Issues with Subnets in my secondary instance - postgresql

Question

Please have some grace because I just started learning AWS. I am trying to launch a PostgreSQL DB from Terraform and I keep running into this subnet error
Error: error creating RDS cluster: DBSubnetGroupNotFoundFault: DB subnet group 'cse-cr' does not exist.
│ status code: 404, request id: 6461e755-8118-41ee-8baf-5678e28c39aa
│
│ with aws_rds_cluster.secondary,
│ on maindb.tf line 67, in resource "aws_rds_cluster" "secondary":
│ 67: resource "aws_rds_cluster" "secondary" {
I tried adding a second resource section specifically for the secondary cluster/instance but that also did not work. What am I doing wrong...?
Here is my code:
provider "aws" {
alias = "primary"
region = "us-east-2"
}
provider "aws" {
alias = "secondary"
region = "us-east-1"
}
resource "aws_rds_global_cluster" "example" {
global_cluster_identifier = "global-test"
engine = "aurora-postgresql"
engine_version = "13.4"
database_name = "example_db"
}
variable "database_name" {
description = "What should RDS name the initial db?"
default = "variableWithSomeDefault"
}
data "aws_subnet" "subnet1" {
id = "subnet-numhere"
}
data "aws_subnet" "subnet2" {
id = "subnet-numhere"
}
resource "aws_db_subnet_group" "default" {
name = "cse-cr"
description = "Private subnets for RDS instance"
subnet_ids = [data.aws_subnet.subnet1.id, data.aws_subnet.subnet2.id]
}
resource "aws_rds_cluster" "primary" {
provider = aws.primary
engine = "aurora-postgresql"
engine_version = "13.4"
cluster_identifier = "test-primary-cluster"
master_username = "username"
master_password = "somepass123"
database_name = var.database_name
global_cluster_identifier = aws_rds_global_cluster.example.global_cluster_identifier
db_subnet_group_name = "cse-cr"
skip_final_snapshot = true
}
resource "aws_rds_cluster_instance" "primary" {
provider = aws.primary
engine = "aurora-postgresql"
engine_version = "13.4"
identifier = "primaryinstancetest"
cluster_identifier = "${aws_rds_cluster.primary.cluster_identifier}"
instance_class = "db.r5.large"
db_subnet_group_name = "cse-cr"
}
resource "aws_rds_cluster" "secondary" {
provider = aws.secondary
engine = "aurora-postgresql"
engine_version = "13.4"
cluster_identifier = "test-secondary-cluster"
global_cluster_identifier = aws_rds_global_cluster.example.global_cluster_identifier
skip_final_snapshot = true
db_subnet_group_name = "cse-cr"
depends_on = [
aws_rds_cluster_instance.primary
]
}
resource "aws_rds_cluster_instance" "secondary" {
provider = aws.secondary
engine = "aurora-postgresql"
engine_version = "13.4"
identifier = "secondaryinstancetest"
cluster_identifier = "${aws_rds_cluster.secondary.cluster_identifier}"
instance_class = "db.r5.large"
db_subnet_group_name = "cse-cr"
}

Answer 1

Terraform tries to create the RDS cluster (aws_rds_cluster.secondary) before the DB subnet group was (aws_db_subnet_group.default) was provisioned. You have to have a dependency between them in order for Terraform to be able to detect the provisioning order.
To solve this, I recommend referencing the aws_db_subnet_group.default.name whenever it asks for the subnet group name. For example:
resource "aws_rds_cluster_instance" "primary" {
provider = aws.primary
engine = "aurora-postgresql"
engine_version = "13.4"
identifier = "primaryinstancetest"
cluster_identifier = aws_rds_cluster.primary.cluster_identifier
instance_class = "db.r5.large"
db_subnet_group_name = aws_db_subnet_group.default.name # there is no need to hardcode "cse-cr" here
}
Or another example in the secondary cluster:
resource "aws_rds_cluster_instance" "secondary" {
provider = aws.primary
engine = "aurora-postgresql"
engine_version = "13.4"
identifier = "secondaryinstancetest"
cluster_identifier = aws_rds_cluster.secondary.cluster_identifier
instance_class = "db.r5.large"
db_subnet_group_name = aws_rds_cluster.primary.cluster_identifier
}
Please note, I removed the secondary provider from the second cluster. You can not have a subnet (or a VPC) which spans to 2 different regions!