Chapter 10 - depends_on

What if one resource needs another to exist before it's created?

Meta Arguments

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• depends on - to handle hidden module dependencies
• count - to create multiple instances based on a count.
• foreach - to create multiple instances according to a map or set of strings
• provider - to select a non default provider config
• lifecycle - resource behavior can be altered using this block
• provisioners and connections (not a true metadata) - - extra action after resource creation -ie, install something on the resource

Depends on

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Available in all module blocks and in all resource blocks, regardless of resource type.

• must be a list of references to other resources or child modules
• arbitrary expressions are not allowed
• This is a last resort. If you are using a lot of depends_on, you're probably doing something you shouldn't be.

Small depends_on demo

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1. Plan and apply this code:

# Create Virtual Network
resource "azurerm_virtual_network" "myvnet" {
  name                = "myvnet-1"
  address_space       = ["10.0.0.0/16"]
  location            = azurerm_resource_group.myrg.location
  resource_group_name = azurerm_resource_group.myrg.name
}

# Create Subnet
resource "azurerm_subnet" "mysubnet" {
  name                 = "mysubnet-1"
  resource_group_name  = azurerm_resource_group.myrg.name
  virtual_network_name = azurerm_virtual_network.myvnet.name
  address_prefixes     = ["10.0.2.0/24"]
}

# Create Public IP Address
resource "azurerm_public_ip" "mypublicip" {
  name                = "mypublicip-1"
  resource_group_name = azurerm_resource_group.myrg.name
  location            = azurerm_resource_group.myrg.location
  allocation_method   = "Static"
  domain_name_label   = "app1-vm-${random_string.myrandom.id}"
  tags = {
    environment = "Dev"
  }

}

# Create Network Interface
resource "azurerm_network_interface" "myvmnic" {
  name                = "vmnic"
  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          = azurerm_public_ip.mypublicip.id
  }
}

2. Notice that the public IP isn't created last
   
3. add the depends on block for the public IP resource
4. Destroy the previous infra, and run a new plan and apply.
5. Notice that the public IP is created after the vnet and subnet