How to Create Auto Scaling Group with Terraform

To create an aws_autoscaling_group in Terraform, define a launch template or configuration, then specify the group with desired capacity and scaling policies. Use resource "aws_autoscaling_group" with parameters like min_size, max_size, and desired_capacity to manage scaling automatically.

Syntax

The aws_autoscaling_group resource manages a group of instances that scale automatically. Key parts include:

• name: Unique name for the group.
• launch_template: Defines the instance configuration.
• min_size, max_size, desired_capacity: Control the number of instances.
• vpc_zone_identifier: List of subnet IDs for instances.
• tags: Optional metadata for instances.

terraform

resource "aws_autoscaling_group" "example" {
  name                      = "example-asg"
  launch_template {
    id      = aws_launch_template.example.id
    version = "$Latest"
  }
  min_size                  = 1
  max_size                  = 3
  desired_capacity          = 2
  vpc_zone_identifier       = ["subnet-12345678", "subnet-87654321"]
  tags = [
    {
      key                 = "Name"
      value               = "example-instance"
      propagate_at_launch = true
    }
  ]
}

Example

This example creates a launch template and an auto scaling group with 2 instances running in specified subnets. It shows how to link the launch template and set scaling sizes.

terraform

provider "aws" {
  region = "us-east-1"
}

resource "aws_launch_template" "example" {
  name_prefix   = "example-launch-template-"
  image_id      = "ami-0c94855ba95c71c99"  # Amazon Linux 2 AMI
  instance_type = "t2.micro"

  tag_specifications {
    resource_type = "instance"
    tags = {
      Name = "example-instance"
    }
  }
}

resource "aws_autoscaling_group" "example" {
  name                      = "example-asg"
  launch_template {
    id      = aws_launch_template.example.id
    version = "$Latest"
  }
  min_size                  = 1
  max_size                  = 3
  desired_capacity          = 2
  vpc_zone_identifier       = ["subnet-12345678", "subnet-87654321"]

  tag {
    key                 = "Name"
    value               = "example-instance"
    propagate_at_launch = true
  }
}

Output

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

Common Pitfalls

Common mistakes when creating auto scaling groups in Terraform include:

• Not specifying vpc_zone_identifier for subnets, causing launch failures.
• Using a launch configuration instead of a launch template, which is now preferred.
• Forgetting to set desired_capacity, leading to zero instances running.
• Not tagging instances with propagate_at_launch = true, making resource tracking harder.

terraform

resource "aws_autoscaling_group" "wrong" {
  name                = "wrong-asg"
  launch_configuration = aws_launch_configuration.example.id
  min_size            = 1
  max_size            = 2
  # Missing vpc_zone_identifier causes errors
}

# Correct way uses launch_template and vpc_zone_identifier
resource "aws_autoscaling_group" "right" {
  name                = "right-asg"
  launch_template {
    id      = aws_launch_template.example.id
    version = "$Latest"
  }
  min_size            = 1
  max_size            = 2
  vpc_zone_identifier = ["subnet-12345678"]
}

Quick Reference

Key parameters for aws_autoscaling_group:

Parameter	Description
name	Unique name for the auto scaling group
launch_template	Reference to launch template with instance config
min_size	Minimum number of instances to run
max_size	Maximum number of instances allowed
desired_capacity	Initial number of instances to launch
vpc_zone_identifier	List of subnet IDs for instance placement
tag	Tags to apply to instances, with propagate_at_launch option

Key Takeaways

Use aws_autoscaling_group with a launch template to create scalable instance groups.

Always specify vpc_zone_identifier to place instances in correct subnets.

Set min_size, max_size, and desired_capacity to control scaling behavior.

Use tags with propagate_at_launch to track and manage instances easily.

Avoid using launch configurations; prefer launch templates for new setups.