Why dynamic blocks reduce repetition in Terraform - See It in Action

Why dynamic blocks reduce repetition

📖 Scenario: You are managing cloud infrastructure using Terraform. You need to create multiple similar nested blocks inside a resource configuration. Writing each block manually causes repetition and makes the code harder to maintain.

🎯 Goal: Learn how to use dynamic blocks in Terraform to reduce repetition by generating multiple nested blocks from a list variable.

📋 What You'll Learn

Create a list variable with exact values

Create a resource with repeated nested blocks

Use a dynamic block to generate nested blocks from the list

Complete the resource configuration with dynamic blocks

💡 Why This Matters

🌍 Real World

Cloud engineers often need to create multiple similar nested blocks in Terraform configurations. Dynamic blocks help reduce manual repetition and errors.

💼 Career

Understanding dynamic blocks is essential for writing clean, maintainable Terraform code in real-world cloud infrastructure projects.

1

Create a list variable with subnet names

Create a Terraform variable called subnet_names with the exact list ["subnet-a", "subnet-b", "subnet-c"].

Terraform

# Your code here

Hint

Use variable block with type = list(string) and default set to the exact list.

2

Create a resource with repeated nested blocks manually

Create a Terraform resource aws_vpc named main with three nested subnet blocks. Each subnet block should have a name attribute set to "subnet-a", "subnet-b", and "subnet-c" respectively.

Terraform

variable "subnet_names" {
  type    = list(string)
  default = ["subnet-a", "subnet-b", "subnet-c"]
}

# Your code here

Hint

Write three subnet blocks inside the aws_vpc resource with the exact name values.

3

Use a dynamic block to generate subnets

Replace the three manual subnet blocks inside aws_vpc.main with a single dynamic block named subnet that iterates over var.subnet_names and sets name = subnet.value inside the block.

Terraform

variable "subnet_names" {
  type    = list(string)
  default = ["subnet-a", "subnet-b", "subnet-c"]
}

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

  # Your code here
}

Hint

Use dynamic "subnet" with for_each = var.subnet_names and set name = subnet.value inside content.

4

Complete the resource configuration with dynamic blocks

Ensure the aws_vpc.main resource has the cidr_block set to "10.0.0.0/16" and uses the dynamic "subnet" block iterating over var.subnet_names with name = subnet.value inside content.

Terraform

variable "subnet_names" {
  type    = list(string)
  default = ["subnet-a", "subnet-b", "subnet-c"]
}

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

  # Your code here
}

Hint

Verify the resource has cidr_block and the dynamic "subnet" block with correct iteration and content.

Practice

1. What is the main benefit of using dynamic blocks in Terraform?

easy

A. They replace the need for variables in Terraform configurations.

B. They improve the speed of Terraform plan and apply operations.

C. They reduce repetition by generating multiple nested blocks automatically.

D. They allow Terraform to run without a backend configuration.

Solution

1. Step 1: Understand the purpose of dynamic blocks

   Dynamic blocks let you create repeated nested blocks from a list or map without manually copying code.

2. Step 2: Compare benefits

   This reduces repetition and keeps code clean and scalable, unlike options about speed, variables, or backend.

3. Final Answer:

   They reduce repetition by generating multiple nested blocks automatically. -> Option C

4. Quick Check:

   Dynamic blocks reduce repetition = C [OK]

Hint: Dynamic blocks create repeated blocks from lists automatically [OK]

Common Mistakes:

• Thinking dynamic blocks speed up Terraform execution
• Confusing dynamic blocks with variables
• Believing dynamic blocks remove backend needs

2. Which of the following is the correct syntax to start a dynamic block in Terraform?

easy

A. dynamic "block_name" { for_each = var.list }

B. dynamic block_name { for_each = var.list }

C. dynamic block_name = var.list { }

D. dynamic "block_name" = var.list { }

Solution

1. Step 1: Recall dynamic block syntax

   Dynamic blocks use the keyword dynamic followed by the block name in quotes, then a block with for_each.

2. Step 2: Match syntax to options

   Only dynamic "block_name" { for_each = var.list } matches the correct syntax: dynamic "block_name" { for_each = var.list }.

3. Final Answer:

   dynamic "block_name" { for_each = var.list } -> Option A

4. Quick Check:

   Correct dynamic block syntax = D [OK]

Hint: Dynamic block name must be in quotes after keyword dynamic [OK]

Common Mistakes:

• Omitting quotes around block name
• Using equals sign instead of braces
• Placing for_each outside the block

3. Given this Terraform snippet:

variable "ports" { default = [80, 443] }

resource "aws_security_group" "example" {
  name = "example"

  dynamic "ingress" {
    for_each = var.ports
    content {
      from_port   = ingress.value
      to_port     = ingress.value
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    }
  }
}

How many ingress blocks will be created in the security group?

medium

A. Depends on AWS region

B. 1

C. 0

D. 2

Solution

1. Step 1: Analyze the for_each list

   The variable ports contains two values: 80 and 443.

2. Step 2: Count dynamic block iterations

   The dynamic block creates one ingress block per item in var.ports, so 2 blocks total.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   List length = number of blocks = 2 [OK]

Hint: Dynamic blocks create one nested block per list item [OK]

Common Mistakes:

• Assuming only one block is created
• Thinking AWS region affects block count
• Confusing dynamic block with static block

4. Identify the error in this Terraform dynamic block snippet:

dynamic "egress" {
  for_each = var.rules
  content = {
    from_port   = egress.value.from
    to_port     = egress.value.to
    protocol    = "tcp"
  }
}

medium

A. Incorrect use of equals sign for content block

B. Missing quotes around block name "egress"

C. for_each must be outside the dynamic block

D. Variable var.rules cannot be used in dynamic blocks

Solution

1. Step 1: Check dynamic block content syntax

   The content must be a block, not assigned with equals sign.

2. Step 2: Identify correct syntax

   Correct syntax uses content { ... } without equals sign. The snippet incorrectly uses content = { ... }.

3. Final Answer:

   Incorrect use of equals sign for content block -> Option A

4. Quick Check:

   content block uses braces, no equals sign = B [OK]

Hint: Use content { } block, not content = { } [OK]

Common Mistakes:

• Using equals sign with content
• Placing for_each outside dynamic block
• Misquoting block name

5. You want to create multiple lifecycle_rule blocks in an aws_s3_bucket resource using a dynamic block. Your variable is a map with keys as rule names and values as rule details. Which approach correctly uses dynamic blocks to reduce repetition and handle the map structure?

hard

A. Use for_each = var.rules and access each rule with lifecycle_rule.key only inside content block.

B. Use for_each = var.rules and access each rule with lifecycle_rule.value inside content block.

C. Use count = length(var.rules) and index into var.rules inside content block.

D. Manually write each lifecycle_rule block without dynamic blocks.

Solution

1. Step 1: Understand dynamic block with map variable

   When using a map, for_each iterates over key-value pairs. The value is accessed with lifecycle_rule.value.

2. Step 2: Identify correct usage

   Use for_each = var.rules and access each rule with lifecycle_rule.value inside content block. correctly uses for_each = var.rules and accesses lifecycle_rule.value inside the content block to get rule details.

3. Final Answer:

   Use for_each = var.rules and access each rule with lifecycle_rule.value inside content block. -> Option B

4. Quick Check:

   Map iteration uses .value for content = A [OK]

Hint: Use .value to access map values inside dynamic block content [OK]

Common Mistakes:

• Using only .key without .value for map data
• Using count instead of for_each for maps
• Writing blocks manually instead of dynamic