View health check status and the reason for health check failures - Amazon EC2 Auto Scaling
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View health check status and the reason for health check failures

Health checks allow Amazon EC2 Auto Scaling to determine when an instance is unhealthy and should be terminated. For warm pool instances kept in a Stopped state, it employs the knowledge that Amazon EBS has of a Stopped instance's availability to identify unhealthy instances. It does this by calling the DescribeVolumeStatus API to determine the status of the EBS volume that's attached to the instance. For warm pool instances kept in a Running state, it relies on EC2 status checks to determine instance health. While there is no health check grace period for warm pool instances, Amazon EC2 Auto Scaling doesn't start checking instance health until the lifecycle hook finishes.

When an instance is found to be unhealthy, Amazon EC2 Auto Scaling automatically deletes the unhealthy instance and creates a new one to replace it. Instances are usually terminated within a few minutes after failing their health check. For more information, see View the reason for health check failures.

Custom health checks are also supported. This can be helpful if you have your own health check system that can detect an instance's health and send this information to Amazon EC2 Auto Scaling. For more information, see Custom health checks.

On the Amazon EC2 Auto Scaling console, you can view the status (healthy or unhealthy) of your warm pool instances. You can also view their health status using the Amazon CLI or one of the SDKs.

To view the status of your warm pool instances (console)

  1. Open the Amazon EC2 console at https://console.amazonaws.cn/ec2/, and choose Auto Scaling Groups from the navigation pane.

  2. Select the check box next to the Auto Scaling group.

    A split pane opens up in the bottom of the Auto Scaling groups page.

  3. On the Instance management tab, under Warm pool instances, the Lifecycle column contains the state of your instances.

    The Health status column shows the assessment that Amazon EC2 Auto Scaling has made of instance health.

    Note

    New instances start healthy. Until the lifecycle hook is finished, an instance's health is not checked.

To view the reason for health check failures (console)

  1. Open the Amazon EC2 console at https://console.amazonaws.cn/ec2/, and choose Auto Scaling Groups from the navigation pane.

  2. Select the check box next to the Auto Scaling group.

    A split pane opens up in the bottom of the Auto Scaling groups page.

  3. On the Activity tab, under Activity history, the Status column shows whether your Auto Scaling group has successfully launched or terminated instances.

    If it terminated any unhealthy instances, the Cause column shows the date and time of the termination and the reason for the health check failure. For example, "At 2021-04-01T21:48:35Z an instance was taken out of service in response to EBS volume health check failure".

To view the status of your warm pool instances (Amazon CLI)

View the warm pool for an Auto Scaling group by using the following describe-warm-pool command.

aws autoscaling describe-warm-pool --auto-scaling-group-name my-asg

Example output.

{
    "WarmPoolConfiguration": {
        "MinSize": 0,
        "PoolState": "Stopped"
    },
    "Instances": [
        {
            "InstanceId": "i-0b5e5e7521cfaa46c",
            "InstanceType": "t2.micro",
            "AvailabilityZone": "us-west-2a",
            "LifecycleState": "Warmed:Stopped",
            "HealthStatus": "Healthy",
            "LaunchTemplate": {
                "LaunchTemplateId": "lt-08c4cd42f320d5dcd",
                "LaunchTemplateName": "my-template-for-auto-scaling",
                "Version": "1"
            }
        },
        {
            "InstanceId": "i-0e21af9dcfb7aa6bf",
            "InstanceType": "t2.micro",
            "AvailabilityZone": "us-west-2a",
            "LifecycleState": "Warmed:Stopped",
            "HealthStatus": "Healthy",
            "LaunchTemplate": {
                "LaunchTemplateId": "lt-08c4cd42f320d5dcd",
                "LaunchTemplateName": "my-template-for-auto-scaling",
                "Version": "1"
            }
        }
    ]
}
To view the reason for health check failures (Amazon CLI)

Use the following describe-scaling-activities command. 

aws autoscaling describe-scaling-activities --auto-scaling-group-name my-asg

The following is an example response, where Description indicates that your Auto Scaling group has terminated an instance and Cause indicates the reason for the health check failure.

Scaling activities are ordered by start time. Activities still in progress are described first. 

{
  "Activities": [
    {
      "ActivityId": "4c65e23d-a35a-4e7d-b6e4-2eaa8753dc12",
      "AutoScalingGroupName": "my-asg",
      "Description": "Terminating EC2 instance: i-04925c838b6438f14",
      "Cause": "At 2021-04-01T21:48:35Z an instance was taken out of service in response to EBS volume health check failure.",
      "StartTime": "2021-04-01T21:48:35.859Z",
      "EndTime": "2021-04-01T21:49:18Z",
      "StatusCode": "Successful",
      "Progress": 100,
      "Details": "{\"Subnet ID\":\"subnet-5ea0c127\",\"Availability Zone\":\"us-west-2a\"...}",
      "AutoScalingGroupARN": "arn:aws-cn:autoscaling:us-west-2:123456789012:autoScalingGroup:283179a2-f3ce-423d-93f6-66bb518232f7:autoScalingGroupName/my-asg"
    },
...
  ]
}