import boto3

def lambda_handler(event, context):
    if 'detail' in event:
        if 'instance-id' in event['detail']:
            instance_id = event['detail']['instance-id']
        else:
            return
    else:
        return
        
    client_ec2 = boto3.client('ec2')
    instance_details = client_ec2.describe_instances(InstanceIds=[instance_id])

    for tag in instance_details['Reservations'][0]['Instances'][0]['Tags']:
        if tag['Key'] == 'Name':
            tag_name = tag['Value']
        if tag['Key'] == 'DNSName':
            tag_dnsname = tag['Value']
        if tag['Key'] == 'HostedZoneId':
            tag_hostedzoneid = tag['Value']
            
    try:
        print('Found tags: {}, {}, {}'.format(tag_name, tag_dnsname, tag_hostedzoneid))
    except:
        print('Required tags missing.')
        return

    client_r53 = boto3.client('route53')
    client_r53.change_resource_record_sets(
        HostedZoneId=tag_hostedzoneid,
        ChangeBatch={
            'Changes': [
                {
                    'Action': 'UPSERT',
                    'ResourceRecordSet': {
                        'Name': tag_dnsname,
                        'Type': 'A',
                        'TTL': 360,
                        'ResourceRecords': [
                            {
                                'Value': instance_details['Reservations'][0]['Instances'][0]['PublicIpAddress']
                            }
                        ]
                    }
                }
            ]
        }
    )
    print('Updated {} for {} to {}'.format(tag_dnsname,instance_id,instance_details['Reservations'][0]['Instances'][0]['PublicIpAddress']))

    return