Auto Scaling Group

Launch Configuration (LC) & Launch templates (LT)

• auto-scaling and self-healing for EC2
• self-healing - create an instance and terminate old problematic one
• ASG defines When & where, uses LT/ LC defines what
• fee and only the resources created are billed
• more effective for more and smaller instances
• Three important value
  • minimum, Desired & maximum (eg 1:2:4)
• keep running instances at the desired capacity by provisioning/ terminating instances
• scaling policies automate based on metrics (eg CPU)
  • rule that adjust the values of Auto Scaling Group
  • AGEs can have no scaling policies
  • 4 scaling method:
    • simple scaling - manually adjust the desired capacity
    • scheduled scaling - time based adjustment - e.g. sales
    • dynamic scaling
      • simple ‘CPU above 50% +1’/ ‘CPU below 50% -1’
      • stepped scaling - multiple range +1/-1
      • target tracking - set a desired aggregated metric (40% CPU on avg)
      • scale based on SQS - based on number of messages
• cooldown periods exists for next scaling

Scaling processes

• Launch & terminate - suspend & resume
  • Launch=suspend: won’t scale out if any alarms / schedule take place
  • terminate =suspend: won’t terminate instances
• AddToLoadBalancer - add to LB on launch
• AlarmNotification - accept notification from cloudwatch
• AZRebalance - balances instances evenly across all of the AZs
• HealthCheck - instance health checks on/ off
• ReplaceUnhealthy - terminate unhealthy and replace
• scheduledActions - scheduled on/off
• Standby - use this for instances ‘inService vs Standby’

Lifecycle Hooks

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• allow custom actions on instances during ASG action during instance launch/ terminate transition
• instances are paused within the flow - wait until a timeout then either continue/ abandon
• Closely work with EventBridge/ SNS

Health Checks

• EC2 (default), ELB (can be enabled) & custom
• EC2 - can be more application aware
  • Unhealthy = stopping, stopped, terminated, shutting down or impaired
  • healthy = running & passing ELB health check
• Custom - by an external system
• health check grace period (default 300s) - delay before starting checks
• allow system launch, bootstrapping and application start