How to use existing OpenSearch reserved instance while creating an OpenSearch domain in Terraform?

Question

Hi AWS community,
I'm working on creating an OpenSearch domain using Terraform and would like to use an existing reserved instance to save on costs. However, I'm not sure how to specify this in my Terraform script.
Can someone provide guidance on how to use an existing OpenSearch reserved instance when creating a new OpenSearch domain with Terraform?

Answer

Hello,

Once you have purchased a reserved instance, your OpenSearch domain using same instance type will start getting billing discounts without any changes in OpenSearch domain.

Similarly, you don’t have to specifically specify the reserved instance while creation of OpenSearch domain. The billing discount is automatically applied to the instances you have reserved.

Amazon OpenSearch RIs are not flexible; they only apply to the exact instance type that you reserve. For example, a reservation for eight `c5.2xlarge.search` instances does not apply to sixteen `c5.xlarge.search` instances or four `c5.4xlarge.search` instances.

Note: OpenSearch reserved instances are different from EC2 reserved instances and are purchased from AWS OpenSearch console/CLI/SDK. Please refer to below document for reference.

[+] Reserved Instances in Amazon OpenSearch Service - https://docs.aws.amazon.com/opensearch-service/latest/developerguide/ri.html

[+] https://aws.amazon.com/opensearch-service/pricing/#Reserved_Instance_pricing

How to use existing OpenSearch reserved instance while creating an OpenSearch domain in Terraform?

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