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Welcome to this lesson on using data sources in OpenTofu. Data sources let you read and reference existing infrastructure or local data, without creating or managing new resources.

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Quiz Questions

  1. Can a data source be used to create, update, and destroy infrastructure?
    Answer: False. A data source only reads resource data and makes it available in OpenTofu.
    For more details, see the official OpenTofu documentation.
  2. Can a data source be created using the data block?
    Answer: True. Data sources are defined with a data block, analogous to resource blocks.

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Hands-On: Local File Data Source

In the directory /root/OpenTofu/project/lexcorp, update main.tf to:
  • Read the contents of /etc/os-release via the local_file data source.
  • Output that content as an OpenTofu output variable.

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Incorrect Configuration

output "os-version" {
  value = data.local_file.content
}

datasource "local_file" "os" {
  filename = "/etc/os-release"
}
  1. The block keyword must be data, not datasource.
  2. The output reference requires the data source name (os).

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Corrected Configuration

data "local_file" "os" {
  filename = "/etc/os-release"
}

output "os-version" {
  value = data.local_file.os.content
}

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Apply the Configuration

opentofu init
opentofu plan
opentofu apply
Expected output: 
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:
os-version = <<EOT
PRETTY_NAME="Ubuntu 22.04.3 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
...
EOT

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AWS Data Sources

Next, we’ll work with AWS data sources. Below is a quick reference table for the examples covered.
AWS Data SourceRequired ArgumentDescription
aws_ebs_volumevolume_idFetches the EBS volume ID
aws_s3_bucketbucketSpecifies the existing bucket name

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EBS Volume

Open ebs.tf to find a data source block for an AWS EBS volume: 
data "aws_ebs_volume" "gpt_volume" {
  # configuration...
}
Question: Which attribute fetches the volume ID?
Answer: volume_id

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S3 Bucket

In s3.tf, you’ll see a data source intended to read an existing S3 bucket: 
data "aws_s3_bucket" "selected" {
  bucket_name = "bucket.test.com"
}
The argument bucket_name is invalid. The correct argument is bucket.
Correct configuration: 
data "aws_s3_bucket" "selected" {
  bucket = "bucket.test.com"
}
That completes this lesson on OpenTofu data sources. In upcoming modules, you’ll get more hands-on practice with AWS data sources and resources. Happy building!

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