#!/bin/bash

# inspired by https://stackoverflow.com/questions/45020323/ecs-asg-scaling-down-policy-recommendations
# needs jq, awscli, root, IAM autoscaling:DescribeAutoScalingInstances, SetInstanceProtection

echo "--------------------------------"
echo "[$(date)] Starting script "

set -x
## self EC2 instance ID
instance_id=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)

## self region
az=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone)
region=${az:0:${#az} - 1}

## self ASG ID
asg_name=$(aws autoscaling describe-auto-scaling-instances --region $region --instance-ids $instance_id | jq .AutoScalingInstances[].AutoScalingGroupName -r)

## number of containers running (except ecs-agent and dd-agent)
containers_running=$(docker ps --format "{{.Image}}" | grep -v "amazon/amazon-ecs-agent" | grep -v "datadog/docker-dd-agent" | wc -l)

## current protection state
scale_protection=$(aws autoscaling describe-auto-scaling-instances --region ${region} --instance-ids ${instance_id} | jq '.AutoScalingInstances[].ProtectedFromScaleIn' -r)

if [ ${containers_running} -ge 1 ]; then
    if [ ${scale_protection} == "true" ]; then
        echo "Containers are running and scale protection is on. Doing nothing."
    else
        aws autoscaling set-instance-protection --region ${region} --instance-ids ${instance_id} --auto-scaling-group-name ${asg_name} --protected-from-scale-in
        echo "Containers are running and scale protection is off. Enabling scale-in protection"
    fi
elif  [ ${containers_running} == 0 ]; then
    if [ ${scale_protection} == "true" ]; then
        echo "No containers running and scale protection is on. Disabling scale-in protection."
        aws autoscaling set-instance-protection --region ${region} --instance-ids ${instance_id} --auto-scaling-group-name ${asg_name} --no-protected-from-scale-in
    else
        echo "No containers running and scale protection is off. Doing nothing."
    fi
fi

set +x

echo "[$(date)] Script finished running successfully."
echo "--------------------------------"
echo