Resolving the variable not allowed Error in Terraform

Learn how to resolve the `variable not allowed` error in Terraform when provisioning a key vault by using locals effectively.
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Understanding the variable not allowed Issue in Terraform

When using Terraform to provision resources, you may encounter the variable not allowed error. This can be frustrating, especially when you're trying to define a variable for local use. For instance, if you’re attempting to provision an Azure Key Vault and want to manage secret values with a variable, you might hit this snag.

The error typically occurs when you try to use terraform variables incorrectly in certain contexts. In this scenario, let's take a closer look at a specific situation where a user wants to use variables to store secrets needed for Azure Cosmos DB in their Key Vault.

The Problem

Consider the following Terraform script:

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

In this code, the intent is clear: you want to store Azure Cosmos DB secrets in a Key Vault. However, the attempt to define secrets as a variable results in the variable not allowed error. This is because the values you are trying to assign within the variable definition are references to resources that can only be evaluated within the execution phase, not the variable phase.

The Solution: Using Locals Instead

Fortunately, there's a straightforward solution to this problem: use locals instead of variables. Using locals allows you to define values that are computed at runtime, which is appropriate for your use case where the azurerm_cosmosdb_account has to be created first.

Implementation Steps

Here’s how you can adjust your Terraform configuration to use locals effectively:

Define Locals:
Replace your variables block with a locals block. This allows you to create a map of secrets that can reference runtime values.

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

Modify the Resource Definition:

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

Benefits of Using Locals

Flexibility: Locals can reference other resources directly, making them very useful for complex configurations.

Runtime Evaluation: Because locals are evaluated at runtime, they can safely reference attributes of resources that were created earlier in the script.

Cleaner Code: Using locals can lead to cleaner, more maintainable code by reducing unnecessary variable declarations.

Conclusion

When faced with the variable not allowed error in Terraform, remember that the use of locals can be a powerful alternative. By transitioning from a variable-based approach to using locals, you gain flexibility and ensures your script runs smoothly. This method not only resolves the issue but also enhances your Terraform scripts for future projects.

By following the guidelines outlined in this post, you'll not only solve your immediate problem but also build a foundation for better Terraform practices moving forward.