AWS Service

ASG

What’s an Auto Scaling Group?
- In real-life, the load on your websites and application can change
- In the cloud, you can create and get rid of servers very quickly
- The goal of an Auto Scaling Group (ASG) is to:
  - Scale out (add EC2 instances) to match an increased load
  - Scale in (remove EC2 instances) to match a decreased load
  - Ensure we have a minimum and a maximum number of EC2 instances running
  - Automatically register new instances to a load balancer
  - Re-create an EC2 instance in case a previous one is terminated (ex: if unhealthy)
- ASG are free (you only pay for the underlying EC2 instances)
Auto Scaling Group in AWS
Auto Scaling Group in AWS With Load Balancer
Auto Scaling Group Attributes
- Launch Template
  - AMI , Instance type
  - EC2 User Data
  - EBS Volumes
  - Security Groups
  - SSH Key Pair
  - IAM Role of EC2 instance
  - Subnet
  - Load Balancer
- Min size, Max Size, Desired Capacity, Initial Capacity
- Scaling Policies
Auto Scaling - CloudWatch Alarms & Scaling
- It is possible to scale an ASG based on CloudWatch alarms
- An alarm monitors a metric (such as Average CPU, or a custom metric)
- Metrics such as Average CPU are computed for the overall ASG instances
- Based on the alarm:
  - We can create scale out policies (increase the number of instances)
  - We can create scale in policies (decrease the number of instances)
Auto Scaling Groups - Dynamic Scaling Policies
- Target Tracking Scaling
  - Most simple and easy to setup
  - Example: I want the average ASG CPU to stay at around 50%
- Simple / Step Scaling
  - When a CloudWatch alarm is triggered (example CPU > 70%), then add 2 instance
  - When a CloudWatch alarm is triggered (example CPU < 30%), then remove 1 instance
- Scheduled Actions
  - Anticipate a scaling based on known usage patterns
  - Example: increase the min capacity to 10 at 5 pm on Fridays
Auto Scaling Groups - Predictive Scaling
- Predictive scaling: continuously forecast load and schedule scaling ahead
Metrics to scale
- CPUUtilization: Average CPU utilization across your instances
- RequestCountPerTarget: to make sure the number of requests per EC2 instances is stable
- Average Network In / Out (if you’re application is network bound)
- Any custom metric (that you push using CloudWatch)
Auto Scaling Groups - Scaling Cooldowns
- After a scaling activity happens, you are in the cooldown period (default 300 seconds)
- During the cooldown period, the ASG will not launch or terminate additional instances (to allow for metrics to stabilize)
- Advice: Use a ready-to-use AMI to reduce configuration time in order to be serving request fasters and reduce the cooldown period