terraform

Best Practice 3

Implicit dependencies should be used whenever possible (see this article from terraform.io website for more information). In this article we will perform the following action with implicit dependencies and with explicit dependencies :

Create 1 load balancer
Create 2 network interfaces and link them to a load balancer

Prerequisite

Item	Description
Azure Subscription	An Azure subscription id
Resource Group	An Azure resource group is available
Storage Account	An Azure storage account is available and is located in the upper resource group, it contains a container named `tfstate`
Service Principal	An Azure service principal is available and has the `owner` privilege on the upper resource group
Terraform file	Clone this repository and fill in the following files with the upper prerequisite items : Variable used for the Terraform `init` : secret/backend-jdld.json Variable used for the Terraform `plan` and `apply` : main.tf & main-jdld.tfvars & secret/main-jdld.json

What should we do?

We will create the upper mentioned element using remote backend (see the previous article BestPractice-1 for more information about remote backend).

Review the code main.tf, as illustrated in the following bracket, the implicit and the explicit methods are highlighted.

module "Az-Vm-Demo" {
  source                  = "JamesDLD/Az-Vm/azurerm"
  version                 = "0.1.2"
  sa_bootdiag_storage_uri = data.azurerm_storage_account.bp3.primary_blob_endpoint #(Mandatory)
  subnets_ids             = module.Az-VirtualNetwork-Demo.subnet_ids               #(Mandatory)
  linux_vms               = {}                                                     #(Mandatory)
  windows_vms             = var.vms                                                #(Mandatory)

  #This an implicit dependency
  internal_lb_backend_ids = module.Create-AzureRmLoadBalancer-Demo.lb_backend_ids

  #This an explicit dependency                 
  #internal_lb_backend_ids = ["/subscriptions/${var.subscription_id}/resourceGroups/infr-jdld-noprd-rg1/providers/Microsoft.Network/loadBalancers/demo-bp3-internal-lb1/backendAddressPools/demo-bp3-internal-bckpool1"]

  vm_resource_group_name          = data.azurerm_resource_group.bp3.name
  vm_prefix                       = "bp3"                               #(Optional)
  windows_storage_image_reference = var.windows_storage_image_reference #(Optional)
  admin_username                  = var.app_admin
  admin_password                  = var.pass
  vm_additional_tags              = { bp = "3" } #(Optional)
}

1. Usage

This step ensures that Terraform has all the prerequisites to build your template in Azure.

terraform init -backend-config="secret/backend-jdld.json" -reconfigure

The Terraform plan command is used to create an execution plan. This step compares the requested resources to the state information saved by Terraform and then gives as an output the planned execution. Resources are not created in Azure.

terraform plan -var-file="secret/main-jdld.json" -var-file="variable/main-jdld.tfvars"

If all is ok with the proposal you can now apply the configuration with both methods (implicit and explicit) to track the impact.

terraform apply -var-file="secret/main-jdld.json" -var-file="variable/main-jdld.tfvars"

We will now destroy what we have done with both methods (implicit and explicit) to track the impact.

terraform destroy -var-file="secret/main-jdld.json" -var-file="variable/main-jdld.tfvars"

2. Analysis

Description	Screenshot
Our 2 network interfaces are linked to the Load Balancer
When using implicit dependencies all is working like a charm
When using explicit dependencies we receive error(s) because some resources doesn’t wait for other to be created (a workaround consists in using the variable `depend on` but this will still cause error when you will proceed Terraform `destroy`)

See you!

JamesDLD