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Chinmay Tonape
Chinmay Tonape

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Implementing Path based routing with Application Load Balancer ALB and EC2 Instances using Terraform

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In this tutorial, we'll leverage Terraform to set up an architecture that includes an Application Load Balancer (ALB) with path-based routing to EC2 instances hosting different web services.

Path-based routing is a method of directing traffic to different backend services based on the URL path of the incoming request. This approach allows you to host multiple applications or services on the same domain or load balancer, each accessible via a unique path. For example, you could route example.com/app1 requests to one set of servers and example.com/app2 requests to another.

Architecture Overview

Before diving into the implementation details, let's outline the architecture we will be working with:

Architecture

Step 1: Create VPC

First, we'll define a Virtual Private Cloud (VPC) and networking components

 ################################################################################ # Create VPC and components ################################################################################ module "vpc" { source = "./modules/vpc" aws_region = var.aws_region vpc_cidr_block = var.vpc_cidr_block enable_dns_hostnames = var.enable_dns_hostnames vpc_public_subnets_cidr_block = var.vpc_public_subnets_cidr_block aws_azs = var.aws_azs common_tags = local.common_tags naming_prefix = local.naming_prefix }
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Step 2: Create Two Web Services on Separate EC2 Instances

Next, we'll provision two groups EC2 instances within our VPC, each running a distinct web service or application. These instances will serve as our backend servers behind the ALB.

First EC2 instance will be at default path and second instance will be at path pattern /other/, therefore keep the web files at /var/www/html/other/ for second EC2 instance.

 ################################################################################ # Get latest Amazon Linux 2 AMI ################################################################################ data "aws_ami" "amazon-linux-2" { most_recent = true owners = ["amazon"] filter { name = "name" values = ["amzn2-ami-hvm*"] } } ################################################################################ # Create the Linux EC2 Web server ################################################################################ resource "aws_instance" "web" { ami = data.aws_ami.amazon-linux-2.id instance_type = var.instance_type key_name = var.instance_key security_groups = var.security_group_ec2 count = length(var.public_subnets) subnet_id = element(var.public_subnets, count.index) user_data = <<-EOF #!/bin/bash yum update -y yum install -y httpd.x86_64 systemctl start httpd.service systemctl enable httpd.service instanceId=$(curl http://169.254.169.254/latest/meta-data/instance-id) instanceAZ=$(curl http://169.254.169.254/latest/meta-data/placement/availability-zone) pubHostName=$(curl http://169.254.169.254/latest/meta-data/public-hostname) pubIPv4=$(curl http://169.254.169.254/latest/meta-data/public-ipv4) privHostName=$(curl http://169.254.169.254/latest/meta-data/local-hostname) privIPv4=$(curl http://169.254.169.254/latest/meta-data/local-ipv4) echo "<font face = "Verdana" size = "5">" > /var/www/html/index.html echo "<center><h1>AWS Linux VM Deployed with Terraform</h1></center>" >> /var/www/html/index.html echo "<center> <b>EC2 Instance Metadata</b> </center>" >> /var/www/html/index.html echo "<center> <b>Instance ID:</b> $instanceId </center>" >> /var/www/html/index.html echo "<center> <b>AWS Availablity Zone:</b> $instanceAZ </center>" >> /var/www/html/index.html echo "<center> <b>Public Hostname:</b> $pubHostName </center>" >> /var/www/html/index.html echo "<center> <b>Public IPv4:</b> $pubIPv4 </center>" >> /var/www/html/index.html echo "<center> <b>Private Hostname:</b> $privHostName </center>" >> /var/www/html/index.html echo "<center> <b>Private IPv4:</b> $privIPv4 </center>" >> /var/www/html/index.html echo "</font>" >> /var/www/html/index.html EOF  tags = merge(var.common_tags, { Name = "${var.naming_prefix}-ec2-${count.index + 1}" }) } ################################################################################ # Create the Linux EC2 Web server - other instance ################################################################################ resource "aws_instance" "web_other" { ami = data.aws_ami.amazon-linux-2.id instance_type = var.instance_type key_name = var.instance_key security_groups = var.security_group_ec2 count = length(var.public_subnets) subnet_id = element(var.public_subnets, count.index) user_data = <<-EOF #!/bin/bash yum update -y yum install -y httpd.x86_64 systemctl start httpd.service systemctl enable httpd.service instanceId=$(curl http://169.254.169.254/latest/meta-data/instance-id) instanceAZ=$(curl http://169.254.169.254/latest/meta-data/placement/availability-zone) pubHostName=$(curl http://169.254.169.254/latest/meta-data/public-hostname) pubIPv4=$(curl http://169.254.169.254/latest/meta-data/public-ipv4) privHostName=$(curl http://169.254.169.254/latest/meta-data/local-hostname) privIPv4=$(curl http://169.254.169.254/latest/meta-data/local-ipv4) mkdir -p /var/www/html/other echo "<font face = "Verdana" size = "5">" > /var/www/html/other/index.html echo "<center><h1>AWS Linux VM Deployed with Terraform - Other Instance</h1></center>" >> /var/www/html/other/index.html echo "<center> <b>EC2 Instance Metadata</b> </center>" >> /var/www/html/other/index.html echo "<center> <b>Instance ID:</b> $instanceId </center>" >> /var/www/html/other/index.html echo "<center> <b>AWS Availablity Zone:</b> $instanceAZ </center>" >> /var/www/html/other/index.html echo "<center> <b>Public Hostname:</b> $pubHostName </center>" >> /var/www/html/other/index.html echo "<center> <b>Public IPv4:</b> $pubIPv4 </center>" >> /var/www/html/other/index.html echo "<center> <b>Private Hostname:</b> $privHostName </center>" >> /var/www/html/other/index.html echo "<center> <b>Private IPv4:</b> $privIPv4 </center>" >> /var/www/html/other/index.html echo "</font>" >> /var/www/html/other/index.html EOF  tags = merge(var.common_tags, { Name = "${var.naming_prefix}-ec2-${count.index + 1}-other" }) }
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Step 3: Configure ALB with Path-Based Routing

The core of our setup involves configuring the ALB to route incoming requests based on their URL paths:

ALB Configuration: We'll define an ALB with multiple target groups, each associated with one of our target group of EC2 instances. This setup allows the ALB to route traffic to different sets of instances based on the URL path.

Make sure health checks for alb target groups are correctly configured.

 ################################################################################ # create application load balancer ################################################################################ resource "aws_lb" "aws-application_load_balancer" { internal = false load_balancer_type = "application" security_groups = [var.security_group_alb[0]] //subnets = [var.public_subnets[0],var.public_subnets[1] ,var.public_subnets[2],var.public_subnets[3]] subnets = tolist(var.public_subnets) enable_deletion_protection = false tags = merge(var.common_tags, { Name = "${var.naming_prefix}-alb" }) } ################################################################################ # create target groups for ALB - Default and Other ################################################################################ resource "aws_lb_target_group" "alb_target_group_default" { target_type = "instance" port = 80 protocol = "HTTP" vpc_id = var.vpc_id health_check { enabled = true interval = 30 path = "/" timeout = 5 matcher = 200 healthy_threshold = 2 unhealthy_threshold = 5 } lifecycle { create_before_destroy = true } tags = merge(var.common_tags, { Name = "${var.naming_prefix}-alb-tg-default" }) } resource "aws_lb_target_group" "alb_target_group_other" { target_type = "instance" port = 80 protocol = "HTTP" vpc_id = var.vpc_id health_check { enabled = true interval = 30 path = "/other/" timeout = 5 matcher = 200 healthy_threshold = 2 unhealthy_threshold = 5 } lifecycle { create_before_destroy = true } tags = merge(var.common_tags, { Name = "${var.naming_prefix}-alb-tg-other" }) }
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Listener Rules: We'll create listener rules using Terraform to specify path-based routing logic. For instance, requests to /* will be forwarded to default target group, while requests to /other/* will be directed to another.

 ################################################################################ # create a listener on port 80 with redirect action - Default and Other ################################################################################ resource "aws_lb_listener" "alb_http_listener_default" { load_balancer_arn = aws_lb.aws-application_load_balancer.id port = 80 protocol = "HTTP" default_action { type = "forward" target_group_arn = aws_lb_target_group.alb_target_group_default.id } } ################################################################################ # create a listener rule for other listener ################################################################################ resource "aws_lb_listener_rule" "alb_rule_other" { listener_arn = aws_lb_listener.alb_http_listener_default.arn priority = 10 action { type = "forward" target_group_arn = aws_lb_target_group.alb_target_group_other.arn } condition { path_pattern { values = ["/other/"] } } } ################################################################################ # Target Group Attachment with Instance ################################################################################ resource "aws_alb_target_group_attachment" "alb_tg_attach_default" { count = length(var.instance_ids) target_group_arn = aws_lb_target_group.alb_target_group_default.arn target_id = element(var.instance_ids, count.index) } resource "aws_alb_target_group_attachment" "alb_tg_attach_other" { count = length(var.instance_ids_other) target_group_arn = aws_lb_target_group.alb_target_group_other.arn target_id = element(var.instance_ids_other, count.index) }
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Steps to Run Terraform

Follow these steps to execute the Terraform configuration:

 terraform init terraform plan terraform apply -auto-approve
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Upon successful completion, Terraform will provide relevant outputs.

 
Apply complete! Resources: 23 added, 0 changed, 0 destroyed.
 
Outputs:
 
alb_dns_name = "tf-lb-20240724121109991900000006-1464435819.us-east-1.elb.amazonaws.com"
 ec2_instance_ids = [
 "i-0c3ec981cbc105993",
 "i-034025ab95d436bcb",
 ]
 public_subnets = [
 "subnet-0bf0fb37d7fe382ac",
 "subnet-086da92937739db51",
 ]
 security_group_alb = [
 "sg-0c1110387f85745d4",
 ]
 security_groups_ec2 = [
 "sg-0ea78de50cb1702e2",
 ]
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Testing

ALB with a listener with 2 rules:

ALB with listener

Listener rules Default and Other

Listener Rules

ALB Resource Map:

ALB Resource Map

Default route EC2 web instances:

EC2

EC2

Path based route EC2 web instances:

EC2

EC2

All running instances:

AllInstances

Cleanup

Remember to stop AWS components to avoid large bills.

 
terraform destroy -auto-approve
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Conclusion

By following this guide and using Terraform for infrastructure as code, you can easily implement path-based routing with ALB and EC2 instances on AWS. This approach enhances scalability and simplifies management by consolidating multiple services under a single load balancer while maintaining clear separation based on URL paths.

Resources

ALB Path Based Routing: https://repost.aws/knowledge-center/elb-achieve-path-based-routing-alb

Github Link: https://github.com/chinmayto/terraform-aws-alb-path-based-routing

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