How to specific elastic instance as reserved instance

When it comes to Reserved Instances (RIs) for AWS OpenSearch (formerly Elasticsearch) Service, there isn't a way to apply RIs to specific instances based on a unique identifier like ARN or instance ID. The Reserved Instance pricing is automatically applied based on the instance type, region, and quantity you purchase.

Here's how it works:

1. You purchase Reserved Instances for a specific instance type and region.

2. AWS automatically applies the RI pricing to any matching instances in your account.

3. The system doesn't distinguish between specific OpenSearch domains or instances when applying the discount.

If you have multiple OpenSearch instances of the same type in the same region, the RI discount will be applied to the matching number of instances you've reserved, but you can't control which specific instances receive the discount.

To purchase Reserved Instances for OpenSearch, you can use the AWS SDK or CLI. The process involves describing the available RI offerings, selecting the appropriate offering, and then purchasing it. You'll need to specify the instance type, the number of instances you want to reserve, and optionally, a reservation name.

Remember, while you can't apply RIs to specific instances, you can still benefit from the cost savings as long as you have the appropriate number and type of instances running in the specified region. If you're looking to optimize costs for specific OpenSearch domains, you might need to consider other strategies in addition to RIs, such as rightsizing instances or using on-demand capacity for less predictable workloads.
Sources
Purchasing Reserved Instances (AWS SDKs) - Amazon OpenSearch Service
Amazon EC2 examples using AWS CLI - AWS SDK Code Examples