Auto Scaling groups

An Auto Scaling group contains a collection of EC2 instances that are treated as a logical grouping for the purposes of automatic scaling and management. An Auto Scaling group also lets you use Amazon EC2 Auto Scaling features such as health check replacements and scaling policies. Both maintaining the number of instances in an Auto Scaling group and automatic scaling are the core functionality of the Amazon EC2 Auto Scaling service.

The size of an Auto Scaling group depends on the number of instances that you set as the desired capacity. You can adjust its size to meet demand, either manually or by using automatic scaling.

An Auto Scaling group starts by launching enough instances to meet its desired capacity. It maintains this number of instances by performing periodic health checks on the instances in the group. The Auto Scaling group continues to maintain a fixed number of instances even if an instance becomes unhealthy. If an instance becomes unhealthy, the group terminates the unhealthy instance and launches another instance to replace it. For more information, see Health checks for instances in an Auto Scaling group.

You can use scaling policies to increase or decrease the number of instances in your group dynamically to meet changing conditions. When the scaling policy is in effect, the Auto Scaling group adjusts the desired capacity of the group, between the minimum and maximum capacity values that you specify, and launches or terminates the instances as needed. You can also scale on a schedule. For more information, see Choose your scaling method.

When creating an Auto Scaling group, you can choose whether to launch On-Demand Instances, Spot Instances, or both. You can specify multiple purchase options for your Auto Scaling group only when you use a launch template. For more information, see Auto Scaling groups with multiple instance types and purchase options.

Spot Instances provide you with access to unused EC2 capacity at steep discounts relative to On-Demand prices. For more information, see Amazon EC2 Spot Instances. There are key differences between Spot Instances and On-Demand Instances:

When a Spot Instance is terminated, the Auto Scaling group attempts to launch a replacement instance to maintain the desired capacity for the group.

When instances are launched, if you specified multiple Availability Zones, the desired capacity is distributed across these Availability Zones. If a scaling action occurs, Amazon EC2 Auto Scaling automatically maintains balance across all of the Availability Zones that you specify.