module.exports.processFile = (event, context, callback) => {
  const csv = require('fast-csv');
  const s3 = new AWS.S3();
  const dynamodb = new AWS.DynamoDB();

  async.waterfall([
    (next) => {
      console.log('Waiting until the uploaded object becomes available',
        '[bucket = ', event.bucketName, ', key = ',
        event.objectKey, '  ]');
      s3.waitFor('objectExists', {
        Bucket: event.bucketName,
        Key: event.objectKey
      }, next);
    },
    (result, next) => {
      console.log('Downloading the CSV file from S3 [bucket = ',
        event.bucketName, ', key = ', event.objectKey, '  ]');

      const csvStream = s3.getObject({
        Bucket: event.bucketName,
        Key: event.objectKey
      }).createReadStream();

      csv.fromStream(csvStream).on('data', (data) => {
        dynamodb.putItem({
          Item: {
            'sensor_id': {
              'S': data[0]
            },
            'timestamp': {
              'N': data[1]
            },
            'value': {
              'N': data[2]
            }
          },
          TableName: "sensor_data"
        });
      });

      next(null);
    },
  ], (err, results) => {
    if (err) {
      console.log('Failed execution');
      return context.fail('Execution failed');
    } else {
      console.log('Successful execution');
      return context.succeed(event);
    }
  });
};