Demystifying The New AWS Regional Benefit Scope: Making EC2 RIs AZ-Agnostic

If you’re an AWS user who counts on Reserved Instances (RIs) for discounts, you’ll know the importance of Availability Zone (AZ) for sure. As you may recall, earlier, each RI had to be bought for a specific AZ of a Region, making it inflexible to reap discounts when the AZ changed. Now with AWS making the new announcement of AWS Regional Benefit Scope, RIs will be AZ-Agnostic — provided you either purchase an RI with Region scope selected or change the current RI’s scope from AZ to Region. In short, the new Regional Benefit will allow you to run RIs in any AZ within a Region with the RI discount applied automatically.

In one of our previous blogs, AWS EC2 RIs are now AZ Agnostic, Convertible RIs are now a Reality, we’d provide the gist of this new AWS feature. Essentially, this new announcement caters to one of the major limitations every AWS user encountered — each reservation tied to a specific AZ of a Region. Let’s walk you through why the new ‘Region’ scope was introduced and how it provides you the capacity reservation benefits irrespective of AZ (however, of the same Region).

Decoding the AWS AZ concept to appreciate the new Region Scope benefits:

The AWS, always, isolates its services into Regions and AZs to ensure an instance’s availability at any given time. Regions, if you are not aware, are nothing but geographic areas, like US West (us-west) or Asia Pacific — Sydney (ap-southeast). Inside each Region, there are multiple AZs, which are nothing but physical datacenters.

Earlier, when you launched an EC2 instance, you had to do so by specifying the AZ. And, when you had to buy an RI, you had to explicitly select the same AZ to get the discounted hourly rate to your active instance.

For example, buying a reservation for use in ‘us-east-1b’ means that you could only see the cost benefits of that reservation in ‘us-east-1b.’

However, more often as you may have experienced, you had to launch instances in multiple AZs for several reasons like load balancing, for redundancy, etc. For instance, in ‘us-east-1a’ instead of ‘us-east-1b.’ And to reap the cost-savings, you had to either purchase a new reservation, manually modify the existing reservation, or use cloud management platform like Botmetric to automatically modify on your behalf.

But now, with the new Regional Benefit feature, RIs are AZ-Agnostic. In essence, all the existing RIs are automatically scoped to an AZ by default and also apply the discounted hourly rate to any instance within a specified region. So, if you purchase a new RI scoped to a Region or modify an existing RI to have the Regional Scope, the cost benefits of this RI will apply to any usage of this instance type within a Region.

For example: Assume you have purchased an ‘All Upfront instance for an m4.xlarge running Windows for us-west-1b with shared tenancy. And your team regularly runs an instance of this type in us-west-1. Hence, it is regularly moved between AZs based on the needs of the application. To reap the discounts for the hours in which an instance of this type was not running in us-west-1a, you will have to modify the RI based on the change in usage. However, with this RI scoped to a Region, you can get the discounts regardless of the AZ in which you run an instance of this type.

How to go about it:

The new Scope feature is available in the AWS console along with the other attributes associated with each instance.

Modify Reserved Instances

What to watch out for:

An RI with Regional Benefit enabled does not have a capacity guarantee. Yes! You heard it right. You can modify the reservations from Availability Zone scope to a Region scope if you don’t need the capacity guarantee accompanying your RIs.

And, if you rely on the capacity guarantee for some or all of your usage, you will need to switch reservations to Availability Zone scope. Note that this capacity guarantee only applies if your instance usage is in the account where the RI was purchased.

The Bottom Line:

With the new Region Scope, you can now run your instance in any AZ in the Region, and experience your RI discounts automatically falling into your pocket. Let us know what you think about this new scope.

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