# Application Load Balancer (ALB)

## What is an Application Load Balancer?

An **Application Load Balancer (ALB)** operates at **OSI Layer 7** (the application layer), meaning it understands HTTP and HTTPS traffic and can make routing decisions based on the content of each request. When a client sends a request to the ALB's DNS endpoint, the ALB inspects the request and forwards it to one of the healthy registered targets in its **target group**, distributing load across multiple backend instances. This means you expose a single DNS name to the internet, and the ALB handles spreading requests across your backend — no single instance bears all the traffic. ALB also supports advanced routing rules (path-based, host-based), sticky sessions, and continuous health checks that automatically remove failed instances from rotation.

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## Lab Overview

| Resource | Name |
|---|---|
| Load Balancer | `demo-ALB` |
| Target Group | `demo-TG-ALB` |
| Security Group (ALB) | `demo-SG-ALB` |
| EC2 Instance 1 | `Demo-EC2-ALB-1` |
| EC2 Instance 2 | `Demo-EC2-ALB-2` |
| Region | `ap-south-1` (Mumbai) |
| Protocol | HTTP port `80` |
| Target type | EC2 instances |

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## Video Demo

[![Application Load Balancer – Full Demo](https://img.youtube.com/vi/JrgvgSGxSdw/0.jpg)](https://youtu.be/JrgvgSGxSdw)

---

## Step-by-Step Walkthrough

### Step 1 — Open the AWS Console

Log in to AWS and navigate to the Console Home. From the recently visited services or the search bar, click **EC2** to start launching instances.

![AWS Console Home – click EC2](./images/application-load-balancer-01.png)

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### Step 2 — Launch EC2 Instances

#### 2a — Set instance name and choose OS

In the EC2 dashboard click **Launch instances**. Enter a name for the instance and select **Amazon Linux 2023** as the AMI.

![EC2 launch wizard – name and OS](./images/application-load-balancer-02.png)

#### 2b — Configure security group and allow HTTP

Under **Network settings**, select an existing security group or create a new one. Make sure **Allow HTTP traffic from the internet** is checked so the web server is reachable on port 80.

![Security group selection – allow HTTP](./images/application-load-balancer-03.png)

#### 2c — Add the user data script

Expand **Advanced details** and scroll down to the **User data** field. Paste the following script:

```bash
#!/bin/bash
# Use this for your user data (script from top to bottom)
# install httpd (Linux 2 version)
yum update -y
yum install -y httpd
systemctl start httpd
systemctl enable httpd
echo "<h1>Hello World from $(hostname -f)</h1>" > /var/www/html/index.html
```

**What this script does:**

| Command | Purpose |
|---|---|
| `yum update -y` | Updates all installed packages to the latest versions |
| `yum install -y httpd` | Installs Apache HTTP Server |
| `systemctl start httpd` | Starts the web server immediately after boot |
| `systemctl enable httpd` | Ensures the web server starts automatically on every reboot |
| `echo ... > /var/www/html/index.html` | Creates a simple HTML page displaying the instance's private hostname — this is how you can tell which backend instance responded when testing the ALB |

Set **Number of instances** to `2`, then click **Launch instance**.

![User data script entered – launching 2 instances](./images/application-load-balancer-04.png)

#### 2d — Rename both instances

Once both instances are running, rename them to `Demo-EC2-ALB-1` and `Demo-EC2-ALB-2` by clicking the pencil icon next to each name.

![Both EC2 instances renamed](./images/application-load-balancer-05.png)

---

### Step 3 — Navigate to Load Balancers

In the EC2 left sidebar, scroll down to the **Load Balancing** section and click **Load Balancers**. Then click **Create load balancer**.

![EC2 instances running](./images/application-load-balancer-06.png)

![Left panel – navigate to Load Balancers](./images/application-load-balancer-07.png)

#### 3a — Choose Application Load Balancer

On the **Compare and select load balancer type** page, click **Create** under **Application Load Balancer**.

![Load balancer type selection – choose ALB](./images/application-load-balancer-08.png)

---

### Step 4 — Configure the ALB

#### 4a — Basic configuration

Enter the following settings:
- **Load balancer name:** `demo-ALB`
- **Scheme:** Internet-facing
- **IP address type:** IPv4
- **VPC:** Select your default VPC (or your custom VPC)

![ALB basic config – name, internet-facing, IPv4, VPC](./images/application-load-balancer-09.png)

#### 4b — Select all availability zones

Under **Mappings**, select all available availability zones. This ensures the ALB can route to instances in any AZ and improves fault tolerance.

![All availability zones selected](./images/application-load-balancer-10.png)

---

### Step 5 — Create and Assign a Security Group

#### 5a — Create a new security group

Click **Create new security group** next to the security group selector. This opens the Security Groups console in a new tab.

![Create new security group form](./images/application-load-balancer-11.png)

#### 5b — Fill in security group details

Enter:
- **Security group name:** `demo-SG-ALB`
- **Description:** Security group for Application Load Balancer
- **VPC:** Select the same VPC chosen for the ALB

![SG name, description, and VPC](./images/application-load-balancer-12.png)

![Security group inbound rules section](./images/application-load-balancer-13.png)

#### 5c — Add inbound rule

Under **Inbound rules**, click **Add rule**. Set:
- **Type:** HTTP
- **Source:** `0.0.0.0/0` (allows all internet traffic on port 80)

Then click **Create security group**.

![Inbound rule – HTTP from 0.0.0.0/0](./images/application-load-balancer-14.png)

![Security group created successfully](./images/application-load-balancer-15.png)

#### 5d — Select the new security group in the ALB config

Go back to the ALB creation tab, refresh the security group list, and select `demo-SG-ALB`. Scroll down to continue.

![New SG selected in ALB config](./images/application-load-balancer-16.png)

---

### Step 6 — Configure Listener and Target Group

#### 6a — Set default listener action

Under **Listeners and routing**, the default listener is HTTP port 80. Set the **Default action** to **Forward to** a target group. Click **Create target group** to open the target group wizard.

![Listener default action – forward to target group](./images/application-load-balancer-17.png)

#### 6b — Configure target group basics

In the target group wizard, set:
- **Target type:** Instances
- **Target group name:** `demo-TG-ALB`
- **Protocol:** HTTP
- **Port:** 80
- **IP address type:** IPv4

![Target group config – instances, HTTP 80, IPv4](./images/application-load-balancer-18.png)

![Target group health check settings](./images/application-load-balancer-19.png)

Scroll down and click **Next**.

![Scroll down and click Next](./images/application-load-balancer-20.png)

#### 6c — Register targets

Select both EC2 instances (`Demo-EC2-ALB-1` and `Demo-EC2-ALB-2`) from the available instances list, then click **Include as pending below**.

![Select all instances and include as pending](./images/application-load-balancer-21.png)

Click **Next** to proceed to the review step.

![Click Next – proceed to review](./images/application-load-balancer-22.png)

Click **Create target group** to finalize.

![Create target group](./images/application-load-balancer-23.png)

![Target group created confirmation](./images/application-load-balancer-24.png)

![Target group detail view](./images/application-load-balancer-25.png)

#### 6d — Select the target group in the ALB config

Return to the ALB creation tab and select `demo-TG-ALB` as the target group for the listener's default action.

![Select newly created target group](./images/application-load-balancer-26.png)

Scroll down and click **Create load balancer**.

![Click Create load balancer](./images/application-load-balancer-27.png)

---

### Step 7 — ALB Created Successfully

The ALB is now created. You will see a success banner and the ALB will appear in the load balancers list with state **Active** once it finishes provisioning.

![ALB created – success banner](./images/application-load-balancer-28.png)

![ALB detail view – Active state](./images/application-load-balancer-29.png)

---

### Step 8 — Test the ALB

#### 8a — Copy the DNS name

In the **Load Balancers** list, select `demo-ALB` and copy the **DNS name** from the details panel at the bottom.

![Load balancer list – locate DNS name](./images/application-load-balancer-30.png)

![DNS name copied](./images/application-load-balancer-31.png)

#### 8b — Open in a browser

Paste the DNS name into your browser's address bar and press Enter. You should see a page displaying one of the instance hostnames.

![Browser – Hello World from instance 1](./images/application-load-balancer-32.png)

#### 8c — Refresh to confirm load distribution

Refresh the page. The ALB routes the second request to the other backend instance, and a different hostname appears — confirming the load balancer is distributing traffic across both EC2 instances.

![Browser after refresh – Hello World from instance 2](./images/application-load-balancer-33.png)

> **ALB in action:** The same DNS endpoint served two different responses from two different backend instances. This is exactly what ALB is designed to do — distribute incoming HTTP requests across all healthy registered targets, preventing any single instance from becoming a bottleneck.

---

### Step 9 — Cleanup

#### 9a — Delete the ALB

In the **Load Balancers** console, select `demo-ALB`, click **Actions > Delete load balancer**, type `confirm` in the confirmation field, and click **Delete**.

![Delete ALB – Actions menu, type confirm](./images/application-load-balancer-34.png)

![ALB deleted successfully](./images/application-load-balancer-35.png)

#### 9b — Delete the security group

Navigate to **EC2 > Security Groups**, select `demo-SG-ALB`, and click **Actions > Delete security groups**.

![Delete security group](./images/application-load-balancer-36.png)

#### 9c — Terminate EC2 instances

Navigate to **EC2 > Instances**, select both `Demo-EC2-ALB-1` and `Demo-EC2-ALB-2`, click **Instance state > Terminate instance**, and confirm.

![Terminate EC2 instances](./images/application-load-balancer-37.png)

All resources used in this lab have been deleted.