The Terraform configuration language allows you to write declarative expressions to create infrastructure. While the configuration language is not a programming language, you can use several built-in functions to perform operations dynamically.
In this tutorial, you will:
- use the templatefile function to dynamically create an EC2 instance user data script.
- use the lookup function to reference values from a map.
- use the file function to read the contents of a file.
»Prerequisites
You can complete this tutorial using the same workflow with either Terraform OSS or Terraform Cloud. Terraform Cloud is a platform that you can use to manage and execute your Terraform projects. It includes features like remote state and execution, structured plan output, workspace resource summaries, and more.
Select the Terraform Cloud tab to complete this tutorial using Terraform Cloud.
»Clone the example repository
Clone the Learn Terraform Functions example repository. This repository contains example configuration for you to use to practice using functions to create dynamic EC2 configuration.
Navigate to the repository directory in your terminal.
»Use templatefile to dynamically generate a script
AWS lets you configure EC2 instances to run a user-provided script – called a user-data script – at boot time. You can use Terraform’s templatefile function to interpolate values into the script at resource creation time. This makes the script more adaptable and re-usable.
Open the user_data.tftpl file, which will be the user data script for your EC2 instance. This template file is a shell script to configure and deploy an application. Notice the ${department} and ${name} references – Terraform will interpolate these values using the templatefile function.
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Next, open the variables.tf file. This file includes definitions for the user_name and user_department input variables, which the configuration uses to set the values for the corresponding template file keys.
Now open main.tf. Add the user_data attribute to the aws_instance resource block as shown below. The templatefile function takes two arguments: the template file name and a map of template value assignments.
Save your changes.
»Create infrastructure
Apply your configuration. Respond yes to confirm the operation.
Terraform provisions your network configuration, instance, and provisioning script necessary to launch the example web app. Your web_public_address output in your terminal is the address of your web app instance. Navigate to that address in your web browser to verify your configuration.
Destroy your infrastructure before moving to the next section.
Enter yes when prompted to accept your changes.
»Use lookup function to select AMI
The lookup function retrieves the value of a single element from a map, given its key.
Add the following configuration to your variables.tf file to declare a new input variable.
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This input variable includes a default value of a map of region-specific AMI IDs for three regions.
Now, open the main.tf file and remove the data source for your AMI ID.
In your aws_instance resource, update the ami attribute to use the lookup function.
The ami is a required attribute for the aws_instance resource, so the lookup function must return a valid value for Terraform to apply your configuration. The lookup function arguments are a map, the key to access in the map, and an optional default value in case the key does not exist.
Next, add the following configuration for an ami_value output to your outputs.tf file. This output lets you verify the AMI returned by the lookup function.
Now run terraform plan to review the execution plan for these changes, using a command-line variable flag to set the region to us-east-2. The output now includes the selected AMI ID, which Terraform determined using the lookup function.
»Use the file function
In this section, you will create a new security group to allow SSH ingress traffic to your instance and configure the instance with an SSH key.
»Create an SSH key and a security group resource
Create a local SSH key to pair with the new instance you create so that you can connect securely to your instance.
Next, add the following configuration to main.tf to create a new security group and AWS key pair.
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In main.tf, add a new aws_security_group resource. Copy and append the resource block below to your main.tf file.
This configuration uses the file function to read the contents of a file to configure an SSH key pair. The file function does not interpolate values into file contents; you should only use it with files that do not need modification.
Next, edit your aws_instance.web resource to use the new security group and key pair. Be sure to save your changes.
Apply your configuration to create the resources. Enter yes when prompted to confirm the operation.
To confirm that your instance now accepts traffic on port 22, SSH into it from your terminal.
»Clean up resources
Now that you have completed this tutorial, destroy the resources to avoid incurring unnecessary costs. Respond yes when prompted to confirm the operation.
If you used Terraform Cloud for this tutorial, after destroying your resources, delete the learn-terraform-functions workspace from your Terraform Cloud organization.
»Next steps
In this tutorial, you learned how to make your Terraform configuration dynamic by using built-in functions. You used the lookup function to access values from maps based on an input variable, the templatefile function to generate a script with interpolated values, and the file function to use the contents of a file as-is within configuration.
Check out the following resources to learn more about how to make your Terraform configuration more flexible:
- Review the functions documentation to learn more about the functions Terraform supports.
- Learn how to manage similar resources using count.
- Learn how to create dynamic expressions in your configuration.
- Use for_each to dynamically configure your resources based on a map.
