Data source block syntax in Terraform - Mini Project: Build & Apply

Data source block syntax

📖 Scenario: You are setting up a Terraform configuration to retrieve information about an existing AWS AMI (Amazon Machine Image) to use in your infrastructure.

🎯 Goal: Build a Terraform configuration that uses a data source block to fetch the latest Amazon Linux 2 AMI ID in the us-east-1 region.

📋 What You'll Learn

Create a data block named aws_ami with the name amazon_linux

Configure the data source to filter for the latest Amazon Linux 2 AMI

Use the owners attribute to specify Amazon's owner ID

Add an output to display the AMI ID

💡 Why This Matters

🌍 Real World

Terraform data sources let you query existing cloud resources to use their information in your infrastructure setup without recreating them.

💼 Career

Understanding data source blocks is essential for cloud engineers and DevOps professionals to integrate existing resources into Terraform-managed infrastructure.

1

Create the data source block skeleton

Create a Terraform data block named aws_ami with the name amazon_linux.

Terraform

# Your code here

Hint

Start with data "aws_ami" "amazon_linux" { } to define the data source block.

2

Add filter and owners attributes

Inside the data "aws_ami" "amazon_linux" block, add a filter block with name = "name" and values = ["amzn2-ami-hvm-*-x86_64-gp2"]. Also add owners = ["137112412989"] to specify Amazon's owner ID.

Terraform

data "aws_ami" "amazon_linux" {
  # Add filter and owners attributes here
}

Hint

Use a filter block with name and values, and add owners outside the filter.

3

Add most_recent attribute

Add the attribute most_recent = true inside the data "aws_ami" "amazon_linux" block to get the latest AMI.

Terraform

data "aws_ami" "amazon_linux" {
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners = ["137112412989"]
  # Add most_recent attribute here
}

Hint

Set most_recent = true to ensure the latest AMI is selected.

4

Add output for the AMI ID

Create an output block named ami_id that outputs the value of data.aws_ami.amazon_linux.id.

Terraform

data "aws_ami" "amazon_linux" {
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners       = ["137112412989"]
  most_recent  = true
}

# Add output block here

Hint

Use output "ami_id" with value = data.aws_ami.amazon_linux.id to show the AMI ID.

Practice

1. What is the main purpose of a data source block in Terraform?

easy

A. To create new cloud resources from scratch

B. To define variables for Terraform configuration

C. To delete existing cloud resources

D. To read information about existing cloud resources without creating new ones

Solution

1. Step 1: Understand the role of data sources

   Data sources in Terraform are used to fetch information about existing resources, not to create or delete them.

2. Step 2: Compare options with this role

   Only To read information about existing cloud resources without creating new ones describes reading existing resource info without creation, matching the data source purpose.

3. Final Answer:

   To read information about existing cloud resources without creating new ones -> Option D

4. Quick Check:

   Data source = read existing info [OK]

Hint: Data sources read existing resources, not create or delete [OK]

Common Mistakes:

• Confusing data sources with resource blocks
• Thinking data sources create resources
• Assuming data sources delete resources

2. Which of the following is the correct syntax to define a data source block for an AWS AMI in Terraform?

easy

A. data "aws_ami" example { filter { name = "name" values = ["my-ami"] } }

B. resource "aws_ami" "example" { filter { name = "name" values = ["my-ami"] } }

C. data "aws_ami" "example" { filter { name = "name" values = ["my-ami"] } }

D. data aws_ami example { filter { name = "name" values = ["my-ami"] } }

Solution

1. Step 1: Identify correct block type and syntax

   Data source blocks start with data, followed by resource type and local name in quotes.

2. Step 2: Check each option

   data "aws_ami" "example" { filter { name = "name" values = ["my-ami"] } } uses correct quotes and block structure. resource "aws_ami" "example" { filter { name = "name" values = ["my-ami"] } } uses resource instead of data. Options A and C miss quotes around local name or resource type.

3. Final Answer:

   data "aws_ami" "example" { filter { name = "name" values = ["my-ami"] } } -> Option C

4. Quick Check:

   Data source syntax = data "type" "name" { ... } [OK]

Hint: Data source block uses data "type" "name" with quotes [OK]

Common Mistakes:

• Using resource instead of data
• Omitting quotes around type or name
• Incorrect block nesting or missing braces

3. Given this Terraform data source block:

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

What will data.aws_vpc.selected.id return?

medium

A. The ID of the VPC tagged with Name=main-vpc

B. The name tag value main-vpc

C. A list of all VPC IDs in the AWS account

D. An error because filter syntax is invalid

Solution

1. Step 1: Understand the filter block

   The filter selects the VPC resource with tag Name equal to main-vpc.

2. Step 2: Identify the returned attribute

   data.aws_vpc.selected.id accesses the ID of the matched VPC, not the tag value or a list.

3. Final Answer:

   The ID of the VPC tagged with Name=main-vpc -> Option A

4. Quick Check:

   Filter selects resource, .id returns its ID [OK]

Hint: Filter narrows resource, .id returns its unique ID [OK]

Common Mistakes:

• Confusing tag value with resource ID
• Expecting a list instead of single ID
• Assuming filter syntax is wrong

4. Identify the error in this Terraform data source block:

data "aws_subnet" "example" {
  filter {
    name = "availability-zone"
    values = "us-west-2a"
  }
}

medium

A. The values attribute should be a list, not a string

B. The filter block is missing a closing brace

C. The data source type aws_subnet does not support filters

D. The local name example must be uppercase

Solution

1. Step 1: Check the values attribute type

   In Terraform, filter values must be a list, even if only one value is given.

2. Step 2: Verify the given values

   The code uses a string instead of a list, which causes a syntax error.

3. Final Answer:

   The values attribute should be a list, not a string -> Option A

4. Quick Check:

   Filter values = list, not string [OK]

Hint: Always use list syntax ["value"] for filter values [OK]

Common Mistakes:

• Using string instead of list for values
• Assuming filter block needs no braces
• Thinking local name case matters

5. You want to use a data source block to get the latest Amazon Linux 2 AMI ID filtered by owner and name pattern. Which of these blocks correctly achieves this?

hard

A. resource "aws_ami" "latest" {
most_recent = true
owners = ["amazon"]
filter {
name = "name"
values = ["amzn2-ami-hvm-*-x86_64-gp2"]
}
}

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

C. data "aws_ami" "latest" {
filter {
name = "owner"
values = ["amazon"]
}
filter {
name = "name"
values = ["amzn2-ami-hvm-*-x86_64-gp2"]
}
}

D. data "aws_ami" "latest" {
most_recent = true
owner = "amazon"
filter {
name = "name"
values = ["amzn2-ami-hvm-*-x86_64-gp2"]
}
}

Solution

1. Step 1: Confirm correct block type and attributes

   Use data block for reading AMI info, with most_recent = true and owners as a list.

2. Step 2: Check filter and owner syntax

   data "aws_ami" "latest" { most_recent = true owners = ["amazon"] filter { name = "name" values = ["amzn2-ami-hvm-*-x86_64-gp2"] } } correctly uses owners list and a filter for name pattern. data "aws_ami" "latest" { filter { name = "owner" values = ["amazon"] } filter { name = "name" values = ["amzn2-ami-hvm-*-x86_64-gp2"] } } incorrectly filters owner as a filter. resource "aws_ami" "latest" { most_recent = true owners = ["amazon"] filter { name = "name" values = ["amzn2-ami-hvm-*-x86_64-gp2"] } } uses resource block instead of data. data "aws_ami" "latest" { most_recent = true owner = "amazon" filter { name = "name" values = ["amzn2-ami-hvm-*-x86_64-gp2"] } } uses singular owner instead of owners list.

3. Final Answer:

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

4. Quick Check:

   Use data block, owners list, most_recent true [OK]

Hint: Use owners list and most_recent true in data block [OK]

Common Mistakes:

• Using resource block instead of data
• Filtering owner instead of using owners attribute
• Using singular owner instead of owners list