Contains a collection of EC2 instances that are treated as a group for the purposes of automatic scaling and management.

Can occur via:

1. Capacity Settings
2. Health Check Replacements
3. Scaling Policies

ASG Capacity Settings

The size of an ASG is based on Min, Max and Desired Capacity.

• Min — how many should at least be running
• Max — how many are allowed to be running
• Desired Capacity — how many you want to ideally run

ASG will always launch instances to meet minimum capacity.

ASG Health Check Replacements

EC2 Health Check

ASG will perform a Health Check on EC2 instances to determine if there is a software or hardware issue.

Is based on the EC2 Status Checks.

If an instance is considered unhealthy. ASG will terminate and launch a new instance.

ELB Health Check

Will perform a health check based on the ELB health check

Can perform health checks by pinging an HTTP(S) endpoint with an expected response.

If ELB determines an instance in unhealthy, it forwards this information to the ASG which will terminate the unhealthy instance.

ELB does not terminate (kill) unhealthy instance. It will just redirect traffic to healthy instances.