100 Days of Cloud – Day 37: Terraform on Azure Cloud Shell Part 3

Its Day 37 of 100 Days of Cloud, and I’m continuing my learning journey on Infrastructure as Code (IaC) by using Terraform.

In the previous post, we looked at how Terraform State stores information and acts as a database of the resources deployed in Azure. We also looked at Interpolation and how this is used to reference different parts of the Terraform configuration and the dependencies needed to deploy parts of our infrastructure (such as deploying a subnet into a vnet).

Todays post is all about variables – we saw a sample of that on our first post where I used some variables when initially deploying our Resource Group. However, we need to go deeper than that and see how powerful variables can be in helping to make our code reusable, because ultimately that is the point of using Infrastructure as Code. So lets dive in and take a look.

Types of Variables

Lets go back to Day 35 when I introduced variables in my resource group deployment. We saw the following 2 variables defined in the variables.tf file:

String Variable

This is the first of our variable types which is a string. This is a sequence of characters representing text, so in our example above it represents the prefix to be used for the resource group name, and also the default location to deploy our resource group into. You could also use this for the likes of admin usernames/passwords, or VM Size/Type we wish to deploy.

List Variable

The next type of variable is a list, which is a sequence of values. In our example, this can be used to define the address space for vnets or subnets. Lists are always surrounded by square brackets.

Map Variable

Next type is a map – this is a list of key value pairs where each value is defined by a label – so in the example above, we are using the map variable to define the type of storage we want to use based on the locations.

Object Variable

Next type is object – this is a structured variable that can contain different types of values where named attributes each have their own type.

There are 2 other types of variables – a number (which can be a whole number or a fraction) or a boolean variable (which is basically true or false).

So thats the list of variable types, and the above examples show how they would be defined in our variables.tf file. But for the most part the above is just a list of definitions of variables, so where do we store the actual values we want to use?

Variable Inputs

There are 3 ways we can input variables for use with our Terraform code:

  • Environment Variables – this uses the “TF_VAR_” prefix for all variables you want to store locally. So for example, to store a password variable, you would run export TF_VAR_PASSWORD="password", and then declare the variable in the variables.tf file.
  • Command Line Switch- we can just use the -var parameter on the command line to input the variables when running terraform apply. So for example, we could run terraform apply -var="location=northeurope" to specify the location we want to deploy to.
  • terraform.tfvars file – this is a file that is deployed in the same location as the main.tf and variables.tf files and contains a set of key value pairs that are the variable and its value. An exmaple of this is shown below:

We can see that the variables are on the left and the values are on the right.

Calling Variables in Main.tf

So now we have our variables defined in our terraform.tfvars file, we can now call them from our main.tf when running terraform apply. If you recall on Day 35, I used the var. syntax in the code to call the variables from variables.tf. If we look at the main.tf file now, we can see where this is now calling all of our variables, for example virtual network and subnet:

And below is the virual machine code block:

Now, the eagle-eyed will have noticed that although I’m calling a var.admin_password variable, I didn’t have that defined in my terraform.tfvars file. What will happen here when I run terraform apply is that I will be prompted to input the password, or indeed any variables that are missing.

There is another safer, more secure and in my opinion much cooler way to call the password in, and thats by calling in a secret that is stored in Azure Key Vault. You can find the code for this here.

Conclusion

So now we’re starting to piece things together and understand how the code can become re-usable. For example, we could just copy the main.tf and variables.tf files out and create separate terraform.tfvars files for multiple deployments across multiple teams or regions depending on requirements.

And thats all for today! In the final post in the Terraform series, we’ll dive into modules and how they can used for multiple resources in your architecture when using Terraform. Until next time!

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