Which solution will meet these requirements?
Modify the CloudFormation template to include an UpdatePolicy attribute for the Auto Scaling group. Include the AutoScalingRollingUpdate policy with MinInstancesInService of 4 and MaxBatchSize of 2. Whenever a software update is needed, alter either or both of the ImageId and UserData of the AWS::EC2::LaunchTemplate and update the stack.
Set the Auto Scaling group’s minimum capacity to 4. Create an AWS CodeDeploy deployment group that has an in-place deployment type. Select Amazon EC2 Auto Scaling group for the environment configuration. Whenever a new revision is available, create a new CodeDeploy deployment that has a deployment configuration of CodeDeployDefault HalfAtATime.
Set the Auto Scaling group’s minimum capacity to 4. Create an AWS CodeDeploy deployment group that has a blue/green deployment type. Select Amazon EC2 Auto Scaling group for the environment configuration. Whenever a new revision is available, create a new CodeDeploy deployment that has a deployment configuration of CodeDeployDefault HalfAtATime.
Modify the CloudFormation template to include a StackPolicy. Designate an AutoScalingReplacingUpdate policy to control the update. Specify MinInstancesInService of 4 and MaxBatchSize of 2. Whenever a software update is needed, alter either or both of the ImageId and UserData of the AWS::EC2::LaunchTemplate and update the stack.
Explanations:
This solution uses the AutoScalingRollingUpdate policy with MinInstancesInService set to 4 and MaxBatchSize set to 2, which meets the requirement to replace two EC2 instances at a time while ensuring a minimum of four EC2 instances are running during the update. Updating the ImageId and/or UserData in the AWS::EC2::LaunchTemplate triggers the update in the stack.
This option uses an in-place deployment in CodeDeploy, which is not suitable for treating infrastructure as immutable. In-place deployments update existing instances rather than replacing them, violating the requirement to replace EC2 instances.
While a blue/green deployment approach with CodeDeploy allows for replacing instances, it doesn’t align with the requirement to replace two instances at a time in an Auto Scaling group. It also introduces a new EC2 instance environment rather than updating existing ones.
The StackPolicy and AutoScalingReplacingUpdate policy are not appropriate for managing EC2 instance replacement in this context. The policy does not directly control instance replacement in the way required by the scenario. The StackPolicy is more focused on resource protection during updates rather than managing the update process itself.
I systematize that the answer is:
Modify the CloudFormation template to include an UpdatePolicy attribute for the Auto Scaling group. Include the AutoScalingRollingUpdate policy with MinInstancesInService of 4 and MaxBatchSize of 2. Whenever a software update is needed, alter either or both of the ImageId and UserData of the AWS::EC2::LaunchTemplate and update the stack.