var AWS = require('aws-sdk');
var domain = '.example.com';
var hostedZone = 'HOSTEDZONEID';

exports.handler = function(event, context, callback) {

    var ec2 = new AWS.EC2();
    var params = {
        InstanceIds : [
            event.detail['instance-id']
        ]
    };
    
    ec2.describeInstances(params, function (err, data) {
        if (err) console.log(err, err.stack);
        else {
            var subdomain = data.Reservations[0].Instances[0].Tags.filter(function(p){return p.Key == "SubDomain"})[0];
            var hostname = data.Reservations[0].Instances[0].PublicDnsName;

            console.log("subdomain: " + (subdomain ? subdomain.Value : null));
            console.log("new hostname: " + hostname);
            
            if (hostname && subdomain) {
                var route53 = new AWS.Route53();
                
                route53.changeResourceRecordSets({
                    HostedZoneId : hostedZone,
                    ChangeBatch : {
                        Changes : [{
                            Action: 'UPSERT',
                            ResourceRecordSet: {
                                Name: subdomain.Value + domain,
                                Type: 'CNAME',
                                ResourceRecords: [
                                    {
                                        Value: hostname
                                    }
                                ],
                                TTL: 300
                            }
                        }]
                    }
                }, function (err, data) {
                    if (err)
                        console.log(err, err.stack);
                    else {
                        console.log('success');
                    }
                });
            }
        }
    });
}