AWS: Highly available infrastructure using ASG and ELB

Building a Highly Available Environment on AWS using Auto Scaling Group and Load Balancer 1. Introduction Highly available infrastructure ensures that applications remain accessible even if one or more servers fail. In AWS, this can be achieved by combining: - EC2 Instances - Auto Scaling Group (ASG) - Elastic Load Balancer (ELB) - Health Checks This architecture automatically replaces unhealthy instances and distributes traffic across multiple servers. --- 2. Prerequisites Before starting the setup, ensure the following components are available. 2.1 AWS Account You must have access to the AWS Management Console. 2.2 VPC Setup A Virtual Private Cloud with: - At least two public subnets - Subnets in different Availability Zones Example: VPC ├── Subnet-A (Availability Zone 1) └── Subnet-B (Availability Zone 2) 2.3 Security Group Allow the following ports: HTTP (80) HTTPS (443) SSH (22) --- 3. Launch Base EC2 Instance First create a base EC2 instance which will later be used to create an AMI. Steps: 1. Go to EC2 Dashboard 2. Click Launch Instance Example configuration: Name: web-server OS: Amazon Linux / Ubuntu Instance type: t2.micro Security group: allow HTTP and SSH --- 4. Install Application on EC2 Login to the EC2 instance. Example: Install Nginx web server. Amazon Linux sudo yum install nginx -y sudo systemctl enable nginx sudo systemctl start nginx Ubuntu sudo apt update sudo apt install nginx -y sudo systemctl enable nginx sudo systemctl start nginx Verify: http://EC2-PUBLIC-IP You should see the default Nginx page. --- 5. Create AMI from EC2 Instance Now create an Amazon Machine Image (AMI) from the configured instance. Steps: 1. Select the EC2 instance 2. Click Actions 3. Image → Create Image Example: Image name: webserver-ami This AMI will be used by the Auto Scaling Group to launch instances. --- 6. Create Target Group A target group registers EC2 instances for the load balancer. Steps: 1. Go to EC2 Console 2. Click Target Groups 3. Click Create Target Group Configuration: Target type: Instances Protocol: HTTP Port: 80 Health check path: / Name: web-target-group --- 7. Create Application Load Balancer (ALB) Steps: 1. Go to EC2 Console 2. Click Load Balancers 3. Click Create Load Balancer Select: Application Load Balancer Configuration: Name: web-alb Scheme: Internet-facing Listeners: HTTP (80) Select the VPC and both subnets. Attach the target group created earlier. --- 8. Create Launch Template Launch Template defines how new instances will be created. Steps: 1. Go to EC2 Console 2. Click Launch Templates 3. Click Create Launch Template Configuration: Name: web-launch-template AMI: webserver-ami Instance type: t2.micro Security group: web-security-group Save the template. --- 9. Create Auto Scaling Group (ASG) Steps: 1. Go to Auto Scaling Groups 2. Click Create Auto Scaling Group Configuration: Name: web-asg Launch template: web-launch-template Select VPC and both subnets. --- Desired Capacity Example: Minimum instances: 2 Desired instances: 2 Maximum instances: 4 This ensures that at least two servers are always running. --- 10. Attach Load Balancer to ASG While creating the Auto Scaling Group: Select: Attach to an existing load balancer Choose: web-alb web-target-group Now the load balancer distributes traffic to ASG instances. --- 11. Configure Health Checks Health checks ensure that unhealthy instances are automatically replaced. ELB Health Check Protocol: HTTP Path: / Interval: 30 seconds ASG Health Check Enable: ELB health check If an instance fails the health check: ASG terminates instance ASG launches new instance --- 12. Enable Auto Scaling Policy Auto Scaling adjusts the number of instances based on load. Example policy: Scale out: If CPU > 70% add 1 instance Scale in: If CPU < 30% remove 1 instance This keeps infrastructure cost-efficient while handling traffic spikes. --- 13. Test High Availability Test the setup. Step 1 Access the application using the Load Balancer DNS name. Example: http://web-alb-123456.ap-south-1.elb.amazonaws.com Step 2 Terminate one EC2 instance manually. Expected behavior: ASG detects instance termination ASG launches a new instance automatically Load balancer routes traffic to healthy instances This confirms high availability. --- 14. Architecture Diagram Internet │ Application Load Balancer │ ┌────────┴────────┐ │ │ EC2 Instance EC2 Instance (AZ-1) (AZ-2) │ │ Auto Scaling Group (ASG) │ Launch Template + AMI --- 15. Benefits of this Architecture High Availability Traffic is distributed across multiple instances in different Availability Zones. Fault Tolerance If one instance fails, a new instance is automatically launched. Auto Scaling Infrastructure automatically adjusts based on demand. Load Distribution The load balancer ensures even traffic distribution. --- 16. Conclusion Using Auto Scaling Groups with Application Load Balancers is a standard approach for building highly available and fault-tolerant applications on AWS. This architecture ensures application uptime, automatically replaces failed instances, and dynamically scales infrastructure to handle changing workloads.