Terraform

 

1️⃣ Variables

Purpose:
Inputs to your Terraform configuration.

variable "region" {
  type    = string
  default = "us-east-1"
}

👉 Used by providers, resources, and modules
👉 Loaded first

---

2️⃣ Providers

Purpose:
Tell Terraform which cloud / service API to talk to and how to authenticate.

provider "aws" {
  region = var.region
}

👉 Depend on variables
👉 Initialize during terraform init

---

3️⃣ Resources

Purpose:
Actual infrastructure objects (EC2, S3, Kafka topic, DB, etc.).

resource "aws_s3_bucket" "data_bucket" {
  bucket = "my-bucket"
}

👉 Depend on providers
👉 Can depend on variables and other resources

---

4️⃣ Data Sources

Purpose:
Read-only existing infrastructure.

data "aws_vpc" "default" {
  default = true
}

👉 Do not create anything
👉 Used by resources and modules

---

5️⃣ Modules

Purpose:
Reusable logical groups of Terraform code (like functions).

module "network" {
  source = "./modules/network"
  vpc_id = data.aws_vpc.default.id
}

👉 Can contain:

• variables

• providers

• resources

• data sources

• outputs

👉 Modules depend on inputs passed to them

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Relationship (Big Picture)

Variables
   ↓
Providers
   ↓
Data Sources ──→ Resources
        ↓           ↓
           Modules

Or more practically:

• Variables feed everything

• Providers enable API communication

• Data sources read existing state

• Resources create/update infrastructure

• Modules organize and reuse all of the above

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Execution Flow (What Terraform Does Internally)

1️⃣ terraform init

• Downloads providers

• Initializes backend

• Loads modules

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2️⃣ terraform plan

Terraform builds a dependency graph:

1. Loads variables

2. Configures providers

3. Reads data sources

4. Calculates resource dependencies

5. Shows what will change

👉 Execution order is determined by dependencies, not file order

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3️⃣ terraform apply

Terraform executes in this order automatically:

1. Providers are initialized

2. Data sources are read

3. Resources are created/updated/destroyed

4. Module outputs are calculated

5. State file is updated

---

Example Execution Order

variable "vpc_id" {}

data "aws_vpc" "existing" {
  id = var.vpc_id
}

resource "aws_subnet" "subnet" {
  vpc_id = data.aws_vpc.existing.id
}

Execution:

1. Read variable vpc_id

2. Read existing VPC (data source)

3. Create subnet (resource)

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Key Rules to Remember

✅ Terraform does NOT execute top to bottom
✅ It executes based on dependency graph
✅ Use references (resource.id) to control order
✅ Use depends_on only when absolutely necessary

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One-Line Summary

Variables provide input → Providers enable APIs → Data sources read → Resources create → Modules organize everything