🐶
Terraform

Terraform Invalid for_each Argument: Troubleshooting Guide

By Filip on 10/07/2024

Learn how to troubleshoot and resolve Terraform errors related to invalid "for_each" arguments in your infrastructure code.

Terraform Invalid for_each Argument: Troubleshooting Guide

Table of Contents

Introduction

This article provides solutions to the common causes of the "Invalid for_each argument" error in Terraform. We'll explore how to ensure the correct input type for for_each, address challenges with resource attributes and sensitive variables, and navigate dynamic attribute references. Additionally, we'll touch upon version compatibility considerations. By understanding these common pitfalls and their solutions, you can effectively utilize the for_each construct to streamline your infrastructure provisioning with Terraform.

Step-by-Step Guide

The for_each construct in Terraform is a powerful way to create multiple instances of a resource or module. However, it requires a specific type of input: a map or a set of strings. If you provide an incorrect input type or a value that Terraform cannot determine before applying the configuration, you'll encounter an "Invalid for_each argument" error.

Here's a breakdown of common causes and solutions:

  1. Incorrect Input Type: The for_each argument must be a map or a set of strings.

    • Solution: Ensure that the variable or expression you're using for for_each evaluates to either a map or a set of strings. You can use functions like tomap or toset to convert other data structures.

  2. Resource Attributes in for_each: You cannot use attributes of a resource that are not known until apply time within the for_each argument. Terraform needs to know how many instances to create before actually creating them.

    • Solution: Refactor your code to use data sources or other mechanisms to retrieve the necessary information before the for_each loop. Alternatively, consider using count instead of for_each if the number of instances can be determined dynamically but doesn't rely on resource attributes.

  3. Sensitive Variables: You cannot directly use a variable marked as sensitive within a for_each argument. Terraform prevents this to avoid potential information leaks.

    • Solution: If you need to use a sensitive value within a for_each loop, consider using the nonsensitive function to temporarily remove the sensitive flag. However, be cautious about where and how you use this function to avoid compromising the security of your sensitive data.

  4. Dynamic Attribute References: When using for_each with nested data structures, ensure that you're correctly referencing attributes within the loop.

    • Solution: Double-check your expressions within the loop to ensure that you're accessing the correct levels of your data structure. Use the each.key and each.value syntax to access the key and value of the current element in the loop.

  5. Version Compatibility: Be aware of potential changes in how for_each behaves across different Terraform versions.

    • Solution: Consult the Terraform documentation for the specific version you're using to understand any version-specific nuances or limitations related to for_each.

If you encounter an "Invalid for_each argument" error, carefully examine the error message and the code surrounding your for_each loop. Pay close attention to the data types involved, the use of resource attributes, and any sensitive variables. By understanding the root cause, you can apply the appropriate solution to resolve the issue and leverage the power of for_each effectively in your Terraform configurations.

Code Example

This code demonstrates common errors and solutions when using the for_each meta-argument in Terraform. It covers issues like incorrect input types, using resource attributes within for_each, handling sensitive variables, and correctly referencing attributes within nested data structures. Each example includes both the incorrect code and the corrected version, along with a brief explanation. The code emphasizes the importance of data type compatibility, pre-fetching resource attributes when needed, carefully handling sensitive information, and using the correct syntax for accessing nested data.

# Example 1: Incorrect Input Type (List instead of Map)

# This will cause an error because `users` is a list, not a map.
variable "users" {
  default = ["user1", "user2"]
}

resource "null_resource" "example" {
  for_each = var.users # Incorrect! Should be a map.
  provisioner "local-exec" {
    command = "echo Creating user ${each.key}"
  }
}

# Solution: Convert the list to a map using `tomap`
resource "null_resource" "example_fixed" {
  for_each = tomap({
    for i, user in var.users : user => i
  })
  provisioner "local-exec" {
    command = "echo Creating user ${each.key}"
  }
}

# Example 2: Resource Attribute in for_each

# This will cause an error because `aws_instance.example.id` is not known until apply time.
resource "aws_instance" "example" {
  # ... instance configuration ...
}

resource "null_resource" "example" {
  for_each = { for id in aws_instance.example.id : id => id } # Incorrect!
  # ... resource configuration ...
}

# Solution: Use a data source or other mechanism to get the IDs before the loop
data "aws_ami" "example" {
  # ... AMI filter ...
}

resource "null_resource" "example_fixed" {
  for_each = { for id in data.aws_ami.example.id : id => id }
  # ... resource configuration ...
}

# Example 3: Sensitive Variable in for_each

variable "db_passwords" {
  type      = map(string)
  sensitive = true
  default = {
    user1 = "password123"
    user2 = "securepassword"
  }
}

# This will cause an error because `var.db_passwords` is sensitive.
resource "null_resource" "example" {
  for_each = var.db_passwords # Incorrect! Cannot directly use sensitive variable.
  # ... resource configuration ...
}

# Solution: Use `nonsensitive` function (with caution!)
resource "null_resource" "example_fixed" {
  for_each = nonsensitive(var.db_passwords)
  # ... resource configuration ...
}

# Example 4: Dynamic Attribute Reference

variable "users" {
  default = {
    user1 = {
      id   = 123
      name = "Alice"
    }
    user2 = {
      id   = 456
      name = "Bob"
    }
  }
}

# Incorrect attribute reference:
resource "null_resource" "example" {
  for_each = var.users
  provisioner "local-exec" {
    # This will cause an error because `each.value.id` is not the correct path.
    command = "echo User ID: ${each.value.id}" 
  }
}

# Solution: Use correct attribute reference:
resource "null_resource" "example_fixed" {
  for_each = var.users
  provisioner "local-exec" {
    command = "echo User ID: ${each.value["id"]}" 
  }
}

Explanation:

  • Example 1: The initial code tries to use a list variable directly in for_each, which is incorrect. The solution converts the list to a map using the tomap function, making it compatible with for_each.
  • Example 2: Using aws_instance.example.id inside for_each is problematic because resource attributes are not known until apply time. The solution uses a data source to fetch the AMI IDs beforehand, allowing for_each to work correctly.
  • Example 3: Directly using a sensitive variable in for_each is prohibited. The solution demonstrates using the nonsensitive function to temporarily remove the sensitive flag, but emphasizes caution to avoid security risks.
  • Example 4: This example highlights the importance of correct attribute referencing within nested data structures. The initial code uses an incorrect path (each.value.id). The solution uses the correct syntax (each.value["id"]) to access the nested "id" attribute.

Remember to consult the Terraform documentation for your specific version to stay updated on any changes or best practices related to for_each.

Additional Notes

General Tips:

  • Error Messages are Key: Terraform's error messages for for_each issues are usually quite informative. Pay close attention to the specific message, as it often points directly to the problematic line and data type mismatch.
  • Simplify for Debugging: If you have a complex for_each loop, try breaking it down into smaller, simpler steps. This can help isolate the source of the error.
  • Use terraform console: The terraform console is a valuable tool for experimenting with expressions and data structures. You can use it to test your for_each input and see how Terraform evaluates it.
  • Consider Alternatives: While for_each is powerful, it's not always the best solution. If you're struggling with for_each, consider whether count or other approaches might be more suitable for your use case.

Deeper Dive into Specific Causes:

  • Resource Attributes: The reason you can't use resource attributes directly in for_each is that Terraform uses a "plan and apply" model. It needs to know the complete configuration, including the number of instances to create, before it can create or modify any resources.
  • Sensitive Variables: The restriction on using sensitive variables in for_each is a security measure. Directly using them could expose sensitive data in logs or state files. The nonsensitive function should be used with extreme caution and only when absolutely necessary.
  • Version Compatibility: Terraform has evolved over time, and the behavior of for_each has undergone some changes, particularly in how it handles errors and edge cases. Always refer to the documentation for your specific version.

Beyond the Basics:

  • Nested for_each: You can nest for_each loops to create more complex resource structures. However, this can quickly become difficult to manage. Consider using modules to encapsulate logic and improve readability.
  • Conditional Logic within for_each: While not directly supported, you can achieve conditional resource creation within a for_each loop using techniques like the splat operator (*) and conditional expressions within resource arguments.

By mastering the nuances of for_each and understanding its limitations, you can write cleaner, more efficient, and less error-prone Terraform code.

Summary

Cause Description Solution
Incorrect Input Type The for_each argument must be a map or a set of strings. Ensure the variable or expression used for for_each evaluates to a map or set of strings. Use functions like tomap or toset for conversion.
Resource Attributes in for_each Using attributes of a resource that are unknown before apply time within the for_each argument. Refactor code to use data sources or other mechanisms to retrieve information before the loop. Consider using count if the number of instances can be determined dynamically without relying on resource attributes.
Sensitive Variables Directly using a variable marked as sensitive within a for_each argument. Use the nonsensitive function to temporarily remove the sensitive flag, but exercise caution to avoid compromising data security.
Dynamic Attribute References Incorrectly referencing attributes within nested data structures in a for_each loop. Double-check expressions within the loop to ensure access to the correct data structure levels. Use each.key and each.value to access the current element's key and value.
Version Compatibility Potential changes in for_each behavior across different Terraform versions. Consult the Terraform documentation for your specific version to understand any nuances or limitations related to for_each.

General Troubleshooting:

  • Carefully examine the error message and code surrounding the for_each loop.
  • Pay close attention to data types, resource attribute usage, and sensitive variables.

By understanding the root cause of the "Invalid for_each argument" error, you can apply the appropriate solution and effectively leverage the power of for_each in your Terraform configurations.

Conclusion

Mastering the use of for_each in Terraform is crucial for efficient and scalable infrastructure provisioning. By understanding the common causes of the "Invalid for_each argument" error, such as incorrect input types, improper use of resource attributes, and handling of sensitive variables, you can avoid configuration pitfalls. Remember to validate data types, utilize data sources effectively, and exercise caution with sensitive information. By adhering to best practices and referring to the Terraform documentation for your specific version, you can unlock the full potential of for_each and streamline your infrastructure as code deployments.

References

Were You Able to Follow the Instructions?

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

Related posts

Terraform: Check if String Contains Substring in Interpolation

Learn how to efficiently check for substrings within strings using Terraform interpolation to enhance your infrastructure-as-code capabilities.

Terraform: Pass Resources as Module Variables?

Learn how to effectively manage your infrastructure by passing Terraform resources as variables into modules for improved code organization and reusability.

Terraform: Get Object From List of Objects

Learn how to efficiently extract specific objects from lists in Terraform using powerful built-in functions and simplify your infrastructure management.