How to Aggregate EC2 Instance IDs from a Module and Attach to Target Groups in AWS ALB using Terraform

Problem Description:

I’m working on a Terraform setup where I have multiple EC2 instances (for Splunk services) managed using a custom module (splunk_instances), and I’m trying to attach these instances to specific target groups in an AWS Application Load Balancer (ALB).

In my root main.tf, I’m using a module that creates several Splunk instances like the search head (splunk_sh), indexers, and others. These instances are created using a custom EC2 module. I want to collect the instance IDs from this module and pass them to the ALB module to attach each instance to its corresponding target group.

My current setup:

Root main.tf (using the splunk_instances module):

module "splunk_instances" {
  source = "./modules/ec2" # Path to the module

  for_each = {
    "splunk_sh"       = "Search Head"
    "splunk_idx1"     = "Indexer 1"
    "splunk_idx2"     = "Indexer 2"
    "splunk_idx3"     = "Indexer 3"
    "splunk_cm"       = "Cluster Manager"
    "splunk_lm_ds_mc" = "License Manager / Deployment Server / Monitoring Console"
  }

  instance_name        = each.value
  instance_type        = var.instance_types[each.key]
  ami_id               = var.ami_id
  key_name             = var.key_name
  network_interface_id = aws_network_interface.splunk_eni[each.key].id
  # Other variables...
}

# ALB configuration
module "load_balancer" {
  source = "./modules/alb"
  instance_ids = module.splunk_instances.instance_ids # Reference to instance IDs from splunk_instances module
  # Other variables...
}

modules/ec2/outputs.tf:

output "instance_id" {
  value = aws_instance.this.id
}

Expected Output:

In my root module’s outputs, I want to aggregate all instance IDs:

output "instance_ids" {
  value = {
    for instance_key, instance_module in module.splunk_instances :
    instance_key => instance_module.instance_id
  }
}

Issue:

Terraform throws an error:

Error: Unsupported attribute │ │ on main.tf line 55, in module "load_balancer": │ 55: instance_ids = module.splunk_instances.instance_ids │ ├──────────────── │ │ module.splunk_instances is object with 6 attributes │ │ This object does not have an attribute named "instance_ids".

It seems the instance_id output is not being correctly passed from each EC2 instance created by the splunk_instances module. I’m unsure how to properly aggregate and reference these instance IDs in the ALB target group attachment.

What I’ve Tried:

• Ensured that the instance_id output is properly defined in the ec2 module.
• Tried aggregating the instance_id outputs in the root outputs.tf.
• Verified that terraform plan shows the instance IDs as outputs, but I can’t reference them as expected when attaching them to the ALB target groups.

Desired Outcome:

I want to be able to:

• Aggregate instance IDs from the splunk_instances module.
• Pass these IDs to the ALB target group attachments in the load_balancer module.

Question:

How can I correctly aggregate the instance_id outputs from the splunk_instances module and use them in the ALB module to attach EC2 instances to target groups?

Since the EC2 module, when called, has created instances based on the values from the for_each meta-argument, this means any outputs should use the key-value pairs:

output "instance_ids" {
  value = values(module.splunk_instances)[*].instance_id
}

Here the built-in values function is used with all the keys and the instance_id output. You can conclude that you have to use the keys to reference the output based on the error:

module.splunk_instances is object with 6 attributes

because six Splunk instances are created when you call the module with for_each.