AWS Auto Scaling with reserved intance

Question

I have a small query on AWS Auto Scaling.

For auto scaling group, we need to set minimum (1 server) and maximum no of instances to scale.

Question:

Lets assume, I already have a reserved instance running 24x7.

I will create an AMI of the reserved instance and use this AMI for auto scaling.

I want to make this reserved instance as part of auto scaling group (this become my minimum 1 server in the auto scaling group).

But I don't want this reserved instance to terminate at all (as I have my elastic IP taken for this) when I scale down, but other instances can terminate as the load comes down.

How can I achieve this?

Kindly suggest. Thanks in advance.

Answer 1

Instances reservations are not tied to a specific EC2 instance. As long as you have an instance running that matches your reservation, you will be charged the reservation's hourly rate.

Answer 2

This reserved instance should not be in the auto-scaling group. You only want it to be in the instances used under the Elastic Load Balancer. The auto-scaling group should only contain instances that are dynamic.

You can set this instance under the load balancer and it will never be terminated.

Remember to set the auto-scaling group minimum to zero, so when the load is low on the reserved instance, the auto-scaling group would call the decrease instance policy, and you would lessen the charges.

Answer 3

The concept of a Reserved Instance is always confusing.

A Reserved Instance is a pre-payment for a certain capacity (Instance Type, OS, optional AZ). For example, let's say you purchase a 1-year Reserved Instance for an m4.large Linux instance. This means that, for every hour of the year, you can run an m4.large Linux instance at no hourly charge, because you have pre-paid it annually or monthly.

Please note that you do not choose which instance receives this billing benefit. Rather, each hour of the year, if an instance is running that matches the Reserved Instance purchased, it is not charged for that hour.

Therefore, you can't really say things like "I want to make this reserved instance as part of auto scaling group" or "create an AMI of the reserved instance" because you have no knowledge nor control over which instance receives the billing benefit. Simply be happy in the knowledge that an instance running that matches the Reserved Instance will receive the benefit.

So, if you have one Reserved Instance and you are running at least one EC2 instance of the matching Instance Type and OS in a given hour, then one of those instances will not receive an hourly charge. It doesn't have to be a specific instance that you nominate.

Side-note: Stopping and Starting an instance triggers a new billing hour. Only one hour is not charged each hour per Reserved Instance purchased. So, if Auto Scaling launching an instance, terminates it, then launches another within the same hour, there will be a charge. Only the first billable hour per Reserved Instance owned will be "not charged".

(I recall seeing something that said that the Reserved Instance benefit is typically applied to the instance with the earliest Launch Time and that if it is stopped/terminated, the benefit goes to the instance with the next earliest Launch Time -- but that might not be accurate.)