Terraform: Reference for_each Resources in Other Resources

• Introduction
• Step-by-Step Guide
• Code Example
• Additional Notes
• Summary
• Conclusion
• References

In Terraform, when you utilize the for_each construct to generate multiple resources, you obtain a map of resources. The keys of this map are determined by the expression you define within for_each. To subsequently reference these resources, you need to employ the corresponding key to access the specific resource instance.

When you use for_each to create multiple resources in Terraform, you get a map of resources where the keys are defined by your for_each expression. To reference these resources later, you need to use the corresponding key to access the specific resource instance.

Let's say you have an aws_route53_zone resource created with for_each:

resource "aws_route53_zone" "example" {
 for_each = {
   "example.com" = {
     tags = {
       Environment = "prod"
     }
   }
   "example.net" = {
     tags = {
       Environment = "dev"
     }
   }
 }
  name = each.key
  tags = each.value.tags
}

This creates two hosted zones, one for "example.com" and one for "example.net".

Now, if you want to reference the zone ID of "example.com" in another resource, you would use:

aws_route53_zone.example["example.com"].zone_id

Here's how it works:

1. aws_route53_zone.example refers to the entire map of aws_route53_zone resources created by the for_each.
2. ["example.com"] accesses the specific resource instance with the key "example.com".
3. .zone_id retrieves the zone_id attribute of that specific hosted zone.

Remember to replace "example.com" with the actual key you need to reference. This approach ensures you are targeting the correct resource instance created by your for_each loop.

This Terraform code defines infrastructure on AWS. It creates two Route53 hosted zones, "example.com" and "example.net", each with specific tags. It then defines an S3 bucket configured for website hosting, using the "example.com" zone ID for website redirection rules. Finally, it outputs the zone IDs of all created zones. This demonstrates creating multiple resources with iteration and referencing specific instances using keys.

# Configure the AWS Provider
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

# Create multiple Route53 zones using for_each
resource "aws_route53_zone" "example" {
 for_each = {
   "example.com" = {
     tags = {
       Environment = "prod"
     }
   }
   "example.net" = {
     tags = {
       Environment = "dev"
     }
   }
 }
  name = each.key
  tags = each.value.tags
}

# Create an S3 bucket and use the zone ID of "example.com"
resource "aws_s3_bucket" "example" {
  bucket = "my-bucket-using-zone-id"

  # Access the zone ID of the "example.com" zone
  website {
    index_document = "index.html"
    error_document = "error.html"

    # Reference the specific zone ID using the key
    routing_rules = <<EOF
[{
  "Condition": {
    "KeyPrefixEquals": "docs/"
  },
  "Redirect": {
    "HostName": "docs.example.com",
    "ReplaceKeyPrefixWith": "",
    "Protocol": "https"
  }
}]
EOF
  }
}

# Output the zone IDs of all created zones
output "zone_ids" {
  value = {
    for k, zone in aws_route53_zone.example : k => zone.zone_id
  }
}

Explanation:

1. Creating Route53 Zones: The aws_route53_zone.example resource uses for_each to create two zones: "example.com" and "example.net".
2. Referencing Zone ID: The aws_s3_bucket.example resource demonstrates how to reference the zone ID of "example.com" using aws_route53_zone.example["example.com"].zone_id. This retrieves the zone_id attribute of the specific zone with the key "example.com".
3. Outputting Zone IDs: The output "zone_ids" block iterates through the created zones and outputs their respective zone IDs.

This example illustrates how to create multiple resources with for_each and then reference specific instances using their corresponding keys. Remember to adapt the code to your specific use case and replace placeholders with your desired values.

Understanding for_each and Resource Maps:

• Data Structure: The for_each construct doesn't just create multiple resources; it organizes them into a map. This map is crucial for referencing individual instances later.
• Key Importance: The keys you define in your for_each expression are not arbitrary; they become the identifiers for accessing specific resources within the created map.
• Dynamic References: The real power of this approach lies in its dynamic nature. You can use variables, locals, or outputs within your for_each keys, making your configurations adaptable and reusable.

Best Practices:

• Meaningful Keys: Choose descriptive keys that clearly reflect the purpose or configuration of the corresponding resource. This enhances code readability and maintainability.
• Consistent Naming: Maintain consistency in how you name your resources created with for_each. For instance, if you use aws_route53_zone.example, stick with a similar pattern for other resource types.
• Comments for Clarity: When referencing resources created with for_each, consider adding comments to explain which specific instance you are targeting and why.

Beyond the Basics:

• Nested for_each: You can nest for_each loops to create more complex resource structures, but be mindful of readability and complexity.
• Conditional Logic: Combine for_each with if statements within your resource blocks to conditionally create or configure resources based on specific keys or values.
• Modules and for_each: Leverage for_each when calling modules to deploy and manage multiple instances of a module with varying configurations.

Troubleshooting:

• Key Errors: The most common errors arise from incorrect key references. Double-check your keys for typos and ensure they match the exact values used in your for_each expression.
• Resource Dependencies: Pay close attention to resource dependencies when using for_each. If one resource relies on another created within the same loop, ensure the dependencies are explicitly defined.

By mastering the concepts of for_each, resource maps, and key-based referencing, you unlock the true potential of Terraform for managing complex and dynamic infrastructure as code.

Explanation:

When using for_each to create multiple resources, Terraform generates a map of resources. Each key in this map corresponds to a unique resource instance. To reference a specific resource, use its corresponding key within square brackets [] after the resource name. Then, access the desired attribute using the dot notation.

In conclusion, mastering the use of for_each in Terraform is crucial for efficiently managing multiple similar resources. By understanding how to reference individual instances within the generated resource map using their corresponding keys, you can create dynamic and adaptable infrastructure configurations. Remember to employ meaningful keys, maintain consistent naming conventions, and leverage comments for clarity. As you delve deeper into Terraform, explore advanced techniques like nested for_each loops and conditional resource creation to further enhance your infrastructure-as-code capabilities. By adhering to best practices and understanding the nuances of for_each, you can effectively harness the power of Terraform to manage complex and scalable infrastructure with ease.

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