Implement JSON template files as input variables for local variables or in for loop

Introduction

This guide demonstrates two methods for using JSON data with Terraform. You can either use a static JSON file as an input variable or dynamically generate a JSON file from a list using a template and a for loop.

Procedure

Follow the steps for the method that best fits your use case.

Method 1: Use a JSON file as an input variable

This method is suitable when you have a complete JSON file that you want to use for variable values.

1. Create a main.tf file that defines an input variable for the JSON file path and uses the jsondecode and file functions to parse its content.

   variable "json_input" {
     description = "Path to the JSON input file"
     type        = string
   }
   
   locals {
     input_data = jsondecode(file(var.json_input))
   }
   
   output "name" {
     value = local.input_data.name
   }

2. Create a JSON file named test.json with the data you want to use.

   {
     "name": "testname",
     "age": 100,
     "city": "San Francisco"
   }

3. Run terraform apply and pass the path to your JSON file using the -var flag.

   $ terraform apply -var="json_input=test.json"

   Terraform will read the file, parse the JSON content, and use the values as defined.

Method 2: Generate a JSON file using a template and a for loop

This method is useful when you need to construct a JSON file dynamically from a Terraform list variable.

1. Create a main.tf file. This configuration defines a list of strings, then uses the template_file data source with a for loop and the jsonencode function to create a JSON structure. Finally, it saves the result to a local file.

   # Define a list of items
   variable "items" {
     type    = list(string)
     default = ["Rahul", "Test1", "Test2"]
   }
   
   # Render the template
   data "template_file" "items_template" {
     template = file("template.tpl")
   
     vars = {
       items_json = jsonencode([for item in var.items : { name = item }])
     }
   }
   
   # Create a local file to save the generated JSON
   resource "local_file" "items_json" {
     filename = "items.json"
     content  = data.template_file.items_template.rendered
   }

2. Create a template file named template.tpl.

   {
     "items": ${items_json}
   }

3. Run terraform apply to generate the items.json file.

   $ terraform apply

   This process iterates over the var.items list, creates a JSON array of objects, and writes it to the items.json file.