LAB: Terraform Dependencies (Implicit vs Explicit)

📁 Project Structure

terraform-dependency-lab/ │ ├── main.tf ├── variables.tf ├── terraform.tfvars ├── outputs.tf └── providers.tf

terraform-dependency-lab/ │ ├── main.tf ├── variables.tf ├── terraform.tfvars ├── outputs.tf └── providers.tf

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🔹 1. providers.tf

terraform {  required_version = ">= 1.5.0"

terraform {  required_version = ">= 1.5.0"

required_providers { aws = { source = "hashicorp/aws" version = "~> 5.0" } } }

provider "aws" { region = var.aws_region }`

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🔹 2. variables.tf (NO HARDCODING)

variable "aws_region" {  description = "AWS region"  type = string }

variable "aws_region" {  description = "AWS region"  type = string }

variable "project_name" { description = "Project name" type = string }

variable "instance_type" { description = "EC2 instance type" type = string }

variable "common_tags" { description = "Common tags" type = map(string) }`

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🔹 3. terraform.tfvars

aws_region = "us-east-2" project_name = "dep-lab" instance_type = "t2.micro"

aws_region = "us-east-2" project_name = "dep-lab" instance_type = "t2.micro"

common_tags = { Owner = "Student" Lab = "Dependencies" }`

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🔹 4. main.tf

🔸 Part 1: Implicit Dependency

resource "aws_vpc" "main" {  cidr_block = "10.0.0.0/16"

resource "aws_vpc" "main" {  cidr_block = "10.0.0.0/16"

tags = merge(var.common_tags, { Name = "${var.project_name}-vpc" }) }

resource "aws_subnet" "subnet" { vpc_id = aws_vpc.main.id # ✅ IMPLICIT DEPENDENCY cidr_block = "10.0.1.0/24"

tags = merge(var.common_tags, { Name = "${var.project_name}-subnet" }) }`

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👉 Explanation:

• aws_subnet depends on aws_vpc automatically

• No depends_on needed

🔸 Part 2: Explicit Dependency (Real Scenario)

resource "aws_security_group" "sg" {  name = "${var.project_name}-sg"  vpc_id = aws_vpc.main.id

resource "aws_security_group" "sg" {  name = "${var.project_name}-sg"  vpc_id = aws_vpc.main.id

ingress { from_port = 22 to_port = 22 protocol = "tcp" cidr_blocks = ["0.0.0.0/0"] }

tags = var.common_tags }`

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🔸 EC2 Instance

resource "aws_instance" "ec2" {  ami = data.aws_ami.amazon_linux.id  instance_type = var.instance_type  subnet_id = aws_subnet.subnet.id  vpc_security_group_ids = [aws_security_group.sg.id]

resource "aws_instance" "ec2" {  ami = data.aws_ami.amazon_linux.id  instance_type = var.instance_type  subnet_id = aws_subnet.subnet.id  vpc_security_group_ids = [aws_security_group.sg.id]

tags = merge(var.common_tags, { Name = "${var.project_name}-ec2" }) }`

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🔸 Data Source (Dynamic AMI)

data "aws_ami" "amazon_linux" {  most_recent = true

data "aws_ami" "amazon_linux" {  most_recent = true

owners = ["amazon"]

filter { name = "name" values = ["al2023-ami--x86_64"] } }`

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🔹 5. Explicit Dependency Example (FORCE ORDER)

⚠️ Simulate hidden dependency

resource "null_resource" "setup" {  provisioner "local-exec" {  command = "echo EC2 should be ready"  }

resource "null_resource" "setup" {  provisioner "local-exec" {  command = "echo EC2 should be ready"  }

depends_on = [aws_instance.ec2] # ✅ EXPLICIT DEPENDENCY }`

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🔹 6. outputs.tf

output "vpc_id" {  value = aws_vpc.main.id }

output "vpc_id" {  value = aws_vpc.main.id }

output "subnet_id" { value = aws_subnet.subnet.id }

output "ec2_id" { value = aws_instance.ec2.id }`

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🔹 🚀 How to Run (Step-by-Step)

cd terraform-dependency-lab

terraform init terraform plan terraform apply`

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✅ Implicit Dependency

• VPC → Subnet → EC2 created in order

• No depends_on used

✅ Parallel Execution

• Security group may create in parallel with subnet

✅ Explicit Dependency

• null_resource runs only after EC2

Show graph:

terraform graph | dot -Tpng > graph.png

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Explain:

• Arrows = dependencies

• Graph = Terraform brain

🔹 💡 Interview-Level Takeaways

• Terraform uses implicit dependencies via references

• Builds dependency graph (DAG)

• Executes parallel when possible

• Uses depends_on when dependency is hidden