Amazon EC2 Auto Scaling
User Guide

Maintaining the Number of Instances in Your Auto Scaling Group

After you have created your Auto Scaling group, the Auto Scaling group starts by launching enough EC2 instances to meet its minimum capacity (or its desired capacity, if specified). If there are no other scaling conditions attached to the Auto Scaling group, the Auto Scaling group maintains this number of running instances even if an instance becomes unhealthy.

To maintain the same number of instances, Amazon EC2 Auto Scaling performs a periodic health check on running instances within an Auto Scaling group. When it finds that an instance is unhealthy, it terminates that instance and launches a new one. If you stop or terminate a running instance, the instance is considered to be unhealthy and replaced. For more information about health check replacements, see Health Checks for Auto Scaling Instances.