AWS Certified Solutions Architect – Professional — Question 816

A Solutions Architect has created an AWS CloudFormation template for a three-tier application that contains an Auto Scaling group of Amazon EC2 instances running a custom AMI.
The Solutions Architect wants to ensure that future updates to the custom AMI can be deployed to a running stack by first updating the template to refer to the new
AMI, and then invoking UpdateStack to replace the EC2 instances with instances launched from the new AMI.
How can updates to the AMI be deployed to meet these requirements?

Answer options

• A. Create a change set for a new version of the template, view the changes to the running EC2 instances to ensure that the AMI is correctly updated, and then execute the change set.
• B. Edit the AWS::AutoScaling::LaunchConfiguration resource in the template, changing its DeletionPolicy to Replace.
• C. Edit the AWS::AutoScaling::AutoScalingGroup resource in the template, inserting an UpdatePolicy attribute.
• D. Create a new stack from the updated template. Once it is successfully deployed, modify the DNS records to point to the new stack and delete the old stack.

Correct answer: C

Explanation

To replace EC2 instances in an Auto Scaling group during a CloudFormation stack update, an UpdatePolicy attribute (such as AutoScalingRollingUpdate) must be defined on the AWS::AutoScaling::AutoScalingGroup resource. Without this attribute, CloudFormation will update the launch configuration but will not automatically cycle the existing instances to use the new AMI. Changing the DeletionPolicy or executing a standard change set without an UpdatePolicy will not trigger the required rolling replacement of the running instances.