Imagine you’re designing your Lego city with Terraform. Some buildings like apartments have repeating patterns: multiple identical floors, each with slightly different details.

Instead of writing the same block over and over, Terraform gives you dynamic blocks. They act like a “loop inside a resource,” letting you generate repeated nested configurations based on variables or lists. Dynamic blocks make your code cleaner, more flexible, and easier to maintain.

Key Concepts

1. What is a Dynamic Block?

• A dynamic block allows you to programmatically generate nested blocks inside a resource.
• Useful when the number of nested blocks depends on variables or lists.

dynamic "<block_name>" {
  for_each = <list or map>
  content {
    # nested arguments
  }
}

2. Example: Security Group Rules

Instead of writing multiple ingress rules manually:

resource "aws_security_group" "example" {
  name = "example-sg"

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

You can use a dynamic block:

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

resource "aws_security_group" "example" {
  name = "example-sg"

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

3. Example: Tags with Dynamic Blocks

variable "tags" {
  default = {
    Environment = "dev"
    Owner       = "Shubham"
  }
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  dynamic "tags" {
    for_each = var.tags
    content {
      key   = tags.key
      value = tags.value
    }
  }
}

Hands‑On Lab / Demo

Lab: Dynamic Subnets

1. Run terraform apply → Subnets are created dynamically inside the VPC.

Use dynamic block in VPC module:

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

  dynamic "subnet" {
    for_each = var.subnets
    content {
      cidr_block = subnet.value
    }
  }
}

Define subnets list:

variable "subnets" {
  default = ["10.0.1.0/24", "10.0.2.0/24"]
}

Pro Tips & Best Practices

• Use dynamic blocks for nested repetition, not for top‑level resources (use for_each or count there).
• Keep dynamic block logic simple - avoid deeply nested loops.
• Always test with terraform plan to confirm generated blocks.
• Document why a dynamic block is used - it can be harder to read than static code.
• Prefer readability over cleverness - sometimes explicit blocks are clearer.

Summary & Cheatsheet

• Dynamic blocks = Loops inside resources.
• Syntax: dynamic "<block_name>" { for_each = <list> content { … } }
• Use cases: Security group rules, tags, nested subnets.
• Best practice: Use for nested repetition, keep logic simple.

Quick mnemonic: Dynamic = Repeat Smartly

The Hackers Notebook

Dynamic blocks are Terraform’s looping mechanism inside resources, letting you generate repeated nested configurations programmatically. They reduce duplication, improve flexibility, and keep your code clean.