AWS: Lambda for S3 processing

const os = require('os');
const fs = require('fs');
const path = require('path');
const readline = require('readline');
const aws = require('aws-sdk');
const s3 = new aws.S3();

async function downloadFileFromS3 (bucket, fileKey, filePath) {
	console.log('downloading', bucket, fileKey, filePath);
	return new Promise(function (resolve, reject) {
		const file = fs.createWriteStream(filePath),
			stream = s3.getObject({
				Bucket: bucket,
				Key: fileKey
			}).createReadStream();
		stream.on('error', reject);
		file.on('error', reject);
		file.on('finish', function () {
			console.log('downloaded', bucket, fileKey);
			resolve(filePath);
		});
		stream.pipe(file);
	});
};

async function processFile(filePath) {
	console.log(`Processing ${filePath}.`);
	return new Promise(function (resolve, reject) {
		const outputPath = `${filePath}.out`;
		const readFile = readline.createInterface({
		  input: fs.createReadStream(filePath),
		  output: fs.createWriteStream(outputPath),
		  terminal: false
		});

		readFile
		  .on('line', transform)
		  .on('close', function() {
		    console.log(`Created "${this.output.path}"`);
		    resolve(outputPath);
		  });

		function transform(line) {
		  this.output.write(line.toUpperCase() + '\n');
		}
	});
}

async function uploadFileToS3 (bucket, fileKey, filePath, contentType) {
	console.log('uploading', bucket, fileKey, filePath);
	return s3.upload({
		Bucket: bucket,
		Key: fileKey,
		Body: fs.createReadStream(filePath),
		ACL: 'private',
		ContentType: contentType
	}).promise();
};

exports.handler = async function (eventObject, context) {
	const OUTPUT_BUCKET = 'demo-s3-output';
	const eventRecord = eventObject.Records && eventObject.Records[0];
	const inputBucket = eventRecord.s3.bucket.name;
	const key = eventRecord.s3.object.key;
	const workdir = os.tmpdir();
	const inputFile = path.join(workdir, key);

	console.log(`Transforming ${key} using ${inputFile}`);
	await downloadFileFromS3(inputBucket, key, inputFile);
	const newFile = await processFile(inputFile);
	await uploadFileToS3(OUTPUT_BUCKET, key, newFile);
};