RDS cluster instance group parameter name has no drift detection

[jar-b]

Description

The DBInstanceParameterGroupName field only appears in ModifyDBCluster, but not CreateDBCluster or DescribeDBClusters.

The current APIs pose potential problems for Terraform users:

1. This value can't be set on create. If provided in a Terraform configuration, users would likely assume that the cluster is created with the designated instance parameter group name. In reality, this only gets set if the resource is subsequently modified.
2. This value can't be read once set. DescribeDBClusters does not include DBInstanceParameterGroupName in the response, so there is no way to detect when changes are made outside of Terraform (ie. via the AWS CLI or console). This prevents customers from verifying their infrastructure is continuously configured as intended.

I've opened a support case with AWS for confirmation that this field is intended to be sent during modification of an existing cluster only. Ideally these fields would be exposed in the Create input and Describe output structs for consistency, providing the ability to properly detect drift. However, given the current state we should send the db_instance_parameter_group_name attribute:

• Any time the value changes (existing behavior)
• Any time it is set in configuration (new behavior)

The new behavior prevents cases where the remote value has changed and the inability to read the existing value causes this parameter to be (incorrectly) omitted. Drift still cannot be detected on this attribute, but by always sending the value when set we at least ensure that the desired configuration is sent every time a modification is made.

References

• aws_rds_cluster
• ModifyDBCluster
• CreateDBCluster
• DescribeDBClusters

Would you like to implement a fix?

None

[github-actions]

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[github-actions]

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