Why Dynamic block syntax in Terraform? - Purpose & Use Cases

The Big Idea

What if you could write one block that magically creates many, saving hours of tedious work?

The Scenario

Imagine you need to create many similar resources in your cloud setup, like multiple firewall rules or storage buckets, each with slightly different settings. Doing this by writing each block manually feels like copying and pasting over and over.

The Problem

Manually repeating similar blocks is slow and boring. It's easy to make mistakes like forgetting to change a name or a setting. If you want to add or remove items, you have to edit many places, which wastes time and causes errors.

The Solution

Dynamic block syntax lets you write one flexible block that repeats itself based on your list of items. This means you write less code, avoid mistakes, and can easily change how many blocks you create by just changing your data.

Before vs After

✗ Before

resource "aws_security_group" "example" {
  ingress {
    from_port = 80
    to_port   = 80
    protocol  = "tcp"
    cidr_blocks = ["10.0.0.1/32"]
  }
  ingress {
    from_port = 443
    to_port   = 443
    protocol  = "tcp"
    cidr_blocks = ["10.0.0.2/32"]
  }
}

✓ After

resource "aws_security_group" "example" {
  dynamic "ingress" {
    for_each = var.rules
    content {
      from_port   = ingress.value.from_port
      to_port     = ingress.value.to_port
      protocol    = ingress.value.protocol
      cidr_blocks = [ingress.value.cidr_blocks]
    }
  }
}

What It Enables

You can create many similar infrastructure blocks easily and safely by just changing your input data, making your code cleaner and more flexible.

Real Life Example

When setting up a firewall, you might have dozens of rules for different ports and IPs. Using dynamic blocks, you define all rules in one list and Terraform creates all the needed blocks automatically.

Key Takeaways

Manual repetition is slow and error-prone.

Dynamic blocks let you loop over data to create repeated blocks.

This makes your infrastructure code simpler, safer, and easier to update.

Practice

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

easy

A. To generate multiple nested blocks from a list or map dynamically

B. To create a new Terraform provider

C. To define variables with default values

D. To execute shell commands during deployment

Solution

1. Step 1: Understand the role of dynamic blocks

   Dynamic blocks allow repeating nested blocks based on a collection like a list or map.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated Terraform features.

3. Final Answer:

   To generate multiple nested blocks from a list or map dynamically -> Option A

4. Quick Check:

   Dynamic block = repeated nested blocks [OK]

Hint: Dynamic blocks repeat nested blocks from lists/maps [OK]

Common Mistakes:

• Confusing dynamic blocks with variable declarations
• Thinking dynamic blocks create providers
• Assuming dynamic blocks run commands

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

easy

A. dynamic ingress { for_each = var.rules content { ... } }

B. dynamic(ingress) { for_each = var.rules content { ... } }

C. dynamic "ingress" { for_each = var.rules content { ... } }

D. dynamic { ingress for_each = var.rules content { ... } }

Solution

1. Step 1: Recall dynamic block syntax

   The block must be declared as dynamic "block_name" { ... } with quotes around the block name.

2. Step 2: Check each option

   dynamic "ingress" { for_each = var.rules content { ... } } uses correct syntax with quotes and proper structure; others miss quotes or have wrong syntax.

3. Final Answer:

   dynamic "ingress" { for_each = var.rules content { ... } } -> Option C

4. Quick Check:

   Dynamic block syntax requires quotes around block name [OK]

Hint: Use quotes around block name in dynamic block [OK]

Common Mistakes:

• Omitting quotes around the block name
• Using parentheses instead of braces
• Placing block name outside dynamic keyword

3. Given this Terraform snippet:

dynamic "tag" {
  for_each = {"env" = "prod", "team" = "devops"}
  content {
    key   = tag.key
    value = tag.value
  }
}

What will be the result in the generated resource configuration?

medium

A. No tag blocks generated because map keys are strings

B. A single tag block with key and value set to the entire map

C. Syntax error due to incorrect for_each usage

D. Two tag blocks with keys "env" and "team" and their values

Solution

1. Step 1: Understand for_each with map in dynamic block

   Using a map with for_each creates one nested block per key-value pair.

2. Step 2: Analyze content block usage

   Inside content, tag.key and tag.value refer to each map entry's key and value, creating two blocks.

3. Final Answer:

   Two tag blocks with keys "env" and "team" and their values -> Option D

4. Quick Check:

   Map for_each creates one block per key-value pair [OK]

Hint: Map in for_each creates one block per key-value pair [OK]

Common Mistakes:

• Thinking only one block is created for whole map
• Confusing key and value references inside content
• Assuming syntax error due to map usage

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

dynamic "egress" {
  for_each = var.egress_rules
  content = {
    from_port = egress.value.from
    to_port   = egress.value.to
  }
}

medium

A. The content block should use braces, not an assignment

B. The variable egress is undefined inside the block

C. Missing quotes around "egress" in dynamic block declaration

D. for_each cannot be used with variables

Solution

1. Step 1: Check dynamic block content syntax

   The content must be a block, not an assignment. It should be content { ... }, not content = { ... }.

2. Step 2: Verify other parts

   Variable egress is valid as the iterator, quotes are present, and for_each can use variables.

3. Final Answer:

   The content block should use braces, not an assignment -> Option A

4. Quick Check:

   content must be a block, not an equals assignment [OK]

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

Common Mistakes:

• Writing content as an assignment instead of a block
• Assuming iterator variable is undefined
• Forgetting quotes around dynamic block name

5. You want to create multiple network_interface blocks dynamically in an AWS instance resource using a list of maps with keys device_index and subnet_id. Which dynamic block correctly achieves this?

hard

A. dynamic "network_interface" {
for_each = var.interfaces
content {
device_index = network_interface.value.device_index
subnet_id = network_interface.value.subnet_id
}
}

B. dynamic "network_interface" {
for_each = var.interfaces
content {
device_index = each.value.device_index
subnet_id = each.value.subnet_id
}
}

C. dynamic "network_interface" {
for_each = var.interfaces
content {
device_index = network_interface.device_index
subnet_id = network_interface.subnet_id
}
}

D. dynamic "network_interface" {
for_each = var.interfaces
content {
device_index = network_interface.key.device_index
subnet_id = network_interface.key.subnet_id
}
}

Solution

1. Step 1: Identify the iterator variable in dynamic block

   Terraform uses each as the iterator inside dynamic blocks for for_each.

2. Step 2: Check how to access map values

   Use each.value.key_name to access map elements inside content block.

3. Step 3: Evaluate options

   dynamic "network_interface" { for_each = var.interfaces content { device_index = each.value.device_index subnet_id = each.value.subnet_id } } correctly uses each.value.device_index and each.value.subnet_id. Others incorrectly use network_interface or network_interface.key.

4. Final Answer:

   dynamic "network_interface" { for_each = var.interfaces content { device_index = each.value.device_index subnet_id = each.value.subnet_id } } -> Option B

5. Quick Check:

   Use each.value to access map in dynamic block [OK]

Hint: Use each.value inside dynamic content block [OK]

Common Mistakes:

• Using block name as iterator variable
• Using .key instead of .value for map access
• Omitting the each keyword