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Difficulty: HardAuto Scaling and Elasticity

A SysOps administrator manages an Amazon EC2 Auto Scaling group (ASG) that hosts a batch processing application. The ASG is configured with a simple scaling policy that launches 22 additional EC2 instances when average CPU utilization exceeds 80%80\% for a single 55-minute period. The policy has a cooldown period of 300300 seconds. During a high-load event, the average CPU utilization reaches 90%90\% and remains constant for 2525 minutes. The ASG successfully launches the first 22 additional instances, but no further scaling actions occur despite the CPU utilization remaining at 90%90\% and the processing queue growing. Which action should the administrator take to ensure the ASG continues to scale out as long as the CPU utilization remains above the threshold?

  1. A
    Decrease the scaling policy's cooldown period to 6060 seconds.
  2. B
    Enable detailed monitoring in the launch template and update the CloudWatch alarm to evaluate the CPU utilization metric at 11-minute intervals.
  3. Replace the simple scaling policy with a step scaling policy.Answer
  4. D
    Configure an Amazon EventBridge rule that detects when the Auto Scaling policy execution status is successful, and use it to invoke an AWS Systems Manager Automation document that resets the alarm state.

Answer

Replace the simple scaling policy with a step scaling policy.
Replacing the simple scaling policy with a step scaling policy is the correct solution. Simple scaling policies are only triggered when the CloudWatch alarm changes its state (e.g., from OK to ALARM). If the metric remains continuously above the threshold, the alarm stays in the ALARM state and does not transition again, preventing further scaling actions. In contrast, step scaling policies can continuously evaluate metrics and perform scaling actions as long as the alarm is in the ALARM state and the instance warm-up periods have expired.

Step-by-Step Solution

1
Analyze the current Auto Scaling configuration and the observed behavior.
The ASG uses a simple scaling policy, which launches 22 instances initially but fails to scale out further when CPU utilization stays high at 90%90\%.
Identify why simple scaling policies fail to trigger subsequent scaling activities when metrics remain elevated.
2
Understand the difference between simple scaling and step scaling trigger mechanisms.
Simple scaling policies are only executed when the CloudWatch alarm transitions into the ALARM state. Step scaling policies can execute multiple scaling actions as long as the metric remains in the ALARM state and the instance warm-up periods are respected.
Select the scaling policy type that supports continuous scaling under sustained load.
3
Select the correct action to modify the Auto Scaling group policy.
Replacing the simple scaling policy with a step scaling policy ensures that the group continues to scale out as long as CPU utilization remains above 80%80\%.
Apply the correct AWS operational practice to resolve the scaling limitation.

Key Concept

Auto Scaling Simple vs. Step Scaling Policies
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Topics

Auto Scaling and Elasticity

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