Terraform AWS provider upgrade issue with RDS

Question

Trying to upgrade AWS provider to version 4, but getting the following error in RDS module:

Error: Conflicting configuration arguments
│
│   with module.my-instance-mysql-eu[0].module.rds.module.db_instance.aws_db_instance.this[0],
│   on .terraform/modules/my-instance-mysql-eu.rds/modules/db_instance/main.tf line 47, in resource "aws_db_instance" "this":
│   47:   db_name                             = var.db_name
│
│ "db_name": conflicts with replicate_source_db

Answer 1

The error is stating that the db_name attribute conflicts with the replicate_source_db attribute; you cannot specify both attributes, it must be one or the other. This is also mentioned in the Terraform documentation .

If you are replicating an existing RDS database, the database name will be the same as the name of the source. If this is a new database, do not set the replicate_source_db attribute at all.

Answer 2

I encountered a similar issue with the engine & engine_version variables:

│ Error: Conflicting configuration arguments
│ 
│   with module.production.module.replica_app_db_production.aws_db_instance.db,
│   on modules/rds/postgres/main.tf line 36, in resource "aws_db_instance" "db":
│   36:   engine                      = var.engine
│ 
│ "engine": conflicts with replicate_source_db
╵
╷
│ Error: Conflicting configuration arguments
│ 
│   with module.production.module.replica_app_db_production.aws_db_instance.db,
│   on modules/rds/postgres/main.tf line 37, in resource "aws_db_instance" "db":
│   37:   engine_version              = var.engine_version
│ 
│ "engine_version": conflicts with replicate_source_db
╵

I found a good example of a solution here: https://github.com/terraform-aws-modules/terraform-aws-rds/blob/v5.2.2/modules/db_instance/main.tf

And I managed to solve this with the below conditions:

  # Replicas will use source metadata
  username       = var.replicate_source_db != null ? null : var.username
  password       = var.replicate_source_db != null ? null : var.password
  engine         = var.replicate_source_db != null ? null : var.engine
  engine_version = var.replicate_source_db != null ? null : var.engine_version

If var.replicate_source_db is not null, then the username/password/engine/engine_version will be set to null (which is what we need as these variables cannot be specified for a replica ). And if it is not a replica, then we will have the variables set accordingly:)