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Auto Scaling Groups Overview

What's an Auto Scaling Group?

  • In real-life, the load on your websites and applications 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 maximum number of machines running
    • Automatically Register new instances to a load balancer

ASGs have the following attributes

  • A launch configuration
    • AMI + Instance type
    • EC2 user data
    • EBS Volumes
    • Security Groups
    • SSH Key Pair
  • Min Size / Max Size / Initial Capacity
  • Network + Subnets Information
  • Load Balancer Information
  • Scaling Policies

Auto Scaling Alarms

  • It is possible to scale an ASG based on CloudWatch alarms
  • An Alarm monitors a metric (such as Average CPU)
  • Metrics 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 New Rules

  • It is now possible to define "better" auto scaling rules that are directly managed by EC2
    • Target Average CPU Usage
    • Number of requests on the ELB per instance
    • Average Network In
    • Average Network Out
  • These rules are easier to set up and can make more sense

Auto Scaling Custom Metric

  • We can auto scale based on a custom metric (ex: number of connected users)

  • Send custom metric from application on EC2 to CloudWatch

  • Create CloudWatch alarm to react to low/high values
  • Use the CloudWatch alaram as the scaling policy for ASG

ASG Brain Dump

  • Scaling policies can be on CPU, Network... and can even be on custom metrics or based on a schedule (if you know your visitors patterns)
  • ASGs use Launch configuration or Launch Templates (newer)
  • To update an ASG, you must provide a new launch configuration / launch template
  • IAM roles attached to an ASG will get assigned to EC2 instances
  • ASG are free. You pay for the underlying resources being launched
  • Having instances under an ASG means that if they get terminated for whatever reason, the ASG will automatically create new ones as a replacement.
  • ASG can terminate instances marked as unhealthy by an LB (and hence replace them)