Reason EC2 Instances Not Counted Towards Auto Scaling Group Metrics

Correct Answer: A.

The AWS Documentation mentions the following.

With step scaling policies, you can specify the number of seconds that it takes for a newly launched instance to warm up.

Until its specified warm-up time has expired, an instance is not counted toward the Auto Scaling group's aggregated metrics.

While scaling out, AWS also does not consider instances warming up as part of the group's current capacity.

Therefore, multiple alarm breaches that fall in the range of the same step adjustment result in a single scaling activity.

This ensures that we don't add more instances than you need.

Since this is clearly given in the AWS Documentation, all other options are invalid.

For more information on step scaling policy, please refer to the below URL-

https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-scaling-simple-step.html#as-step-scaling-warmup

The best possible reason for the newly launched EC2 instances not being counted towards the Auto Scaling group's aggregated metrics could be because the warm-up period has not expired.

Explanation:

When an EC2 instance is launched, it takes some time to become fully operational. During this time, the instance is said to be in the warm-up period. The warm-up period is the time taken by the instance to become fully operational and to start accepting traffic.

During the warm-up period, the Auto Scaling group does not consider the newly launched instances for scaling purposes. This is because the instances are not yet fully operational and may not be able to handle the traffic.

The warm-up period can be set using the cooldown period option in the Auto Scaling group's configuration. The cooldown period is the time to wait after a scaling activity before another scaling activity can start. During this time, the Auto Scaling group waits for the instances to become fully operational and to stabilize.

Therefore, the best possible reason for the newly launched EC2 instances not being counted towards the Auto Scaling group's aggregated metrics could be because the warm-up period has not expired. The other options listed, Instances have not completed their boot sequence, Instances have not been attached to the Autoscaling Group, and wrong launch configuration used, could also cause issues but they are not the most likely reasons in this scenario.