What “desired instances” is needed for? AWS Amazon Webservices AutoScaling group

Question

I am facing a strange situation on Amazon AWS. I don´t understand for what the desired Instances number is used for? I have an autoscaling group that contains scale up and scale down actions configured.

I have a custom PHP file that run actions Scale up and Scale down depending on some external factors. I want to know which number I have to write in desired instances to not affect my autoscaling actions.

For example:

1. I set desired to 2
2. I have 2 instances running
3. I run Scale Down action
4. Instances is 1
5. Autoscaling group will automatically start another instance, so my scale down is not useful because I ended by having 2 running

What can I do?

Many thanks!

Answer 1

The Desired amount represents the "current amount at any time" of the instances in your autoscaling group. Your autoscaling group will start by launching as many instances as the specified desired capacity and then this desired capacity is adjusted between the minimum and maximum amount.