AWS EC2 Auto Scaling Groups: I get Min and Max, but what's Desired instances limit for?

Question

When you setup an Auto Scaling groups in AWS EC2 Min and Max bounds seem to make sense:

• The minimum number of instances to scale down to based on policies
• The maximum number of instances to scale up to based on policies

However, I've never been able to wrap my head around what the heck Desired is intended to affect.

I've always just set Desired equal to Min, because generally, I want to pay Amazon the minimum tithe possible, and unless you need an instance to handle load it should be at the Min number of instances.

I know if you use ElasticBeanstalk and set a Min to 1 and Max to 2 it sets a Desired to 2 (of course!)--you can't choose a value for Desired.

What would be the use case for a different Desired number of instances and how does it differ? When you expect AWS to scale lower than your Desired if desired is larger than Min?

Answer 1

Min: This is the minimum number of instances that have to be there in your Autoscaling Group at all times. Your autoscaling group always maintains this number and never terminates instances below this number.

Max: This is the maximum number of instances that your autoscaling group can have. Your autoscaling will never increase the number of instances more than the specified Max number.

Desired: The desired amount represents the "current amount" of instances in your autoscaling group. An autoscaling group will start by launching as many instances as specified as the desired capacity. When scaling policies are set, the desired capacity is adjusted between the minimum and maximum amount.

Desired capacity should be set greater than or equal to the min value and less than or equal to max value.

Learn more about Amazon Elastic Compute Cloud on AWS EC2.