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AWS Solutions Architect – Professional
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](/aws-solutions-architect-professional/04-compute/images/ASGArchitecture3.png)