Terraform Null Resource

Step-01: Introduction¶

Understand about Null Provider
Understand about Null Resource
Understand about Time Provider
Usecase: Force a resource to update based on a changed null_resource
Create time_sleep resource to wait for 90 seconds after Azure Linux VM Instance creation
Create Null resource with required provisioners
File Provisioner: Copy apps/app1 folder to /tmp
Remote Exec Provisioner: Copy app1 folder from /tmp to /var/www/htnl
Over the process we will learn about
null_resource
time_sleep resource
We will also learn how to Force a resource to update based on a changed null_resource using timestamp function and triggers in null_resource

Step-02: Define null provider in Terraform Settings Block¶

Update null provider info listed below in c1-versions.tf

    null = {
      source = "hashicorp/null"
      version = ">= 3.0.0"
    }

Step-03: Define Time Provider in Terraform Settings Block¶

Update time provider info listed below in c1-versions.tf

    time = {
      source = "hashicorp/time"
      version = ">= 0.6.0"
    }

Step-04: Create / Review the c8-null-resource.tf terraform configuration¶

Step-04-01: Create Time Sleep Resource¶

This resource will wait for 90 seconds after VM Instance creation.
This wait time will give VM Instance to provision the Apache Webserver and create all its relevant folders

Primarily if we want to copy static content we need Apache webserver static folder /var/www/html

# Wait for 90 seconds after creating the above Azure Virtual Machine Instance 
resource "time_sleep" "wait_90_seconds" {
  depends_on = [azurerm_linux_virtual_machine.mylinuxvm]
  create_duration = "90s"
}

Step-04-02: Create Null Resource¶

Create Null resource with triggers with timestamp() function which will trigger for every terraform apply
This Null Resource will help us to sync the static content from our local folder to VM Instnace as and when required.
Also only changes will applied using only null_resource when terraform apply is run. In other words, when static content changes, how will we sync those changes to VM Instance using terraform - This is one simple solution.
Primarily the focus here is to learn the following
null_resource
null_resource trigger
How trigger works based on timestamp() function ?

Provisioners in Null Resource

# Terraform NULL RESOURCE
# Sync App1 Static Content to Webserver using Provisioners
resource "null_resource" "sync_app1_static" {
  depends_on = [ time_sleep.wait_90_seconds ]
  triggers = {
    always-update =  timestamp()
  }

  # Connection Block for Provisioners to connect to Azure VM Instance
  connection {
    type = "ssh"
    host = azurerm_linux_virtual_machine.mylinuxvm.public_ip_address 
    user = azurerm_linux_virtual_machine.mylinuxvm.admin_username
    private_key = file("${path.module}/ssh-keys/terraform-azure.pem")
  }  

 # Copies the app1 folder to /tmp
  provisioner "file" {
    source      = "apps/app1"
    destination = "/tmp"
  }

# Copies the /tmp/app1 folder to Apache Webserver /var/www/html directory
  provisioner "remote-exec" {
    inline = [
      "sudo cp -r /tmp/app1 /var/www/html"
    ]
  }
}

Step-05: Execute Terraform Commands¶

# Terraform Initialize
terraform init

# Terraform Validate
terraform validate

# Terraform Format
terraform fmt

# Terraform Plan
terraform plan

# Terraform Apply
terraform apply -auto-approve

# Verify
ssh -i ssh-keys/terraform-azure.pem azureuser@<PUBLIC-IP>
ls -lrt /tmp
ls -lrt /tmp/app1
ls -lrt /var/www/html
ls -lrt /var/www/html/app1
http://<public-ip>/app1/app1-file1.html
http://<public-ip>/app1/app1-file2.html

Step-06: Create new file locally in app1 folder¶

Create a new file named app1-file3.html
Also updated app1-file1.html with some additional info
file3.html
```
<h1>>App1 File3</h1
```
file1.html
```
<h1>>App1 File1 - Updated</h1
```

Sample terraform plan Output

# Terraform Plan Output
Terraform used the selected providers to generate the following execution plan. Resource actions
are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

  # null_resource.sync_app1_static must be replaced
-/+ resource "null_resource" "sync_app1_static" {
      ~ id       = "256904776759333943" -> (known after apply)
      ~ triggers = {
          - "always-update" = "2021-06-14T05:44:33Z"
        } -> (known after apply) # forces replacement
    }

Plan: 1 to add, 0 to change, 1 to destroy.

───────────────────────────────────────────────────────────────

Step-07: Execute Terraform plan and apply commands¶

# Terraform Plan
terraform plan
Observation: You should see changes for "null_resource.sync_app1_static" because trigger will have new timestamp when you fired the terraform plan command

# Terraform Apply
terraform apply -auto-approve

# Verify
ssh -i ssh-keys/terraform-azure.pem azureuser@<PUBLIC-IP>
ls -lrt /tmp
ls -lrt /tmp/app1
ls -lrt /var/www/html
ls -lrt /var/www/html/app1
http://<public-ip>/app1/app1-file1.html
http://<public-ip>/app1/app1-file3.html

Step-08: Clean-Up Resources & local working directory¶

# Terraform Destroy
terraform destroy -auto-approve

# Delete Terraform files 
rm -rf .terraform*
rm -rf terraform.tfstate*

Step-09: Roll back to Demo State¶

# Change-1: Delete app1-file3.html in apps/app1 folder
# Change-2: app1-file1.html - Remove updated text