What is an Auto Scaling Group?

• In real-life, the load of your websites and applications can be change

• In the cloud, you can create and get rid of servers very quickly

• The goal of an Auto Scaling Groups (ASG) is to:

  • Scale out (add EC2 intances) 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)

• Auto Scaling Group are free - you only pay for the underlying EC2 intances.

Auto Scaling Group Attributes

• A Launch Template, includes:

  • AMI + Instance Type
  • EC2 User Data
  • EBS Volumes
  • Security Groups
  • SSH Key Pair
  • IAM Role for your EC2 instance
  • Network + Subnet Information
  • Load Balancer Information

• Min Size / Max Size / Initial Capacity

• Scaling Policies

CloudWatch Alarm and Scaling

• It is posible to scale an ASG based on CloudWatch alarms

• An alarm monitors a metric (such as Average CPU, or a custom metric)

• Metric 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 Policies

Dynamic Scaling

• Target Tracking Scaling

  • Simple to set-up
  • Example: I want the average ASG CPU to stay at around 40%

• Simple / Step Scaling

  • When a CloudWatch alarm is triggered (ex: CPU > 70%) then add 2 units
  • When a CloudWatch alarm is triggerd (ex: CPU < 30%) then remove 1

Scheduled Scaling

• Anticipate a scaling based on known usage patterns
• Example: increase the min capacity to 10 at 5pm on Fridays

Predictive Scaling: continously forecast load and schedule scaling ahead

Good metrics to scale on

• 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 your application is network bound

• Any custom metric (that you push using CloudWatch)