An Elastic Load Balancer (ELB) is typically used to load balance requests across two (or more) different EC2 instances. 

This assumes you have setup Terraform with the Amazon Web Services (AWS) provider. If not, check out my article Amazon Web Services (AWS) Getting Started with Terraform.

Let's say you have the following files on your Terraform server.

├── required_providers.tf
├── application_load_balancers (directory)
│   ├── provider.tf
│   ├── listeners.tf
│   ├── listener_rules.tf
│   ├── load_balancers.tf
│   ├── target_groups.tf

required_providers.tf will almost always have this.

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
    }
  }
}

Let's say provider.tf has the following. In this example, the "default" profile in /home/username/.aws/config and /home/username/.aws/credentials is being used. This assumes you have setup Terraform as described in Amazon Web Services (AWS) - Getting Started with Terraform.

provider "aws" {
  alias   = "default"
  profile = "default"
  region  = "default"
}

This assume you have already:

• Created an Application Load Balancer using Terraform
• Created an Application Load Balancer Target Group using Terraform
• Created an Application Load Balancer Listener using Terraform

Let's say you want to forward requests onto different Target Groups based on the URL. For example:

• www.example.com -> www Target Group
• foo.example.com -> foo Target Group
• bar.example.com -> bar Target Group

HTTP Host Header can be used. listener_rules.tf could have the following to create an Application Load Balancer Listener Rule to forward requests based on the HTTP Host Header.

resource "aws_lb_listener_rule" "alb_http_listener_rules" {

  for_each = {
    "www.example.com" = ["1", aws_lb_target_group.target-groups["www-target-group"].arn]
    "foo.example.com" = ["2", aws_lb_target_group.target-groups["foo-target-group"].arn]
    "bar.example.com" = ["3", aws_lb_target_group.target-groups["bar-target-group"].arn]
  }

  listener_arn = aws_lb_listener.https_listener.arn
  priority     = each.value[0]

  action {
    type = "forward"
    target_group_arn = each.value[1]
  }

  condition {
    host_header {
      values = [each.key]
    }
  }

  tags = {
    Name = each.key
  }

}

The terraform plan command can be used to see what Terraform will try to do.

terraform plan

And the terraform apply command can be used to create the Elastic Load balancer Target Group.

terraform apply