ASG: Auto Scaling Group

• In real-life, the load on your websites and applications can change. 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 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 a scale-out policies (increase the number of instances)
  • We can create a scale-in policies (decrease the number of instances)

Auto Scaling 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)
  1. Send custom metrics from an application on EC2 to CloudWatch (PutMetric API)
  2. Create a CloudWatch alarm to react to low / high values
  3. Use the CloudWatch Alarm as the scaling policy for ASG

ASG Summary

• 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 configurations and you update an ASG by providing a new launch configuration
• 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 restart them. Extra safety
• ASG can terminate instances marked as unhealthy by an LB (and hence replace them)