Terraform variable referencing locals not working - amazon-web-services

Question

I need to pass the database host name (that is dynamically generated) as an environmental variable into my task definition. I thought I could set locals and have the variable map refer to a local but it seems to not work, as I receive this error: “error="failed to check table existence: dial tcp: lookup local.grafana-db-address on 10.0.0.2:53: no such host". I am able to execute the terraform plan without issues and the code works when I hard code the database host name, but that is not optimal.
My Variables and Locals
//MySql Database Grafana Username (Stored as ENV Var in Terraform Cloud)
variable "username_grafana" {
description = "The username for the DB grafana user"
type = string
sensitive = true
}
//MySql Database Grafana Password (Stored as ENV Var in Terraform Cloud)
variable "password_grafana" {
description = "The password for the DB grafana password"
type = string
sensitive = true
}
variable "db-port" {
description = "Port for the sql db"
type = string
default = "3306"
}
locals {
gra-db-user = var.username_grafana
}
locals {
gra-db-password = var.password_grafana
}
locals {
db-address = aws_db_instance.grafana-db.address
}
locals {
grafana-db-address = "${local.db-address}.${var.db-port}"
}
variable "app_environments_vars" {
type = list(map(string))
description = "Database environment variables needed by Grafana"
default = [
{
"name" = "GF_DATABASE_TYPE",
"value" = "mysql"
},
{
"name" = "GF_DATABASE_HOST",
"value" = "local.grafana-db-address"
},
{
"name" = "GF_DATABASE_USER",
"value" = "local.gra-db-user"
},
{
"name" = "GF_DATABASE_PASSWORD",
"value" = "local.gra-db-password"
}
]
}
Task Definition Variable reference
"environment": ${jsonencode(var.app_environments_vars)},
Thank you to everyone who has helped me with this project. I am new to all of this and could not have done it without help from this community.

Answer 1

You can't use dynamic references in your app_environments_vars. So your default values "value" = "local.grafana-db-address" will never get resolved by TF. If will be just a literal string "local.grafana-db-address".
You have to modify your code so that all these dynamic references in app_environments_vars get populated in locals.
UPDATE
Your app_environments_vars should be local variable for it to be resolved:
locals {
app_environments_vars = [
{
"name" = "GF_DATABASE_TYPE",
"value" = "mysql"
},
{
"name" = "GF_DATABASE_HOST",
"value" = local.grafana-db-address
},
{
"name" = "GF_DATABASE_USER",
"value" = local.gra-db-user
},
{
"name" = "GF_DATABASE_PASSWORD",
"value" = local.gra-db-password
}
]
}
then you pass that local to your template for the task definition.