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The alb module places an Application Load Balancer in front of the two Node.js EC2 instances, distributing HTTP traffic evenly across both application servers. The ALB spans both public subnets across two Availability Zones, providing fault tolerance at the load-balancing layer. Target registration uses a for_each over a map of instance IDs, so adding more Node.js instances in the future only requires extending that map.

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Traffic Flow

         Internet
             │
   Application Load Balancer
             │  (HTTP :80 listener)
     Target Group (HTTP:80)
      /                  \
     /                    \
Node 1 (:80)         Node 2 (:80)
     │                    │
Nginx → Node.js      Nginx → Node.js
Requests enter through the ALB’s public DNS name, hit the HTTP listener on port 80, and are forwarded to one of the two registered targets. Each target runs Nginx as a reverse proxy in front of the Node.js process.

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Resources Created

ResourceTerraform nameDescription
aws_lbthisInternet-facing Application Load Balancer; deletion protection disabled
aws_lb_target_groupthisHTTP:80 target group with instance targets and /health health checks
aws_lb_listenerhttpListens on port 80 and forwards all traffic to the target group
aws_lb_target_group_attachmentthisfor_each over target_instances — one attachment per Node.js instance

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Health Check Configuration

The target group is configured with conservative health check thresholds to avoid unnecessary instance flapping:
SettingValue
Path/health
ProtocolHTTP
Expected status code200
Check interval30 seconds
Timeout5 seconds
Healthy threshold2 consecutive successes
Unhealthy threshold2 consecutive failures
Your Node.js application must expose a GET /health endpoint that returns HTTP 200 for the ALB to mark instances as healthy. An instance that fails two consecutive health checks is removed from rotation automatically.

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Input Variables

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project_name
string
required
Used to name the ALB (<project_name>-alb) and the target group (<project_name>-tg).
​
vpc_id
string
required
The VPC ID in which the target group is created. Sourced from module.network.vpc_id.
​
public_subnet_ids
list(string)
required
List of public subnet IDs to attach to the ALB. Must contain subnets from at least two different Availability Zones. Sourced from module.network.public_subnet_1_id and module.network.public_subnet_2_id.
​
alb_security_group_id
string
required
The ID of the security group that allows HTTP (80) and HTTPS (443) inbound traffic. Sourced from module.security.alb_security_group_id.
​
target_instances
map(string)
required
A map of arbitrary keys to EC2 instance IDs to register as targets on port 80. Example: { node1 = "i-0abc123", node2 = "i-0def456" }. Each map entry produces one aws_lb_target_group_attachment resource.

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Outputs

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dns_name
string
The public DNS hostname of the ALB (e.g. terraform-mean-alb-1234567890.us-east-1.elb.amazonaws.com). Use this URL to access your MEAN stack application after deployment.
​
target_group_arn
string
The ARN of the target group. Useful if you need to attach additional listeners or configure auto-scaling policies that reference the target group.
​
alb_arn
string
The ARN of the Application Load Balancer itself. Can be used to reference the ALB in WAF associations or AWS Shield configurations.

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Module Call

module "alb" {
  source = "./modules/alb"

  project_name = var.project_name

  vpc_id = module.network.vpc_id

  public_subnet_ids = [
    module.network.public_subnet_1_id,
    module.network.public_subnet_2_id
  ]

  alb_security_group_id = module.security.alb_security_group_id

  target_instances = {
    node1 = module.node_1.instance_id
    node2 = module.node_2.instance_id
  }
}
After terraform apply, retrieve the application URL with terraform output -raw alb_dns_name (if exposed as a root output) or by navigating to the ALB in the EC2 console. The DNS name is stable for the lifetime of the load balancer.

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