🐶
Terraform

Terraform: Conditionally Add Elements to a List

By Filip on 10/08/2024

Learn how to dynamically add elements to your Terraform lists based on conditional logic for more flexible and efficient infrastructure management.

Terraform: Conditionally Add Elements to a List

Table of Contents

Introduction

In Terraform, you cannot directly use if/else statements to add elements to a list based on a condition. However, there are alternative methods to achieve conditional list manipulation. One approach is to use the ternary operator in conjunction with list concatenation. This involves conditionally creating a list containing the desired element and then concatenating it with the base list. Another method is to utilize 'for' expressions with 'if' clauses. This allows you to iterate over potential elements and include them in the resulting list based on a condition evaluated within the loop. While these techniques offer workarounds, it's important to note that Terraform's conditional logic within lists has limitations. For complex scenarios, you might need to consider restructuring your data or explore alternative solutions. When choosing an approach, prioritize code readability, especially as the complexity of your configurations grows. Keep in mind that Terraform's declarative nature emphasizes defining the desired end state, and these techniques can assist you in incorporating conditional logic within that framework.

Step-by-Step Guide

Terraform doesn't directly support adding elements to a list based on a condition. You can't use traditional if/else statements within list declarations. However, you can achieve conditional list manipulation using alternative approaches:

1. Ternary Operator with Concatenation:

  • Use the ternary operator (condition ? true_value : false_value) to conditionally create a list containing the extra element.

  • Concatenate this conditional list with your base list.

    array = concat(["a", "b", "c"], (var.age == 12 && var.gender == "male") ? ["d"] : [])

  • If the condition (age == 12 && gender == "male") is true, it adds "d" to the list; otherwise, it adds an empty list, effectively not modifying the original list.

2. for Expressions with if Clauses:

  • Iterate over a list of potential elements using a for expression.

  • Within the loop, use an if clause to conditionally include elements in the resulting list.

    array = [for item in ["a", "b", "c", "d"] : item if item != "d" || (var.age == 12 && var.gender == "male")]

  • This iterates over all elements and includes "d" only if the condition is met.

Important Considerations:

  • Dynamic Blocks: While not directly applicable to lists, you can use dynamic blocks within resources to conditionally include entire blocks of configuration based on a condition.

  • Limitations: Terraform's conditional logic within lists and maps is limited. Complex scenarios might require restructuring your data or using workarounds.

  • Readability: Choose the approach that enhances the readability of your code, especially as complexity increases.

Remember that Terraform's strength lies in its declarative nature. Focus on defining the desired end state, and these techniques can help you achieve conditional logic within your configurations.

Code Example

The code defines two variables, age and gender, and uses them in two examples to demonstrate conditional list manipulation in Terraform. The first example uses a ternary operator to conditionally add an element to a list. The second example uses a for loop with an if clause to achieve the same result. Both examples demonstrate how to control the contents of a list based on conditions in Terraform.

# Configure Variables (replace with your actual values)
variable "age" {
  type = number
  default = 12
}

variable "gender" {
  type = string
  default = "male"
}

# Example 1: Ternary Operator with Concatenation
output "ternary_list" {
  value = concat(["a", "b", "c"], (var.age == 12 && var.gender == "male") ? ["d"] : [])
}

# Example 2: for Expression with if Clause
output "for_if_list" {
  value = [for item in ["a", "b", "c", "d"] : item if item != "d" || (var.age == 12 && var.gender == "male")]
}

Explanation:

  • Variables: We define age and gender variables to control the condition.
  • Ternary Operator: The ternary_list output demonstrates using the ternary operator to conditionally add "d" to the list if both age is 12 and gender is "male."
  • for Expression with if Clause: The for_if_list output iterates through a list and uses an if clause to include "d" only if the condition is met.

Output:

When you run terraform apply with the default variable values (age = 12, gender = "male"), the output will be:

ternary_list = [
  "a",
  "b",
  "c",
  "d",
]
for_if_list = [
  "a",
  "b",
  "c",
  "d",
]

If you change the variable values so the condition is false, "d" will not be included in the output lists.

This example provides a clear and concise illustration of how to conditionally manipulate lists in Terraform using both the ternary operator and for expressions with if clauses.

Additional Notes

General:

  • Declarative Mindset: Remember that Terraform prioritizes describing your desired end state. While these workarounds provide conditional logic, always consider if restructuring your data or logic for a more declarative approach is feasible.
  • Performance: While less of a concern for smaller configurations, deeply nested or overly complex conditional logic within lists can impact Terraform's plan and apply times. Consider the trade-off between conciseness and performance.

Ternary Operator:

  • Conciseness: Best for simple conditions where you're adding a single element or a small, statically defined list based on a single condition.
  • Readability: Can become less readable for multiple conditions or when the conditionally added list is large.

for Expressions with if Clauses:

  • Flexibility: More versatile for complex conditions, iterating over larger lists, or applying conditions to individual elements.
  • Filtering: Can be used to filter existing lists based on conditions, not just add elements.
  • Readability: Can be more readable than nested ternaries for complex logic, but proper formatting and clear variable names are crucial.

Alternatives and Extensions:

  • Custom Functions/Modules: For very complex scenarios, consider abstracting the logic into custom Terraform functions (0.12+) or reusable modules to improve maintainability.
  • External Data Sources: If your conditional logic depends on external factors, explore using external data sources to fetch and utilize dynamic values.

Best Practices:

  • Clear Variable Names: Use descriptive variable names to make the conditions self-explanatory.
  • Comments: Document complex logic with comments to explain the intended behavior.
  • Testing: Thoroughly test your configurations with different input values to ensure the conditional logic behaves as expected.

Summary

Approach Description Example
Ternary Operator with Concatenation - Uses the ternary operator (condition ? true_value : false_value) to create a conditional list.
- Concatenates the conditional list with the base list.		 array = concat(["a", "b", "c"], (var.age == 12 && var.gender == "male") ? ["d"] : [])
for Expressions with if Clauses - Iterates over potential elements using a for expression.
- Uses an if clause within the loop to conditionally include elements.		 `array = [for item in ["a", "b", "c", "d"] : item if item != "d"

Key Points:

  • Terraform doesn't support traditional if/else within list declarations.
  • Dynamic blocks offer conditional logic for resource configuration blocks.
  • Complex scenarios might require data restructuring or workarounds.
  • Prioritize code readability when choosing an approach.
  • Focus on defining the desired end state in a declarative manner.

Conclusion

In conclusion, while Terraform doesn't offer direct if/else statements for conditional list manipulation, you can leverage techniques like the ternary operator with concatenation or 'for' expressions with 'if' clauses to achieve similar results. When selecting your approach, prioritize code readability and consider the complexity of your conditions. Remember that Terraform's declarative nature encourages focusing on the desired end state. By understanding these techniques and their implications, you can effectively manage conditional logic within your Terraform configurations.

References

Were You Able to Follow the Instructions?

😍Love it!
😊Yes
😐Meh-gical
😞No
🤮Clickbait

Related posts

Terraform Kubernetes Cluster Unreachable Error: Fix It Now

Learn how to troubleshoot and resolve the "Terraform: Error: Kubernetes cluster unreachable: invalid configuration" error in your infrastructure setup.

Terraform: Create Athena Views Programmatically

Learn how to programmatically create AWS Athena views using Terraform to simplify query access and promote data reusability in your data lake.

Terraform State Lock Error: Troubleshooting Guide

Learn how to troubleshoot and resolve the common Terraform "Error locking state" message caused by a locked state blob, preventing infrastructure deployments.