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Terraform

Terraform Conditionals: Handle Missing Variables

By Filip on 10/06/2024

Learn how to use Terraform conditionals to handle cases where a variable is not defined and prevent unexpected infrastructure deployment issues.

Terraform Conditionals: Handle Missing Variables

Table of Contents

Introduction

In Terraform, you can't use familiar "if" statements for conditional logic. This article explains how to work with situations where you need to check if a variable exists and make decisions based on that, using functions like try() and coalesce(). You'll also learn about important considerations for handling data sources and creating resources conditionally in the absence of traditional if/else structures.

Step-by-Step Guide

Terraform doesn't have a traditional "if" statement. Instead, we use conditional expressions within resource arguments and other Terraform constructs. Here's how to handle situations where you want to check if a variable exists:

1. Checking for Variable Existence:

  • Use the try() function: The most reliable way to check if a variable is defined is with the try() function. It attempts to evaluate an expression and returns a default value if the expression would cause an error.

    resource "example_resource" "example" {
  count = try(var.my_variable != "", 0)
  # ... other configurations
}

    In this example, if var.my_variable is not defined or is an empty string, the try() function will return 0, effectively creating no instances of the resource. If var.my_variable has a value, the expression evaluates to true, and count will be set to 1, creating the resource.

2. Providing Default Values:

  • Use the coalesce() function: This function lets you specify a list of values. It returns the first value in the list that is not null.

    variable "environment" {
  # No default value here
}

resource "example_resource" "example" {
  name = "example-${coalesce(var.environment, "default")}"
}

    If var.environment is not provided, the resource will be named "example-default".

Important Considerations:

  • Data Sources: When using data sources, you can't directly check for the non-existence of a resource. If a data source doesn't find a match, it will throw an error. Consider using techniques like fetching data into a list and checking its length, or using the try() function to handle potential errors gracefully.

  • Conditional Resource Creation: You can conditionally create resources based on variable values. The count argument (as shown in the first example) is a common way to achieve this.

  • No Traditional if/else: Remember that Terraform evaluates expressions, not imperative code blocks. You can't directly translate traditional if/else logic into Terraform.

By understanding these concepts and functions, you can effectively manage conditional logic and handle situations where variables may or may not be defined in your Terraform configurations.

Code Example

This document provides examples of how to manage variable existence and conditional logic in Terraform. It demonstrates using the try() function to conditionally create an AWS EC2 instance based on the existence of a variable. It also shows how to use the coalesce() function to provide a default value for a variable used in an AWS S3 bucket name. Finally, it illustrates how to handle data source results that might not return any values using try() and list length, ensuring resources are created only when appropriate data is available.

This document provides practical examples demonstrating how to handle variable existence and implement conditional logic in Terraform.

1. Checking for Variable Existence and Conditionally Creating Resources

This example demonstrates how to use the try() function to check if a variable is defined and conditionally create an AWS EC2 instance.

# Define a variable for the instance type, without a default value
variable "instance_type" {
}

# Define a resource for an EC2 instance
resource "aws_instance" "example" {
  # Conditionally create the instance based on the 'instance_type' variable
  count = try(var.instance_type != "", 0)

  # Set the instance type if the variable is defined
  instance_type = var.instance_type

  # ... other EC2 instance configurations
}

Explanation:

  • The try(var.instance_type != "", 0) expression checks if var.instance_type is defined and not empty.
  • If var.instance_type is defined and not empty, the expression evaluates to true, setting count to 1 and creating the EC2 instance.
  • If var.instance_type is not defined or empty, the try() function returns 0, effectively creating no instances.

2. Providing Default Values with coalesce()

This example shows how to use the coalesce() function to provide a default value for a variable used in an AWS S3 bucket name.

# Define a variable for the environment, without a default value
variable "environment" {
}

# Define a resource for an S3 bucket
resource "aws_s3_bucket" "example" {
  # Use coalesce() to set a default environment if not provided
  bucket = "my-bucket-${coalesce(var.environment, "prod")}"

  # ... other S3 bucket configurations
}

Explanation:

  • The coalesce(var.environment, "prod") expression checks the value of var.environment.
  • If var.environment is defined, its value is used in the bucket name.
  • If var.environment is not defined, the default value "prod" is used, resulting in a bucket name like "my-bucket-prod".

3. Handling Data Source Results with try() and List Length

This example demonstrates how to use try() and list length to handle cases where a data source might not return any results.

# Data source to fetch an AMI based on a filter
data "aws_ami" "example" {
  most_recent = true
  filter {
    name   = "name"
    values = ["my-custom-ami*"]
  }
}

# Resource for an EC2 instance
resource "aws_instance" "example" {
  # Create the instance only if an AMI matching the filter is found
  count = try(length(data.aws_ami.example.id) > 0, 0)

  # Use the fetched AMI ID
  ami = data.aws_ami.example.id

  # ... other EC2 instance configurations
}

Explanation:

  • The data.aws_ami.example data source fetches an AMI based on the provided filter.
  • The try(length(data.aws_ami.example.id) > 0, 0) expression checks if the data source returned any results.
  • If the data source finds a matching AMI, its ID is stored in data.aws_ami.example.id, and the length() function returns a value greater than 0, creating the EC2 instance.
  • If no matching AMI is found, the data source would typically throw an error. However, the try() function catches this error and returns 0, preventing the instance from being created.

These examples demonstrate how to effectively manage conditional logic and handle situations where variables may or may not be defined in your Terraform configurations. Remember to leverage functions like try() and coalesce() to create robust and flexible infrastructure deployments.

Additional Notes

General:

  • Think Declarative: Terraform's strength is its declarative nature. Instead of writing step-by-step instructions (if this, then that), focus on describing the desired end state. Let Terraform figure out how to get there.
  • Error Handling: While try() is great for avoiding errors, sometimes you want an error to stop a misconfigured deployment. Use try() strategically.
  • Readability: Complex conditional expressions can become hard to read. Use comments and break down logic into smaller variables or locals for clarity.

Alternatives and Advanced Techniques:

  • For Modules: When passing variables to modules, you can use the ? operator to make an input variable optional. This avoids the need for try() in many cases.
  • Ternary Operator: For simple conditions, the ternary operator (condition ? true_val : false_val) can be more concise than try().
  • Dynamic Blocks: Use dynamic blocks within resources to conditionally create nested blocks based on variable values.
  • External Data Sources: For very complex logic, consider fetching decisions from external sources like configuration files or APIs.

Beyond the Basics:

  • Testing: Automated testing is crucial when using conditional logic. Tools like Terratest can help ensure your configurations behave as expected.
  • State Management: Be mindful of how conditional logic affects your Terraform state. Unexpected changes in state can lead to issues during subsequent applies.

Remember:

  • The examples provided are simplified for illustration. Real-world scenarios might require more complex combinations of these techniques.
  • Always refer to the official Terraform documentation for the most up-to-date information on functions and best practices.

Summary

This table summarizes how to handle situations where you need to check if a variable exists in Terraform, given that it lacks a traditional "if" statement:

Situation Approach Example Explanation
Check if a variable exists Use the try() function count = try(var.my_variable != "", 0) - Attempts to evaluate var.my_variable != "".
- Returns 0 (false) if the variable is undefined or empty, effectively creating no resources.
- Returns 1 (true) if the variable has a value, creating the resource.
Provide default values for potentially undefined variables Use the coalesce() function name = "example-${coalesce(var.environment, "default")}" - Takes a list of values and returns the first non-null value.
- If var.environment is not provided, the resource name defaults to "example-default".
Handle data sources that might not find a match Use try() or list length checks - try(): Wrap data source access within try() to handle potential errors gracefully.
- List Length: Fetch data into a list and check its length to determine if the resource exists.

Key Takeaways:

  • Terraform relies on conditional expressions within resource arguments.
  • try() and coalesce() are essential functions for managing variable existence and default values.
  • Data source handling requires specific techniques due to potential errors when resources are not found.
  • Remember that Terraform evaluates expressions, not imperative code blocks like traditional if/else statements.

Conclusion

Mastering conditional logic in Terraform, despite the absence of traditional "if" statements, is crucial for building flexible and robust infrastructure. By understanding how to leverage functions like try() and coalesce(), you can effectively manage variable existence and create resources conditionally. Remember to consider the nuances of data sources and embrace Terraform's declarative nature for optimal results. As you delve deeper, explore advanced techniques like dynamic blocks and external data sources to handle complex scenarios. Always prioritize readability and maintainability in your code, and remember that comprehensive testing is paramount when implementing conditional logic in your infrastructure as code.

References

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