HCL Language - 9 - Terraform Data Sources

Terraform Data Sources in HCL

Terraform is a popular infrastructure as code (IaC) tool that allows you to define and provision resources on various cloud platforms. In this guide, we’ll explore how to use Terraform data sources in HashiCorp Configuration Language (HCL). Data sources enable you to fetch information from existing resources and use it in your configurations.

Understanding Data Sources

Data sources in Terraform provide a way to access information about existing resources outside of your configuration. They are read-only and allow you to query attributes or properties of resources that have already been created, such as Amazon Web Services (AWS) instances, databases, or virtual networks.

Declaring Data Sources

To declare a data source in HCL, you use a data block with the type of data source and a unique name. You can also specify the required configuration attributes, such as filters or criteria to identify the desired resource.


data "aws_ami" "example" {
  most_recent = true
  owners = ["self"]
  filter {
    name   = "name"
    values = ["my-ami-name"]
  }
}

In this example, a data source of type “aws_ami” is declared with the name “example.” It fetches the most recent Amazon Machine Image (AMI) owned by “self” with the specified name.

Using Data in Configurations

Once you’ve declared a data source, you can access its attributes in other parts of your configuration. This allows you to use the fetched data in various ways, such as configuring resources or setting variables.


resource "aws_instance" "example" {
  ami           = data.aws_ami.example.id
  instance_type = "t2.micro"
  key_name      = "my-key"
}

In this code snippet, the data source “data.aws_ami.example” is used to set the “ami” attribute for an AWS EC2 instance. The “data.aws_ami.example.id” reference retrieves the ID of the AMI fetched by the data source.

Dynamic Data Queries

Data sources can also be queried dynamically based on variables or other configuration values. This allows you to fetch information that matches specific criteria or changes based on your needs.


variable "ami_name" {
  type    = string
  default = "my-ami-name"
}

data "aws_ami" "example" {
  most_recent = true
  owners = ["self"]
  filter {
    name   = "name"
    values = [var.ami_name]
  }
}

In this example, the data source filters AMIs based on the “ami_name” variable, allowing you to fetch different AMIs based on the provided name.

Best Practices for Data Sources

To make the best use of Terraform data sources in your HCL configurations, consider the following best practices:

Descriptive Names: Use meaningful names for your data sources to ensure clarity and readability.
Parameterized Queries: Leverage variables and dynamic queries to make your configurations more flexible and reusable.
Documentation: Include comments and documentation to explain the purpose and usage of data sources.

Conclusion

Terraform data sources in HCL offer a powerful way to access existing resource information and incorporate it into your infrastructure configurations. By understanding how to declare data sources, use fetched data, and query resources dynamically, you can create flexible and data-driven IaC solutions.

HCL Language – 9 – Terraform Data Sources