Terraform Resource Meta-Argument for_each Chaining

Step-01: Introduction

• Understand about Meta-Argument for_each
• Implement for_each Chaining
• Because a resource using for_each appears as a map of objects or set of strings when used in expressions elsewhere, you can directly use one resource as the for_each of another in situations where there is a one-to-one relationship between two sets of objects.
• In our case, we will use the azurerm_network_interface.myvmnic resource directly in azurerm_linux_virtual_machine.mylinuxvm Resource.

Step-02: Review Terarform Manifests

• Copy the terraform-manifests from Section 11-Meta-Argument-count\terraform-manifests-v2 and re-implement this usecase using for_each.
• Also apply for_each chaining concept
• c1-versions.tf
• c2-resource-group.tf
• c3-virtual-machine.tf: Changes for Public IP and Network Interface Resources with for_each same argument in both resources.
• c4-linux-virtual-machine.tf: for_each using Network Interface Resource

Step-03: c3-virtual-machine.tf - Azure Public IP Resource

# Create Azure Public IP Address
resource "azurerm_public_ip" "mypublicip" {
  #count = 2  
  for_each = toset(["vm1", "vm2"])
  name                = "mypublicip-${each.key}"
  resource_group_name = azurerm_resource_group.myrg.name
  location            = azurerm_resource_group.myrg.location
  allocation_method   = "Static"
  domain_name_label = "app1-${each.key}-${random_string.myrandom.id}"  
}

Step-04: c3-virtual-machine.tf - Azure Network Interface Resource

# Create Network Interface
resource "azurerm_network_interface" "myvmnic" {
  #count = 2
  for_each = toset(["vm1", "vm2"])  
  name                = "vmnic-${each.key}"
  location            = azurerm_resource_group.myrg.location
  resource_group_name = azurerm_resource_group.myrg.name

  ip_configuration {
    name                          = "internal"
    subnet_id                     = azurerm_subnet.mysubnet.id
    private_ip_address_allocation = "Dynamic"
    #public_ip_address_id = element(azurerm_public_ip.mypublicip.*.id, each.key)
    public_ip_address_id = azurerm_public_ip.mypublicip[each.key].id
  }
}

Step-05: c4-linux-virtual-machine.tf

# Resource: Azure Linux Virtual Machine

resource "azurerm_linux_virtual_machine" "mylinuxvm" {
  #count = 2
  #for_each = toset(["vm1", "vm2"])  
  for_each = azurerm_network_interface.myvmnic #for_each chaining
  # Define Explicit Dependency that if VM Nic exists, then only create VM
  depends_on = [ azurerm_network_interface.myvmnic ]
  name                = "mylinuxvm-${each.key}"
  computer_name       = "devlinux-${each.key}" # Hostname of the VM
  resource_group_name = azurerm_resource_group.myrg.name
  location            = azurerm_resource_group.myrg.location
  size                = "Standard_DS1_v2"
  admin_username      = "azureuser"
  #network_interface_ids = [element(azurerm_network_interface.myvmnic.*.id, each.key)]
  network_interface_ids = [azurerm_network_interface.myvmnic[each.key].id]
  admin_ssh_key {
    username   = "azureuser"
    public_key = file("${path.module}/ssh-keys/terraform-azure.pub")
  }
  os_disk {
    name = "osdisk${each.key}"
    caching              = "ReadWrite"
    storage_account_type = "Standard_LRS"
    #disk_size_gb = 20
  }
  source_image_reference {
    publisher = "RedHat"
    offer     = "RHEL"
    sku       = "83-gen2"
    version   = "latest"
  }
  custom_data = filebase64("${path.module}/app-scripts/app1-cloud-init.txt")
}

Step-06: Observe Linux Virtual Machine for_each Argument

• In azurerm_linux_virtual_machine resource we are using the for_each argument by referring to Network Interface Resource named azurerm_network_interface.myvmnic. This is called for_each chaining.

# for_each chaining
  for_each = azurerm_network_interface.myvmnic 

Step-07: Execute Terraform Commands

# Terraform Init
terraform init

# Terraform Validate
terraform validate

# Terraform Format
terraform fmt

# Terraform Plan
terraform plan
Observation: 
1) 2 Public IP, 2 Network Interface and 2 Linux VM Resources will be generated in plan
2) Review Resource Names ResourceType.ResourceLocalName[each.key]
3) Review Resource Names

# Terarform Apply
terraform apply

# Verify
1. Azure Resource Group
2. Azure Virtual Network
3. Azure Subnet
4. Azure Public IP - 2 Resources created as specified in count
5. Azure Network Interface - 2 Resources created as specified in count
6. Azure Linux Virtual Machine - - 2 Resources created as specified in count

# Access Application
http://<PUBLIC_IP-1>
http://<PUBLIC_IP-2>

Step-08: Destroy Terraform Resources

# Destroy Terraform Resources
terraform destroy

# Remove Terraform Files
rm -rf .terraform*
rm -rf terraform.tfstate*