Terraform error creating subnet dependency - amazon-web-services

Question

I'm trying to get a documentdb cluster up and running and have it running from within a private subnet I have created.
Running the config below without the depends_on i get the following error message as the subnet hasn't been created:
Error: error creating DocDB cluster: DBSubnetGroupNotFoundFault: DB subnet group 'subnet-0b97a3f5bf6db758f' does not exist.
status code: 404, request id: 59b75d23-50a4-42f9-99a3-367af58e6e16
Added the depends on setup to wait for the subnet to be created but are running into an issue.
cluster_identifier = "my-docdb-cluster"
engine = "docdb"
master_username = "myusername"
master_password = "mypassword"
backup_retention_period = 5
preferred_backup_window = "07:00-09:00"
skip_final_snapshot = true
apply_immediately = true
db_subnet_group_name = aws_subnet.eu-west-3a-private
depends_on = [aws_subnet.eu-west-3a-private]
}
On running terraform apply I an getting an error on the config:
Error: error creating DocDB cluster: DBSubnetGroupNotFoundFault: DB subnet group 'subnet-0b97a3f5bf6db758f' does not exist.
status code: 404, request id: 8b992d86-eb7f-427e-8f69-d05cc13d5b2d
on main.tf line 230, in resource "aws_docdb_cluster" "docdb":
230: resource "aws_docdb_cluster" "docdb"

Answer 1

A DB subnet group is a logical resource in itself that tells AWS where it may schedule a database instance in a VPC. It is not referring to the subnets directly which is what you're trying to do there.
To create a DB subnet group you should use the aws_db_subnet_group resource. You then refer to it by name directly when creating database instances or clusters.
A basic example would look like this:
resource "aws_vpc" "example" {
cidr_block = "10.0.0.0/16"
}
resource "aws_subnet" "eu-west-3a" {
vpc_id = aws_vpc.example.id
availability_zone = "a"
cidr_block = "10.0.1.0/24"
tags = {
AZ = "a"
}
}
resource "aws_subnet" "eu-west-3b" {
vpc_id = aws_vpc.example.id
availability_zone = "b"
cidr_block = "10.0.2.0/24"
tags = {
AZ = "b"
}
}
resource "aws_db_subnet_group" "example" {
name = "main"
subnet_ids = [
aws_subnet.eu-west-3a.id,
aws_subnet.eu-west-3b.id
]
tags = {
Name = "My DB subnet group"
}
}
resource "aws_db_instance" "example" {
allocated_storage = 20
storage_type = "gp2"
engine = "mysql"
engine_version = "5.7"
instance_class = "db.t2.micro"
name = "mydb"
username = "foo"
password = "foobarbaz"
parameter_group_name = "default.mysql5.7"
db_subnet_group_name = aws_db_subnet_group.example.name
}
The same thing applies to Elasticache subnet groups which use the aws_elasticache_subnet_group resource.
It's also worth noting that adding depends_on to a resource that already references the dependent resource via interpolation does nothing. The depends_on meta parameter is for resources that don't expose a parameter that would provide this dependency information directly only.

Answer 2

It seems value in parameter is wrong. db_subnet_group_name created somewhere else gives the output id/arn. So u need to use id value. although depends_on clause looks okie.
db_subnet_group_name = aws_db_subnet_group.eu-west-3a-private.id
So that would be correct/You can try to use arn in place of id.
Thanks,
Ashish