Data source vs resource difference in Terraform - Hands-On Comparison

Understanding Data Source vs Resource in Terraform

📖 Scenario: You are setting up cloud infrastructure using Terraform. You need to understand the difference between data sources and resources in Terraform to manage your infrastructure effectively.

🎯 Goal: Build a simple Terraform configuration that uses a data block to read existing information and a resource block to create new infrastructure.

📋 What You'll Learn

Create a data source block to fetch existing AWS AMI information

Create a resource block to launch an AWS EC2 instance using the fetched AMI

Use exact block names and attributes as specified

Follow Terraform best practices for configuration

💡 Why This Matters

🌍 Real World

Terraform data sources let you read existing cloud information, like AMIs, without creating them. Resources let you create and manage cloud infrastructure, like EC2 instances.

💼 Career

Understanding data sources vs resources is essential for cloud engineers and DevOps professionals to write efficient and reusable Terraform configurations.

1

Create a data source to fetch the latest Amazon Linux 2 AMI

Write a Terraform data block named aws_ami with the name amazon_linux that fetches the latest Amazon Linux 2 AMI using the filter name = "amzn2-ami-hvm-*-x86_64-gp2" and owners set to ["amazon"].

Terraform

# Your code here

Hint

Use a data block with type aws_ami and name amazon_linux. Set most_recent to true. Use a filter block with name and values. Set owners to ["amazon"].

2

Create a resource to launch an EC2 instance using the fetched AMI

Add a Terraform resource block named aws_instance with the name example. Use the AMI ID from the data source data.aws_ami.amazon_linux.id and set the instance type to t2.micro.

Terraform

data "aws_ami" "amazon_linux" {
  most_recent = true
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners = ["amazon"]
}

# Your code here

Hint

Use a resource block with type aws_instance and name example. Set ami to data.aws_ami.amazon_linux.id and instance_type to t2.micro.

3

Add a tag to the EC2 instance resource

Inside the aws_instance.example resource block, add a tags attribute with a map containing Name = "ExampleInstance".

Terraform

data "aws_ami" "amazon_linux" {
  most_recent = true
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners = ["amazon"]
}

resource "aws_instance" "example" {
  ami           = data.aws_ami.amazon_linux.id
  instance_type = "t2.micro"
  # Your code here
}

Hint

Add a tags block inside the resource with Name = "ExampleInstance".

4

Add provider configuration for AWS

At the top of the file, add a provider block for aws with the region set to us-east-1.

Terraform

# Your code here

data "aws_ami" "amazon_linux" {
  most_recent = true
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners = ["amazon"]
}

resource "aws_instance" "example" {
  ami           = data.aws_ami.amazon_linux.id
  instance_type = "t2.micro"
  tags = {
    Name = "ExampleInstance"
  }
}

Hint

Add a provider block for aws with region = "us-east-1" at the top of the file.

Practice

1. What is the main difference between a resource and a data source in Terraform?

easy

A. Resources and data sources both create infrastructure but in different clouds.

B. Resources create or manage infrastructure; data sources only read existing information.

C. Resources only read existing infrastructure; data sources create new infrastructure.

D. Resources are temporary; data sources are permanent.

Solution

1. Step 1: Understand resource role

   Resources in Terraform are used to create or manage cloud infrastructure components.

2. Step 2: Understand data source role

   Data sources are used to read information about existing infrastructure without changing it.

3. Final Answer:

   Resources create or manage infrastructure; data sources only read existing information. -> Option B

4. Quick Check:

   Resource = create/manage, Data source = read [OK]

Hint: Resources build; data sources just look up info [OK]

Common Mistakes:

• Thinking data sources create resources
• Confusing resource and data source roles
• Assuming both modify infrastructure
• Believing data sources are temporary

2. Which of the following is the correct syntax to declare a data source in Terraform?

easy

A. data "aws_ami" "example" {}

B. resource "aws_instance" "example" {}

C. source "aws_vpc" "example" {}

D. resource "data_aws_subnet" "example" {}

Solution

1. Step 1: Identify data source syntax

   Data sources use the keyword data followed by the type and name.

2. Step 2: Check options

   data "aws_ami" "example" {} uses data "aws_ami" "example" {}, which is correct syntax for a data source.

3. Final Answer:

   data "aws_ami" "example" {} -> Option A

4. Quick Check:

   Data source starts with 'data' keyword [OK]

Hint: Data sources start with 'data' keyword [OK]

Common Mistakes:

• Using 'resource' keyword for data sources
• Mixing 'source' keyword which doesn't exist
• Prefixing resource with 'data_' incorrectly
• Confusing resource and data source syntax

3. Given this Terraform snippet:

data "aws_vpc" "selected" {
  filter {
    name   = "tag:Name"
    values = ["main-vpc"]
  }
}

resource "aws_subnet" "example" {
  vpc_id     = data.aws_vpc.selected.id
  cidr_block = "10.0.1.0/24"
}

What does the data.aws_vpc.selected.id represent?

medium

A. The ID of an existing VPC filtered by tag "Name=main-vpc".

B. The ID of a subnet created inside the VPC.

C. A random VPC ID generated by Terraform.

D. The ID of a new VPC created by Terraform.

Solution

1. Step 1: Analyze data source usage

   The data "aws_vpc" "selected" block reads an existing VPC filtered by the tag "Name=main-vpc".

2. Step 2: Understand reference in resource

   The resource aws_subnet.example uses data.aws_vpc.selected.id to get the ID of that existing VPC to create a subnet inside it.

3. Final Answer:

   The ID of an existing VPC filtered by tag "Name=main-vpc". -> Option A

4. Quick Check:

   Data source reads existing VPC ID [OK]

Hint: Data source reads existing resource info [OK]

Common Mistakes:

• Thinking data source creates new VPC
• Confusing subnet ID with VPC ID
• Assuming random ID generation
• Using resource instead of data source for existing info

4. You wrote this Terraform code but get an error:

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

data "aws_vpc" "main" {
  id = aws_vpc.main.id
}

What is the problem?

medium

A. The data source block is missing a filter instead of id.

B. Data sources cannot reference resources in the same configuration.

C. You cannot use the same name for resource and data source.

D. The resource block is missing a provider configuration.

Solution

1. Step 1: Check naming conflict

   The resource and data source both use the name "main" which causes a naming conflict in Terraform.

2. Step 2: Terraform rules

   Terraform does not allow a resource and a data source to share the same type and name within the same module.

3. Final Answer:

   You cannot use the same name for resource and data source. -> Option C

4. Quick Check:

   Resource and data source names must be unique [OK]

Hint: Resource and data source names must differ [OK]

Common Mistakes:

• Thinking same names cause conflicts
• Believing you always need filters over id
• Assuming data sources can depend on resources
• Blaming missing provider config

5. You want to create a subnet in an existing VPC without recreating the VPC. Which Terraform approach is best?

hard

A. Use a resource block for the VPC and a resource block for the subnet.

B. Use a resource block for the subnet and ignore the VPC.

C. Use a data source block for both VPC and subnet.

D. Use a data source block to read the existing VPC and a resource block to create the subnet.

Solution

1. Step 1: Identify existing infrastructure

   The VPC already exists, so it should not be recreated or managed as a resource.

2. Step 2: Use data source for existing VPC

   Use a data source block to safely read the existing VPC's details without changing it.

3. Step 3: Create subnet resource

   Create a resource block for the subnet to add new infrastructure inside the existing VPC.

4. Final Answer:

   Use a data source block to read the existing VPC and a resource block to create the subnet. -> Option D

5. Quick Check:

   Data source reads existing; resource creates new [OK]

Hint: Read existing with data source; create new with resource [OK]

Common Mistakes:

• Trying to recreate existing VPC as resource
• Using data source to create subnet (not possible)
• Ignoring existing VPC and creating subnet without VPC ID
• Using resource for both causing conflicts