Documentation about aws_dms_replication_instance seems wrong regarding replication_subnet_group_id

[Imanuel-Mizrahi]

Inside the docs of aws_dms_replication_instance, there seems to be a mistake.
For the parameter: "replication_subnet_group_id", the value is: aws_dms_replication_subnet_group.test.id.
However, inside the docs for "aws_dms_replication_subnet_group", there is no 'id' field at all!
I guess that the value should be: "replication_subnet_group_id" instead, as this one is a required field for aws_dms_replication_subnet_group.

[anGie44]

Hi @Imanuel-Mizrahi , thank you raising this issue. While the aws_dms_replication_instance resource documentation is correct to reference the aws_dms_replication_subnet_group.test.id, you've raised a good point in that the aws_dms_replication_subnet_group documentation is missing details on the id attribute, which happens to be the replication_subnet_group_id as seen here:

terraform-provider-aws/aws/resource_aws_dms_replication_subnet_group.go

Line 72 in 078a334

d.SetId(d.Get("replication_subnet_group_id").(string))

Adding a row with the id attribute in the documentation should fix this 👍

[Imanuel-Mizrahi]

Thank you @anGie44 !!
So, now I come to the part how it all began...
I was using this TF template, based on the docs:

resource "aws_dms_replication_subnet_group" "test" {
  replication_subnet_group_description = "Test replication subnet group"
  replication_subnet_group_id          = "test-dms-replication-subnet-group-tf"

  subnet_ids = [
    aws_subnet.nondefault_1.id,
    aws_subnet.nondefault_2.id
  ]

  tags = {
    Name = "test"
  }
}

resource "aws_dms_replication_instance" "test" {
  allocated_storage            = 20
  apply_immediately            = true
  auto_minor_version_upgrade   = true
  multi_az                     = true
  preferred_maintenance_window = "sun:10:30-sun:14:30"
  publicly_accessible          = true
  replication_instance_class   = "dms.t2.micro"
  replication_instance_id      = "test-dms-replication-instance-tf"
  replication_subnet_group_id  = aws_dms_replication_subnet_group.test.id

  tags = {
    Name = "test"
  }

I was sure that the "terraform plan" will import the replication_subnet_group_id from the "aws_dms_replication_subnet_group" resource, so the "aws_dms_replication_instance" resource will show it as the actual name ("test-dms-replication-subnet-group-tf"), once I run the plan.
However, the result was:

Terraform will perform the following actions:

  # aws_dms_replication_instance.test will be created
  + resource "aws_dms_replication_instance" "test" {
      + allocated_storage                = 20
      + apply_immediately                = true
      + auto_minor_version_upgrade       = true
      + availability_zone                = (known after apply)
      + engine_version                   = (known after apply)
      + id                               = (known after apply)
      + kms_key_arn                      = (known after apply)
      + multi_az                         = true
      + preferred_maintenance_window     = "sun:10:30-sun:14:30"
      + publicly_accessible              = true
      + replication_instance_arn         = (known after apply)
      + replication_instance_class       = "dms.t2.micro"
      + replication_instance_id          = "test-dms-replication-instance-tf"
      + replication_instance_private_ips = (known after apply)
      + replication_instance_public_ips  = (known after apply)
      + replication_subnet_group_id      = (known after apply)
      + tags                             = {
          + "Name" = "test"
        }
      + vpc_security_group_ids           = (known after apply)
    }

Note that replication_subnet_group_id will only be known after apply.

Now, if I change the TF to use the replication_subnet_group_id from resource "aws_dms_replication_subnet_group", as follows:

resource "aws_dms_replication_instance" "test" {
  allocated_storage            = 20
  apply_immediately            = true
  auto_minor_version_upgrade   = true
  multi_az                     = true
  preferred_maintenance_window = "sun:10:30-sun:14:30"
  publicly_accessible          = true
  replication_instance_class   = "dms.t2.micro"
  replication_instance_id      = "test-dms-replication-instance-tf"
  replication_subnet_group_id  = aws_dms_replication_subnet_group.test.replication_subnet_group_id

  tags = {
    Name = "test"
  }

}

The plan result is as expected, and the data is imported:

Terraform will perform the following actions:

  # aws_dms_replication_instance.test will be created
  + resource "aws_dms_replication_instance" "test" {
      + allocated_storage                = 20
      + apply_immediately                = true
      + auto_minor_version_upgrade       = true
      + availability_zone                = (known after apply)
      + engine_version                   = (known after apply)
      + id                               = (known after apply)
      + kms_key_arn                      = (known after apply)
      + multi_az                         = true
      + preferred_maintenance_window     = "sun:10:30-sun:14:30"
      + publicly_accessible              = true
      + replication_instance_arn         = (known after apply)
      + replication_instance_class       = "dms.t2.micro"
      + replication_instance_id          = "test-dms-replication-instance-tf"
      + replication_instance_private_ips = (known after apply)
      + replication_instance_public_ips  = (known after apply)
      + replication_subnet_group_id      = "test-dms-replication-subnet-group-tf"
      + tags                             = {
          + "Name" = "test"
        }
      + vpc_security_group_ids           = (known after apply)
    }

I am pretty new with github, should I create a new issue for this...?

Appreciate your assistance @anGie44 !!

[Imanuel-Mizrahi]

Turns out this behavior changes, depends on the terraform version you use.
So, using v0.12.30 the plan include the information correctly.
I think that initially, I was running with a higher version while experiencing the issue with the plan version 14 I think.