Auto Scaling:

• Auto Scaling is a service (and method) provided by AWS that automates the process of increasing or decreasing the number of provisioned on-demand instances available for your application.

• Auto Scaling will increase or decrease the amount instances based on chosen CloudWatch metrics.

• For example, if your applications's demand increases un-expectantly, auto scaling can automatically scale up (add instance) to meet the demand and terminate instances when demand decreases.

  • This is known as "elasticity" in the AWS environment.

Auto Scaling has two main components:

• Launch Configuration:

  • The EC2 "template" used when the auto scaling group needs to provision an additional instance (i.e. AMI, instance type, user-data, storage, security groups, etc.).

• Auto Scaling Group:

  • All the rules and settings that govern if/when an EC2 instance is automatically provisioned or terminated.

    • Number of MIN & MAX allows instances.

    • VPC & AZs to launch instances into.

    • If provisioned instances should receive traffic from a ELB.

    • Scaling policies (CloudWatch metrics thresholds that trigger scaling).

    • SNS notifications (to keep you informed when scaling occurs).

NOTE: For architecture to be considered highly available and fault tolerant - it MUST have an ELB serving traffic to and ASG (Auto Scaling Group) with a MIN of two instances located in separate Availability Zones.