Terraform doesn’t support passing dynamic variables for state files or backend blocks.
For example, let us consider we have a backend defined like the below,
Now, we have a scenario where we do not want to pass the values in the backend block in a static format, instead, we want the value to be dynamic.
We will rewrite the code and create two variables to pass the bucket name and bucket object (key) and pass these values in the backend block as variables instead of static values.
The above terraform configuration when we run the terraform init command will throw errors, as variables not allowed in the backend block.
E:\Projects\terraform>terraform init -migrate-state
Initializing the backend...
Backend configuration changed!
Terraform has detected that the configuration specified for the backend
has changed. Terraform will now check for existing state in the backends.
╷
│ Error: Variables not allowed
│
│ on main.tf line 9, in terraform:
│ 9: bucket = var.backend_bucket #"s3-backend-demo"
│
│ Variables may not be used here.
╵
╷
│ Error: Variables not allowed
│
│ on main.tf line 10, in terraform:
│ 10: key = var.backend_key #"backend"
│
│ Variables may not be used here.
╵
To resolve the issue, terraform allows us to pass the backend values dynamically through backend-config arguments via command prompt.
In your backend configuration block, remove the details which you want to pass dynamically. Like in our example we are using s3 as the backend, passing bucket name and object (key) values.
We will remove those attributes from the backend configuration block and instead we will pass them through backend-config arguments.
Passing backend configuration arguments values through a command prompt,
terraform init -backend-config="bucket=s3-backend-demo" -backend-config="key=backend"
In case we have an existing state file and we want to update the new backend arguments, you will need to run the init command with -migrate-state arguments.
terraform init -migrate-state -backend-config="bucket=s3-backend-demo" -backend-config="key=backend"