How to Load Input Data from a File in Terraform

TLDR

To load input data from a file in Terraform, use the file function for plain text files or the jsondecode function for JSON files. For more complex scenarios, use the external data source.

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Terraform allows you to load input data from external files, making it easier to manage configurations and reuse data. This guide will show you how to load input data from files in different formats.

Why Load Input Data from Files?

• Reusability: Share data across multiple configurations.
• Separation of Concerns: Keep data separate from Terraform code.
• Dynamic Configurations: Load data that changes frequently without modifying the code.

Loading Plain Text Files

Use the file function to load the contents of a plain text file.

Example: Loading a Plain Text File

This example demonstrates how to read a configuration file and output its contents.

variable "config_file" {
  default = "path/to/config.txt"
}

output "file_contents" {
  value = file(var.config_file)
}

In this example, the file function reads the contents of config.txt and outputs it.

Loading JSON Files

Use the file and jsondecode functions to load and parse JSON files.

Example: Loading a JSON File

variable "config_file" {
  default = "path/to/config.json"
}

output "json_data" {
  value = jsondecode(file(var.config_file))
}

If config.json contains:

{
  "region": "us-east-1",
  "instance_type": "t2.micro"
}

The output will be a map:

{
  "region" = "us-east-1"
  "instance_type" = "t2.micro"
}

Using the external Data Source

For more complex scenarios, use the external data source to load data from a script or external system.

Example: Using the external Data Source

Here's how to use the external data source to run a script that returns data in JSON format.

data "external" "example" {
  program = ["python3", "path/to/script.py"]
}

output "external_data" {
  value = data.external.example.result
}

The script should return data in JSON format:

{
  "key": "value"
}

Best Practices

• Validate Data: Use validation blocks to enforce constraints on input data.
• Secure Sensitive Data: Avoid storing sensitive data in plain text files.
• Document File Formats: Clearly document the expected format of input files.

By loading input data from files, you can create more dynamic and reusable Terraform configurations, simplifying your infrastructure management.