Terraform Failing with Invalid for_each Argument / The Given "for_each" Argument Value is Unsuitable

TLDR

The "Invalid for_each argument" error in Terraform occurs when the value provided to for_each is not a valid map or set. Learn how to troubleshoot and resolve this issue.

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Understanding the Error

The for_each meta-argument in Terraform requires a map or a set of strings. If the provided value does not meet these criteria, Terraform will throw an error like:

Error: Invalid for_each argument

The given "for_each" argument value is unsuitable.

This error often occurs due to:

• Using a list instead of a set.
• Providing a value that is not iterable.
• Incorrect data types.

Common Scenarios and Fixes

As you work with Terraform, you may encounter the "Invalid for_each argument" error. This typically happens when the value provided to for_each is not a valid map or set. Here are some common scenarios and how to fix them:

1. Using a List Instead of a Set

Terraform requires a set or map for for_each. If you use a list, you need to convert it to a set:

variable "instances" {
  default = ["web", "db", "cache"]
}

resource "aws_instance" "example" {
  for_each = toset(var.instances)

  ami           = "ami-12345678"
  instance_type = "t2.micro"
  tags = {
    Name = each.key
  }
}

Here, toset converts the list to a set, making it compatible with for_each.

2. Using a Map for Key-Value Pairs

If you need to iterate over key-value pairs, use a map:

variable "instance_types" {
  default = {
    web   = "t2.micro",
    db    = "t2.small",
    cache = "t2.medium"
  }
}

resource "aws_instance" "example" {
  for_each = var.instance_types

  ami           = "ami-12345678"
  instance_type = each.value
  tags = {
    Name = each.key
  }
}

3. Handling Null or Empty Values

If the value for for_each is null or empty, Terraform will throw an error. Use a default value to handle this:

variable "instances" {
  default = null
}

resource "aws_instance" "example" {
  for_each = var.instances != null ? toset(var.instances) : {}

  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

4. Debugging Data Types

Use terraform console to inspect the data type of your variable:

> var.instances
> toset(var.instances)

This helps identify issues with the input data.

Best Practices

• Validate Input Data: Use validation blocks in variables to ensure correct data types.
• Use Default Values: Provide sensible defaults to avoid null or empty values.
• Test Iterations: Use terraform console to test your for_each logic before applying changes.

By understanding and addressing the root cause of the "Invalid for_each argument" error, you can create more robust and error-free Terraform configurations.