dict['ue_px']){
        dict['unstruct_event'] = Buffer.from(dict['ue_px'], 'base64').toString('utf8');
    }       
    access['snowplowData'] = dict;
    return access;
};

let parseCloudfrontSnowplowLogs = (fileContents) => {
    const accesses = CloudFrontParser.parse(fileContents, { format: 'web' });
    let snowplowEvents = accesses.map(mapCloudFrontObjectToSnowPlowObject);
    return snowplowEvents;
};

exports.handler = async (event, context) => {
    const bucket = event.Records[0].s3.bucket.name;
    const key = decodeURIComponent(event.Records[0].s3.object.key.replace(/\+/g, ' '));
    var fileContents = await getS3FileContents(bucket, key);   
    var snowplowEvents = await parseCloudfrontSnowplowLogs(fileContents);
    let stagedContent = JSON.stringify(snowplowEvents, null, 0)
    
    console.log('snowplowEvents');
    console.log(stagedContent);

    await putJsonObjAsGzipToS3(bucket, 'staged/' + key, stagedContent);   

    return 'Ok'
};

package.json

{
  "name": "clickstream-log-reader",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "prepackage": "rm -f lambda.zip && mkdir -p lambda/node_modules/ && rm -rf ./node_modules && npm install --prod",
    "package": "cp ./index.js ./lambda/ && cp -r ./node_modules/. ./lambda/node_modules/ && cd lambda && zip -r ../lambda.zip *",
    "postpackage": "rm -rf lambda && rm -rf ./node_modules && npm install",
    "clean": "rm -fr ./output"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "cloudfront-log-parser": "^1.0.0",
    "zlib": "^1.0.5"
  }
}

Please change the error handling code to ensure a useful error message is returned.

From:
const message = Error getting object ${objectKey} from bucket ${bucketName} or decoding object. Make sure they exist and your bucket is in the same region as this function.;
console.log(message);

To:
console.log(err, err.stack);
throw new Error(err);

Amazingly this exact error message has spread across 50 GitHub Gists. #copypasta