update-private-dns.sh

#!/bin/bash
export AWS_ACCESS_KEY_ID="..."
export AWS_SECRET_ACCESS_KEY="..."
export AWS_SESSION_TOKEN="..."

should_terminate_ec2_instance=no
sleep_after_terminate_ec2_instance=30s

usage() {
    echo "Usage: ./update-private-dns.sh --terminate-ec2 no --sleep 40s --service a-alias"
}

while [ "$#" -gt 0 ]
do
	case "$1" in
	--help)
		usage
        exit 0
		;;
	--terminate-ec2)
		should_terminate_ec2_instance="$2"
		;;
	--sleep)
		sleep_after_terminate_ec2_instance="$2"
		;;
	--service)
		service="$2"
		;;
	--)
		break
		;;
	# an option argument, continue
	*)	;;
	esac
	shift
done

if [[ -z "$service" ]]; then
  echo "service can not be empty"
  exit 1
fi

HOSTED_ZONE_ID=an-example

# ec2 mapping
declare -A SERVICE_NAMES_TO_EC2_NAMES=(
    ["a-alias"]="a-instance-name" 
    ["b-alias"]="b-instance-name" 
    ["c-alias"]="c-instance-name"
)

# route53 mapping
declare -A SERVICE_NAMES_TO_RECORD_NAMES=(
    ["a-alias"]="a-record-name"
    ["b-alias"]="b-record-name"
    ["c-alias"]="c-record-name"
)

instance_name=${SERVICE_NAMES_TO_EC2_NAMES[$service]}
record_name=${SERVICE_NAMES_TO_RECORD_NAMES[$service]}

if [[ -z "$instance_name" || -z "$record_name" ]]; then
  echo "invalid service"
  exit 1
fi


get_ec2_instance_by_ec2_instance_name() {
    local name=$1

    aws ec2 describe-instances --region ap-southeast-1 --no-cli-pager \
    --filters Name=tag:Name,Values=${name} Name=instance-state-name,Values=running \
    --query 'Reservations[0].Instances[0]' --output json
}

terminate_ec2_instance_by_instance_id() {
    local instance_id=$1
    aws ec2 terminate-instances --instance-ids $instance_id --no-cli-pager
}


update_private_dns() {
    local value=$1
    local record_name=$2

    aws route53 change-resource-record-sets --no-cli-pager --hosted-zone-id $HOSTED_ZONE_ID \
    --change-batch file://<(cat << EOF
{
    "Comment": "Update record to reflect private dns of ec2 instance",
    "Changes": [
        {
            "Action": "UPSERT",
            "ResourceRecordSet": {
                "Name": "${record_name}",
                "Type": "CNAME",
                "TTL": 60,
                "ResourceRecords": [
                    {
                        "Value": "${value}"
                    }
                ]
            }
        }
    ]
}
EOF
)
}

echo "get ec2 instance details: $instance_name"
instance_details=$(get_ec2_instance_by_ec2_instance_name $instance_name)

instance_id=$(echo $instance_details | jq -r '.InstanceId')
private_dns_name=$(echo $instance_details | jq -r '.PrivateDnsName')

echo "ec2 instance id: $instance_id, private dns name: $private_dns_name"

if [[ $should_terminate_ec2_instance == 'yes' ]]; then
    echo "terminate ec2 ${instance_id}"
    terminate_ec2_instance_by_instance_id $instance_id
    echo "waiting for new ec2 instance provison"
    sleep $sleep_after_terminate_ec2_instance

    echo "get new ec2 instance details: $instance_name"

    instance_details=$(get_ec2_instance_by_ec2_instance_name $instance_name)

    instance_id=$(echo $instance_details | jq -r '.InstanceId')
    private_dns_name=$(echo $instance_details | jq -r '.PrivateDnsName')

    echo "new ec2 instance id: $instance_id, private dns name: $private_dns_name"
fi

echo "update private dns name ${private_dns_name}, record name ${record_name}"
update_private_dns $private_dns_name $record_name