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Difficulty: MediumAuto Scaling and Elasticity

An Amazon EC2 Auto Scaling group (ASG) is registered with an Application Load Balancer (ALB). The ASG has a minimum size of 3, a desired capacity of 3, and a maximum size of 5. To troubleshoot a database connectivity issue, a SysOps administrator temporarily suspends the Launch process for the ASG. Shortly after, one of the three EC2 instances fails its ALB health check and is marked unhealthy.

What will happen to the unhealthy instance and the capacity of the Auto Scaling group?

  1. A
    The Auto Scaling group will leave the unhealthy instance running and take no action until the Launch process is resumed.
  2. The Auto Scaling group will terminate the unhealthy instance, leaving the group with 2 running instances, and will not launch a replacement.Answer
  3. C
    The Auto Scaling group will automatically resume the Launch process to maintain the minimum capacity of 3 and then replace the instance.
  4. D
    The Auto Scaling group will wait for the scale-in cooldown period to expire before terminating the instance and launching a replacement.

Answer

The Auto Scaling group will terminate the unhealthy instance, leaving the group with 2 running instances, and will not launch a replacement.
The correct answer is that the Auto Scaling group will terminate the unhealthy instance, leaving the group with 2 running instances, and will not launch a replacement. When the Launch process is suspended, the ReplaceUnhealthy process is still allowed to terminate instances that fail health checks, but it cannot launch new ones to replace them. This can temporarily reduce the capacity of the Auto Scaling group below its minimum size limit.

Step-by-Step Solution

1
Identify the active and suspended processes in the Auto Scaling group.
The Launch process is suspended, but other processes like Terminate and ReplaceUnhealthy remain active.
Suspending a specific process only pauses actions associated with that process; other processes continue to run normally unless they explicitly depend on the suspended process.
2
Determine how the ReplaceUnhealthy process behaves when an instance fails health checks.
The ReplaceUnhealthy process will proceed to terminate the unhealthy instance.
The termination phase of health check replacement does not require the Launch process to be active.
3
Determine how the suspended Launch process affects the replacement step.
No replacement instance is launched, resulting in a net loss of one instance.
Launching a replacement instance requires the Launch process. Because it is suspended, the Auto Scaling group cannot launch a new instance, even though the group size falls below the minimum limit of 3.

Key Concept

Auto Scaling Process Suspension Behavior
Estimated Time:1m 30s
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Topics

Auto Scaling and Elasticity

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