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ASG - Auto Scaling Groups

  • Auto Scaling Groups provide auto scaling for EC2
  • Provide the ability to implement a self-healing architecture
  • ASGs make use of configurations defined in launch templates or launch configurations
  • ASGs are using one version of a launch template/configuration
  • ASG have 3 important values defined: Minimum, Desired and Maximum size
  • ASG provides on foundational job: keeps the size of running instances at the desired size
  • Scaling Policies: update the desired capacity based on some metric (CPU usage, number of connections, etc.)
    • They are essentially rules defined by us which can adjust the values of an ASG
    • Scaling types:
      • Manual Scaling
      • Scheduled Scaling: scheduling based on know time window
      • Dynamic Scaling
  • Dynamic Scaling has 3 subtypes:
    • Simple Scaling: example "CPU above 50% +1", "CPU Below 50% -1"
    • Stepped Scaling: scaling based on difference, allowing to react quicker
    • Target Tracking: example desired aggregate CPU = 40%. Not all metrics are supported by target tracking scaling
  • Cooldown Period: a value in seconds, controls how long to wait after a scaling action happened before starting another action
  • ASG monitor the health of instances, by default using the EC2 health checks
  • ASG can integrate with load balancers: ASG can add/remove instances from a LB target group
  • ASG can use the LB health checks in case of EC2 health checks

Scaling Processes

  • Launch and Terminate: if Launch is suspended, the ASG wont scale out / if Terminate is suspended the ASG wont scale in
  • AddToLoadBalancer: add instance to LB
  • AlarmNotification: control is the ASG reacts to CloudWatch alarms
  • AZRebalance: balances instances evenly across all of AZs
  • HealthCheck: controls if instance health checks are on/off
  • ReplaceUnhealthy: controls if instances are replaced in case there are unhealthy
  • ScheduledActions: controls if scheduled actions are on/off
  • Standby: suspend any activities of ASG in a specific instance

ASG Consideration

  • ASG are free, we pay only for the instances provisioned
  • We should use cool downs to avoid rapid scaling
  • We should use smaller instances for granularity
  • ASG integrates with ALBs
  • ASG defines when and where, LT defines what

ASG Lifecycle Hooks

  • Allow to configure custom actions which can occur during ASG actions
  • When an ASG scales out/in instances may pause within the flow to allow execution of lifecycle hooks
  • We can specify a timeout for the lifecycle action, after the pause the system can decide if the ASG process continues or is abandoned
  • We can resume the ASG process by calling CompleteLifecycleAction
  • Lifecycle event hooks can be integrated with EventBridge, SQS or SNS ASG Lifecycle Hooks