Intro

• Regional Service (cannot span multiple regions)
• Supports Multi AZ
• Automatically add or remove instances (scale horizontally) based on the load
• Free (pay for the underlying resources)
• IAM roles attached to an ASG will get assigned to the launched EC2 instances
• ASG can terminate instances marked as unhealthy by an ELB (and hence replace them)

<aside> 💡 If you have an ASG with running instances and you delete the ASG, the instances will be terminated and the ASG will be deleted.

</aside>

Scaling Policies

• Scheduled Scaling
  • Scale based on a schedule
  • Used when the load pattern is predictable
• Simple Scaling
  • Scale to certain size on a CloudWatch alarm
  • Ex. when CPU > 90%, then scale to 10 instances
• Step Scaling
  • Scale incrementally in steps using CloudWatch alarms
  • Ex. when CPU > 70%, then add 2 units and when CPU < 30%, then remove 1 unit
  • Specify the instance warmup time to scale faster
• Target Tracking Scaling
  • ASG maintains a CloudWatch metric and scale accordingly (automatically creates CW alarms)
  • Ex. maintain CPU usage at 40%
• Predictive Scaling
  • Historical data is used to predict the load pattern using ML and scale automatically

Launch Configuration & Launch Template

• Defines the following info for ASG
  • AMI (Instance Type)
  • EC2 User Data
  • EBS Volumes
  • Security Groups
  • SSH Key Pair
  • Min / Max / Desired Capacity
  • Subnets (where the instances will be created)
  • Load Balancer (specify which ELB to attach instances)
  • Scaling Policy
• Launch Configuration (legacy)
  • Cannot be updated (must be re-created)
  • Does not support Spot Instances
• Launch Template (newer)
  • Versioned
  • Can be updated
  • Supports both On-Demand and Spot Instances
  • Recommended by AWS