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

View the reason for health check failures

Using the following procedure, you can view information about any instances replaced due to a health check.

By default, Amazon EC2 Auto Scaling creates a new scaling activity for terminating the unhealthy instance and then terminates it. While the instance is terminating, another scaling activity launches a new instance. You can change this behavior to start launching a new instance as soon as possible by using an instance maintenance policy. For more information, see Instance maintenance policies.

Console
Viewing the reason for health check failures

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/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 2022-05-14T20:11:53Z an instance was taken out of service in response to a user health-check. This message indicates that a custom health check marked the instance unhealthy.

    For help with health check failures, see Troubleshoot unhealthy instances.

AWS CLI
Viewing the reason for health check failures

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 Cause contains the reason for the health check failure.

{
  "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 a user health-check.",
      "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:autoscaling:us-west-2:123456789012:autoScalingGroup:283179a2-f3ce-423d-93f6-66bb518232f7:autoScalingGroupName/my-asg"
    },
...
  ]
}

For a description of the fields in the output, see Activity in the Amazon EC2 Auto Scaling API Reference.

To describe the scaling activities after the Auto Scaling group has been deleted, add the --include-deleted-groups option to the describe-scaling-activities command.