# How to Dynamically Retrieve EC2 Instances IDs/Private IPs from ASG in Terraform to Trigger Ansible Tower Jobs?

**URL:** <https://discuss.hashicorp.com/t/how-to-dynamically-retrieve-ec2-instances-ids-private-ips-from-asg-in-terraform-to-trigger-ansible-tower-jobs/70983>\
**Category:** Terraform\
**Tags:** hcp-terraform\
**Created:** [October 28, 2024, 5:37pm UTC](https://discuss.hashicorp.com/t/how-to-dynamically-retrieve-ec2-instances-ids-private-ips-from-asg-in-terraform-to-trigger-ansible-tower-jobs/70983 "2024-10-28T17:37:35Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![yogeshselvarajan](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/yogeshselvarajan/32/43633_2.png) [@yogeshselvarajan](https://discuss.hashicorp.com/u/yogeshselvarajan)\
**Post date:** [October 28, 2024, 5:37pm UTC](https://discuss.hashicorp.com/t/how-to-dynamically-retrieve-ec2-instances-ids-private-ips-from-asg-in-terraform-to-trigger-ansible-tower-jobs/70983/1 "2024-10-28T17:37:35Z")

</div>

Hello,

I am developing a Terraform module to automate the provisioning of EC2 instances using AWS Auto Scaling Groups (ASG) and to trigger Ansible Tower jobs for each dynamically created instance. The Ansible jobs enable compilance for the instances.

### Problem

I am facing an issue where, during the Terraform `plan`, I get the following error:

```auto
Error: Invalid count argument
   on ../main.tf line 9, in resource "null_resource" "asg_instance_provisioning":
    9: count = length(data.aws_instances.auto_scaling_instances.ids)
   The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created.

```

## # Current Setup:

**1. ASG Configuration:** I am using the `aws_autoscaling_group` resource to create EC2 instances dynamically. The ASG assigns a tag with the group name to the instances.

**2. Data Block to Fetch Instances:** I’m using the following `data` block to fetch the EC2 instance details:

```auto
data "aws_instances" "asg_instances" {
  filter {
    name = "tag:aws:autoscaling:groupName"
    values = [aws_autoscaling_group.ec2_asg.name]
  }
  depends_on = [aws_autoscaling_group.ec2_asg]
}

```

**3. Local Variables for Ansible Tower** : I generate custom names for the instances:

```auto
locals {
  vm_name = [for id in data.aws_instances.asg_instances.ids : lower(format("vm-%s", id))]
  vm_computer_name = [for name in local.vm_name : substr(name, 0, 64)]
  vm_computer_object_name = [for name in local.vm_computer_name : substr(name, 0, 15)]
}

```

**4 : Null Resource for Ansible Tower Jobs:** The `null_resource` triggers the Ansible Tower job to register EC2 instances:

```auto
resource "null_resource" "asg_instance_provisioning" {
  count = length(data.aws_instances.auto_scaling_instances.ids)

  triggers = {
    auth_token = var.auth_token
    inventory_id = var.inventory_id
    instance_ip = data.aws_instances.auto_scaling_instances.private_ips[count.index]
    instance_name = lower(local.instance_names[count.index])
    domain_name = lower(var.domain_name)
  }

  provisioner "local-exec" {
    environment = {
      AUTH_TOKEN = self.triggers.auth_token
    }
    command = "cli-tool job launch --token $AUTH_TOKEN --inventory ${self.triggers.inventory_id} --job-template=123 --extra-vars 'instance_ip=${self.triggers.instance_ip} domain=${self.triggers.domain_name} instance_name=${self.triggers.instance_name}'"
  }
}

```

### Issue:

Terraform throws the error during the `plan` phase because the `count` value relies on attributes that aren’t available until the `apply` phase. This is because there isn’t a native Terraform attribute from the ASG resource to directly get the EC2 instance IDs and private IPs at the planning stage.

### Question

How can I dynamically retrieve the EC2 instance IDs and private IPs from the Auto Scaling Group in a way that can be used during the `plan` phase, allowing the `null_resource` to trigger Ansible Tower jobs?

I’m looking for any workaround or alternate approach to handle this. I need to ensure that my `null_resource` triggers the Ansible jobs for all the EC2 instances created by ASG without running into this issue.

Any help or suggestions would be greatly appreciated!

---

<div class="post-metadata">

**Author:** ![system](https://global.discourse-cdn.com/hashicorp/original/1X/79bf76adfd92a75631996933098f9ce29d37abf3.svg) [@system](https://discuss.hashicorp.com/u/system)\
**Post date:** [April 26, 2025, 5:37pm UTC](https://discuss.hashicorp.com/t/how-to-dynamically-retrieve-ec2-instances-ids-private-ips-from-asg-in-terraform-to-trigger-ansible-tower-jobs/70983/2 "2025-04-26T17:37:37Z")

</div>

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.