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Coping with Accounts in Constrained Availability Zones

The Reserved Instance Planner (RI Planner) can make recommendations as to which RIs you should buy, modify and sell, based on your usage in a specified period of time (see help article on RI Planner). An organization would typically have many Amazon Web Services (AWS) accounts, all linked together for billing purposes. While you could buy an RI from any account you own, any modifications to an existing RI would need to be done by the account that purchased that RI.

AWS provides its services across multiple regions around the globe, ensuring your application could be supported quickly anywhere in the world. Within each region, AWS has numerous data centers, physical warehouses hosting networked computers. When purchasing an RI, you would also specify an availability zone, which maps to one or more datacenters within a region. As more and more applications are launched in a specific availability zone, a point in time might come when AWS would restrict you from launching additional capacity in that zone. This is known as a constrained availability zone, one in which you are unable to buy additional capacity in. To learn more about Amazon’s regions and availability zones, refer to the following document.

Since availability zones are logical mappings to data centers, you may have two accounts with applications running in the same availability zone, yet they do not correspond to the same physical data center. Or, using the table, two different accounts could have applications running in us-west-1a and us-west-2, yet they both correspond to the same data center, namely data center #212. This logically mapping allows AWS to distribute workloads across their data centers, even when customers choose to run their applications in the same availability zone. 

Continuing from the example above, the same availability zone might be constrained for one of your accounts yet unconstrained for another.  

Using the table below, Account 1 is constrained in us-west-1a, yet Account 2 is unconstrained in the same availability zone.

How do constrained availability zones affect the RI Planner?

  • Buying recommendations are not affected by a constrained availability zone, as you can have an account that is not constrained in any given availability zone.
  • Modifications are affected, as they need to remain in the same account that purchased them. As such, the RI Planner should not make modification suggestions that increase capacity.

If you do have accounts that are constrained in certain availability zones, contact our Customer Success team by sending them a CSV file, listing all accounts and whether or not they are constrained or unconstrained in a specific availability zone (see sample CSV below).Note accounts constrained in multiple availability zones should appear in multiple lines, that is to say one line in the CSV file would pertain to an account and an availability zone. Once Customer Success receives the CSV file, we will update the app to refrain from making modified recommendations that lead to increased capacity for an account in a constrained availability zone.

Alternatively, you could share a CSV file, listing only those accounts that are constrained in an availability zone.  Note, once an account has been marked as constrained in a particular availability zone, Cloudability will treat it as constrained up until Customer Success receives another CSV indicating otherwise (see table above).

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