Bilel Khaled

If you set null to Terraform’s Optional attribute value, it will skip the process nicely.

Introduction

It’s been a long time since i wrote articles for personal reasons, i’ll try to be more active and share my tips with you .

When creating Terraform resources, there were cases where you wanted to skip or omit processing depending on the combination of settings while assigning values ​​to Optional attributes.

In such a case, I was at a loss as to how to create a general-purpose module, so I will leave it on the blog.

Environmental config

$ terraform -v
Terraform v1.2.8
on linux_amd64
+ provider registry.terraform.io/hashicorp/aws v4.22.0

InvalidParameterCombination

What I encountered was aws_db_instance the case of creating a resource that is an RDS DB instance.

modules/rds.tf

resource "aws_db_instance" "mysql" {
  // ...
  availability_zone = "eu-central-1"
  multi_az          = true
  // ...
}

The following error will occur as it is.

Error creating DB Instance: InvalidParameterCombination: Requesting a specific availability zone is not valid for Multi-AZ instances.

If you enable Multi-AZ, specifying AZ will be disabled.
The CloudFormation documentation makes this clear.

AWS::RDS::DBInstance – AWS CloudFormation

MultiAZ Specifies whether the database instance is a Multi-AZ DB instance deployment. You can’t set the AvailabilityZone parameter if the MultiAZ parameter is set to true.

multi_azI want to ignore true the specification when availability_zone

Workaround

You can omit it in Terraform null by assigning it to the value of the Optional attribute.

defaults – Functions – Configuration Language | Terraform by HashiCorp

When you define an attribute as optional and the caller doesn’t provide an explicit value for it, Terraform will set the attribute to null to represent that it was omitted. If you want to use a placeholder value other than null when an attribute isn’t set, you can use the defaults function to concisely assign default values only where an attribute value was set to null.

When not to use Multi-AZ deployment

modules/rds.tf

resource "aws_db_instance" "mysql" {
  // ...
  availability_zone = "eu-central-1"
  multi_az          = false
  // ...
}

For Multi-AZ deployment

modules/rds.tf

resource "aws_db_instance" "mysql" {
  // ...
  availability_zone = null
  multi_az          = true
  // ...
}

If you want to write generically

modules/rds.tf

resource "aws_db_instance" "mysql" {
  // ...
  availability_zone = var.multi_az ? null : var.availability_zone
  multi_az          = var.multi_az
  // ...
}

With this, you can specify the placement AZ of the DB instance in the case of single AZ, and avoid errors even when multi-AZ is enabled.

In conclusion

By using the example introduced this time, it seems possible to set the default value of the Optional variable to null and create a variable that does not require assignment if processing is not required.

I felt like i could get along well with Terraform’s null. I enjoy Terraform because it makes it easier to code the behavior I expect.

reference

• defaults – Functions – Configuration Language | Terraform by HashiCorp
• Terraform Optional Variables and Attributes — Using Null and Optional Flag | by Jason Zhong | Servian
• AWS::RDS::DBInstance – AWS CloudFormation