Gain expert insights into AWS-SysOps with a detailed analysis of Auto Scaling behavior in response to Cloudwatch Alarms, and learn how instance counts are managed during cool down periods.
Table of Contents
Question
An Auto Scaling group is running at the desired capacity of 5 instances and receives a trigger from the Cloudwatch Alarm to increase the capacity by 1. The cool down period is 5 minutes.
Cloudwatch sends another trigger after 2 minutes to decrease the desired capacity by 1. What will be the count of instances at the end of 4 minutes?
A. 7
B. 6
C. 4
D. 5
Answer
B. 6
Explanation
The cool down period is the time difference between the end of one scaling activity (can be start or terminate) and the start of another one (can be start or terminate). During the cool down period, Auto Scaling does not allow the desired capacity of the Auto Scaling group to be changed by any other CloudWatch alarm. Thus, in this case the trigger from the second alarm will have no effect.
When the Auto Scaling group receives the first trigger to increase capacity, it adds 1 instance, making the total count 6. The cool down period is 5 minutes, which means no scaling activities will occur until this period ends. Even though a second trigger to decrease capacity is received after 2 minutes, it will not be acted upon until the cool down period has elapsed. Therefore, at the end of 4 minutes, the instance count remains at 6.
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