Obtaining External Data with Data Sources in Terraform

In this tutorial, we will cover how to use data sources in Terraform to obtain information from external resources. Data sources allow you to query existing infrastructure and use the data in your Terraform configurations.

What are Data Sources?

Data sources in Terraform provide a way to query and retrieve information from outside of Terraform's control. This can include querying existing infrastructure, external APIs, or other data that you might need to configure your resources.

Using Data Sources

Let's go through an example where we use a data source to obtain the latest Amazon Machine Image (AMI) for a given operating system. We will use the aws_ami data source to achieve this.

Step 1: Define the Data Source

In your Terraform configuration file, define the aws_ami data source to find the latest AMI for Amazon Linux 2:

provider "aws" {
  region = "us-west-2"
}

data "aws_ami" "latest_amazon_linux" {
  most_recent = true

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["amazon"]
}

In this configuration:

• most_recent: Ensures that the most recent AMI is selected.
• filter: Filters the AMIs by name and virtualization type.
• owners: Specifies the owner of the AMI, in this case, Amazon.

Step 2: Use the Data Source in a Resource

Now that we have defined the data source, we can use it to configure a resource. Let's create an EC2 instance using the AMI ID obtained from the data source:

resource "aws_instance" "example" {
  ami           = data.aws_ami.latest_amazon_linux.id
  instance_type = "t2.micro"

  tags = {
    Name = "example-instance"
  }
}

Here, the ami attribute of the aws_instance resource is set to the ID of the AMI obtained from the aws_ami data source.

Applying the Configuration

To apply your configuration and create the EC2 instance using the latest Amazon Linux 2 AMI, follow these steps:

Step 1: Initialize Terraform

Initialize your Terraform configuration:

terraform init

Step 2: Validate the Configuration

Validate your configuration to ensure there are no syntax errors:

terraform validate

Step 3: Apply the Configuration

Apply your configuration to create the resources:

terraform apply

Review the plan and confirm the apply action. Terraform will query the data source to retrieve the latest AMI ID and use it to create the EC2 instance.

Other Common Data Sources

Terraform provides many data sources for different providers. Some common data sources include:

• aws_vpc: Retrieve information about an existing VPC.
• aws_subnet: Retrieve information about an existing subnet.
• aws_security_group: Retrieve information about an existing security group.
• http: Make HTTP requests to external APIs and use the response data.

Conclusion

Using data sources in Terraform allows you to dynamically query and use information from existing infrastructure and external systems. This tutorial covered how to define and use a data source to obtain the latest Amazon Linux 2 AMI and use it to create an EC2 instance. By leveraging data sources, you can make your Terraform configurations more flexible and adaptable to changes in your environment.