Reference to other module resource in Terraform

I have a hierarchy like this in my Terraform Cloud git project:

├── aws
│   ├── flavors
│   │   └── main.tf
│   ├── main.tf
│   ├── security-rules
│   │   └── sec-rule1
│   │       └── main.tf
│   └── vms
│   │   └── vm1
│   │       └── main.tf
└── main.tf

All main main.tf files contain module definitions with child folders:

/main.tf:

terraform {
  required_version = "~> 0.12.0"

  backend "remote" {
    hostname = "app.terraform.io"    
    organization = "foo"

    workspaces {
      name = "bar"
    }
  }
  required_providers {
    openstack = "~> 1.24.0"
  }
}

module "aws" {
  source = "./aws"
}

/aws/main.tf:

module "security-rules" {
  source = "./security-rules"
}

module "flavors" {
  source = "./flavors"
}

module "vms" {
  source = "./vms"
}

/aws/security-rules/main-tf:

module "sec-rule1" {
  source = "./sec-rule1"
}

/aws/vms/main-tf:

module "vm1" {
  source = "./vm1"
}

Then I have this security rule defined.

/aws/security-rules/sec-rule1/main-tf:

resource "openstack_compute_secgroup_v2" "sec-rule1" {
  name        = "sec-rule1"
  description = "Allow web port"
  rule {
    from_port   = 80
    to_port     = 80
    ip_protocol = "tcp"
    cidr        = "0.0.0.0/0"
  }
  lifecycle {
            prevent_destroy = false
    }
}

And I want to reference it from one or more VMs, but I don't know how to reference by resource ID (or name). I use plain names instead of reference.

/aws/vms/vm1/main-tf:

resource "openstack_blockstorage_volume_v3" "vm1_volume" {
  name     = "vm1_volume"
  size     = 30
  image_id = "foo-bar"
}

resource "openstack_compute_instance_v2" "vm1_instance" {
  name        = "vm1_instance"
  flavor_name = "foo-bar"
  key_pair    = "foo-bar keypair"
  image_name  = "Ubuntu Server 18.04 LTS Bionic"
  block_device {
    uuid                  = "${openstack_blockstorage_volume_v3.vm1_volume.id}"
    source_type           = "volume"
    destination_type      = "volume"
    boot_index            = 0
    delete_on_termination = false
  }

  network {
    name = "SEG-tenant-net"
  }

  security_groups = ["default", "sec-rule1"]
  config_drive    = true
}

resource "openstack_networking_floatingip_v2" "vm1_fip" {
  pool = "foo-bar"
}

resource "openstack_compute_floatingip_associate_v2" "vm1_fip" {
  floating_ip = "${openstack_networking_floatingip_v2.vm1_fip.address}"
  instance_id = "${openstack_compute_instance_v2.vm1_instance.id}"
}

I want to use security-rules (and more stuff) referencing by name or ID, because it would be more consistent. Besides when I create a new security rule and, at the same time, a VM, Terraform OpenStack provider plans it without error, but when applying it, an error is produced because VM is created first and it doesn't find not-yet created new security rule.

How can I do this?

You should make an output of sec_rule_allow_web_name for sec-rule1 and security-rules/ modules, then set the output of the security-rules/ module as an input of the vm1 and vms modules. This way you can keep a dependency of the vm1 module with the output of security_rules which is called Dependency Inversion.

# ./security-rules/<example>/outputs.tf

output "sec_rule_allow_web_name" {
  value = "<some-resource-to-output>"
}

# ./vms/variables.tf

variable "security_rule_name" {}

Provided the outputs and inputs are defined in the correct modules.

# /aws/main.tf

# best practice to use underscores instead of dashes in names
# so security-roles/ directory is now called security_rules
module "security_rules" {
  source = "./security-rules"
}

module "flavors" {
  source = "./flavors"
}

module "vms" {
  source = "./vms"

  security_rule_name = module.security_rules.sec_rule_allow_web_name
}