|Using Auto Scaling / Auto Scaling Concepts|
Auto scaling periodically performs health checks on the instances in your Auto Scaling group. By default, Auto Scaling group determines the health state of each instance by periodically checking the results of Eucalyptus instance status checks. When Auto Scaling determines that an instance is unhealthy, it terminates that instance and launches a new one.
If your Auto Scaling group is using elastic load balancing, Auto Scaling can determine the health status of the instances by checking the results of both Eucalyptus instance status checks and elastic load balancing instance health.
Auto scaling determines an instance is unhealthy if the calls to either Eucalyptus instance status checks or elastic load balancing instance health status checks return any state other than OK (or InService).
If there are multiple elastic load balancers associated with your Auto Scaling group, Auto Scaling will make health check calls to each load balancer. If any of the health check calls return any state other than InService, that instance will be marked as unhealthy. After Auto Scaling marks an instance as unhealthy, it will remain marked in that state, even if subsequent calls from other load balancers return an InService state for the same instance.
For more information, go to Configuring Health Checks.