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Amazon EC2 Auto Scaling
User Guide

AWS Documentation » Amazon EC2 Auto Scaling » User Guide » Monitoring Your Auto Scaling Instances and Groups » Health Checks for Auto Scaling Instances

Health Checks for Auto Scaling Instances

The health status of an Auto Scaling instance is either healthy or unhealthy. After an instance is fully configured and passes the initial health checks, it is considered healthy by Amazon EC2 Auto Scaling and enters the InService state. Amazon EC2 Auto Scaling periodically performs health checks on the instances in your Auto Scaling group and identifies any instances that are unhealthy. After Amazon EC2 Auto Scaling marks an instance as unhealthy, it is scheduled for replacement. For more information, see Replacing Unhealthy Instances.

Instance Health Status

Amazon EC2 Auto Scaling determines the health status of an instance using one or more of the following:

By default, Amazon EC2 Auto Scaling health checks use the results of the EC2 status checks to determine the health status of an instance. Amazon EC2 Auto Scaling marks an instance as unhealthy if its instance fails one or more of the status checks.

If you attached a load balancer or target group to your Auto Scaling group, you can configure Amazon EC2 Auto Scaling to mark an instance as unhealthy if Elastic Load Balancing reports the instance as OutOfService. If connection draining is enabled for your load balancer, Amazon EC2 Auto Scaling waits for in-flight requests to complete or the maximum timeout to expire, whichever comes first, before terminating instances due to a scaling event or health check replacement. For more information, see Using Elastic Load Balancing Health Checks with Auto Scaling.

Health Check Grace Period

Frequently, an Auto Scaling instance that has just come into service needs to warm up before it can pass the health check. Amazon EC2 Auto Scaling waits until the health check grace period ends before checking the health status of the instance. While the EC2 status checks and ELB health checks can complete before the health check grace period expires, Amazon EC2 Auto Scaling does not act on them until the health check grace period expires. To provide ample warm-up time for your instances, ensure that the health check grace period covers the expected startup time for your application. Note that if you add a lifecycle hook to perform actions as your instances launch, the health check grace period does not start until the lifecycle hook is completed and the instance enters the InService state.

Custom Health Checks

If you have custom health checks, you can send the information from your health checks to Amazon EC2 Auto Scaling so that Amazon EC2 Auto Scaling can use this information. For example, if you determine that an instance is not functioning as expected, you can set the health status of the instance to Unhealthy. The next time that Amazon EC2 Auto Scaling performs a health check on the instance, it will determine that the instance is unhealthy and then launch a replacement instance.

Use the following set-instance-health command to set the health state of the specified instance to Unhealthy: 

aws autoscaling set-instance-health --instance-id i-123abc45d --health-status Unhealthy

Use the following describe-auto-scaling-groups command to verify that the instance state is Unhealthy: 

aws autoscaling describe-auto-scaling-groups --auto-scaling-group-names my-asg

The following is an example response that shows that the health status of the instance is Unhealthy and that the instance is terminating: 

{
    "AutoScalingGroups": [
        {
            ....
            "Instances": [
                {
                    "InstanceId": "i-123abc45d",
                    "AvailabilityZone": "us-west-2a",
                    "HealthStatus": "Unhealthy",
                    "LifecycleState": "Terminating",
                    "LaunchConfigurationName": "my-lc"
                },
                ...
            ]
        }
    ]
}