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

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