SOA-C02: How will number of EC2 instances in Auto Scaling group be affected by RequestCountPerTarget metric

Question

ASysOps administrator configures an application to run on Amazon EC2 instances behind an Application Load Balancer (ALB) in a simple scaling Auto Scaling group with the default settings. The Auto Scaling group is configured to use the RequestCountPerTarget metric for scaling. The SysOps administrator notices that the RequestCountPerTarget metric exceeded the specified limit twice in 180 seconds. How will the number of EC2 instances in this Auto Scaling group be affected in this scenario?

A. The Auto Scaling group will launch an additional EC2 instance every time the RequestCountPerTarget metric exceeds the predefined limit.
B. The Auto Scaling group will launch one EC2 instance and will wait for the default cooldown period before launching another instance.
C. The Auto Scaling group will send an alert to the ALB to rebalance the traffic and not add new EC2 instances until the load is normalized.
D. The Auto Scaling group will try to distribute the traffic among all EC2 instances before launching another instance.

Answer

B. The Auto Scaling group will launch one EC2 instance and will wait for the default cooldown period before launching another instance.

Explanation

In this scenario, the number of EC2 instances in the Auto Scaling group will be affected as follows:

Option B: The Auto Scaling group will launch one EC2 instance and will wait for the default cooldown period before launching another instance.

Here’s a detailed explanation of why option B is the correct choice:

• When an Auto Scaling group is configured to use the RequestCountPerTarget metric for scaling, it monitors the number of requests per target (in this case, the EC2 instances behind the ALB).
• The metric value exceeded the specified limit twice in 180 seconds, indicating increased traffic or load on the application.
• By default, the Auto Scaling group responds to this increased load by launching one EC2 instance to handle the additional traffic.
• After launching an instance, the Auto Scaling group enters into a cooldown period, which is the time interval during which any further scaling actions are suspended. The default cooldown period is 300 seconds.
• During the cooldown period, the Auto Scaling group waits to ensure that the newly launched instance becomes fully operational and can handle the load effectively.
• Once the cooldown period is over, if the RequestCountPerTarget metric continues to exceed the limit, the Auto Scaling group may launch additional instances based on the scaling policy and configuration.
• However, in this scenario, since the RequestCountPerTarget metric exceeded the limit twice in 180 seconds, the Auto Scaling group will launch only one EC2 instance and wait for the default cooldown period before launching another instance.

Option A: The Auto Scaling group will launch an additional EC2 instance every time the RequestCountPerTarget metric exceeds the predefined limit.

This option is not accurate because the Auto Scaling group does not launch additional instances every time the RequestCountPerTarget metric exceeds the limit. It launches one instance and waits for the cooldown period.

Option C: The Auto Scaling group will send an alert to the ALB to rebalance the traffic and not add new EC2 instances until the load is normalized.

This option is not accurate because the Auto Scaling group itself is responsible for scaling the number of EC2 instances based on the metric. It does not send alerts to the ALB to rebalance traffic. The ALB itself is designed to distribute traffic among instances.

Option D: The Auto Scaling group will try to distribute the traffic among all EC2 instances before launching another instance.

This option is not accurate because the Auto Scaling group does not distribute traffic among instances before launching another instance. It launches one instance based on the scaling policy and configuration.

In summary, in this scenario, the Auto Scaling group will launch one EC2 instance and wait for the default cooldown period before considering any further scaling actions.

Reference

• Step and simple scaling policies for Amazon EC2 Auto Scaling – Amazon EC2 Auto Scaling
• Target tracking scaling policies for Amazon EC2 Auto Scaling – Amazon EC2 Auto Scaling
• amazon web services – AWS AutoScaling – how does the “Request count per target” measurement works? – Stack Overflow
• amazon web services – AWS Auto Scaling Group – Application Load Balancer Request Count Per Target – Stack Overflow

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