How to Handle Null Values in Terraform Lists When Creating AWS ACM Certificates

Learn how to create an AWS ACM certificate with a dynamic subject alternative names list in Terraform while handling null values effectively.
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Introduction

When working with Terraform, particularly in AWS environments, you might encounter scenarios where you need to define resources conditionally based on the deployment environment. One common challenge is dealing with null values within lists, especially when creating resources like AWS ACM certificates.

In this guide, we will explore a specific question regarding how to create a subject alternative names (SANs) list that adapts based on whether you are deploying to a production environment or not. The key issue arises from Terraform's restriction against "null values" in lists, which can lead to errors if not handled properly.
The Problem

Imagine you’re trying to create an AWS ACM certificate that has a subject alternative names list relying on the deployment environment. When deploying to production (indicated by var.env_name == "prod"), you might want to include the base domain, while in non-production environments, this value should not be included. However, you encounter errors when including null in a list.

Here’s the code snippet causing the issue:

[[See Video to Reveal this Text or Code Snippet]]

Trying to include a null value directly results in an error message: "Null values are not allowed for this attribute value." The post delves into different attempts made to resolve this issue, but all returned errors.
The Solution

After some experimentation, a working solution was identified that effectively handles the conditional logic for subject alternative names while removing null values seamlessly. Below, we’ll break down the solution into easy-to-understand sections.

Using compact and concat

To avoid issues with null values in your list, you can use compact and concat functions together. Here’s a successful attempt:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown

tolist([...]): Wraps the conditional expression in a list.

concat(...): Combines the list created by the tolist function with an array containing the second SAN.

compact(...): Removes any null values from the combined list, resulting in a valid list of SANs.

A Simpler Alternative

Upon realizing the initial complexity was not entirely necessary, the following concise solution was also found effective:

[[See Video to Reveal this Text or Code Snippet]]

Explanation

Here, you simply create the list directly with compact, which will automatically clean up any null entries, ensuring a clean list of subject alternative names.
Conclusion

Handling null values in a list within Terraform might seem daunting, particularly when managing dynamic resources like AWS certificates. However, using functions like compact and concat, you can manage conditional logic efficiently while avoiding the pitfalls of null entries.

By adopting these practices, you can streamline your Terraform configurations, creating robust solutions that adapt based on the environment without errors.

Feel free to test out these suggested solutions in your own Terraform scripts and share your experiences or questions in the comments!

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