AWS EC2 Auto Scaling Groups: I get Min and Max, but what's Desired instances limit for?
Here are the explanations for the "min, desired and max" values from AWS support:
MIN: This will be the minimum number of instances that can run in your auto scale group. If your scale down CloudWatch alarm is triggered, your auto scale group will never terminate instances below this number
DESIRED: If you trip a CloudWatch alarm for a scale up event, then it will notify the auto scaler to change it's desired to a specified higher amount and the auto scaler will start an instance/s to meet that number. If you trip a CloudWatch alarm to scale down, then it will change the auto scaler desired to a specified lower number and the auto scaler will terminate instance/s to get to that number.
MAX: This will be the maximum number of instances that you can run in your auto scale group. If your scale up CloudWatch alarm stays triggered, your auto scale group will never create instances more than the maximum amount specified.
Think about it like a sliding range UI element.
With min and max, you are setting the lower bound of your instance scaling. Withe desired capacity, you are setting what you'd currently like the instance count to hover.
Example: You know your application will have heavy load due to a marketing email or product launch...simply scale up your desired capacity beforehand:
aws autoscaling set-desired-capacity --auto-scaling-group-name my-auto-scaling-group --desired-capacity 2 --honor-cooldown
Source
When you expect AWS to scale lower than your Desired if desired is larger than Min?
This happens when you set a CloudWatch alarm based on some AutoScaling policy. Whenever that alarm is triggered it will update the DesiredCount to whatever is mentioned in config.
e.g., If an AutoScalingGroup config has Min=1, Desired=3, Max=5 and there is an Alarm set on an AutoScalingPolicy which says if CPU usage is <50% for consecutive 10 mins then Remove 1 instances then it will keep reducing the instance count by 1 whenever the alarm is triggered until the DesiredCount = MinCount.
Lessons Learnt: Set the MinCount to be > 0 or = DesiredCount. This will make sure that the application is not brought down when the mincount=0 and CPU usage goes down.