s3gz-lambda.js

var aws = require('aws-sdk');
var fs = require('fs');
var zlib = require('zlib');

exports.handler = function(event, context) {
    console.log(JSON.stringify(event, null, 2));
    console.log(JSON.stringify(context, null, 2));

    var bucket = event.Records[0].s3.bucket.name;
    var key = event.Records[0].s3.object.key;
    if ( key.match(/\.gz$/) ) {
        context.done(key + ' alread gzipped');
        return;
    }
    var s3 = new aws.S3();
    var gzStream = s3.getObject({ Bucket: bucket, Key: key }).
        createReadStream().pipe(zlib.createGzip());

    var s3obj = new aws.S3({ params: { Bucket: bucket, Key: 'gz/' + key + '.gz' }});
    s3obj.upload({Body: gzStream}).
        send(function(err, data) {
            console.log(err, data);
            if (err) {
                context.done('error putting object', err);
            } else {
                context.done();
            }
        });
};