Application Load Balancer?

A load balancer serves as the single point of contact for clients. The load balancer distributes incoming application traffic across multiple targets, such as EC2 instances, in multiple Availability Zones. This increases the availability of your application. You add one or more listeners to your load balancer.

[ Reference Source: AWS documentation ]

Let's start the Lab session in AWS account

In this lab you will learn the following:

How to create an EC2 instance with user data
How to make simple VPC
How to make a security group suit for ELB
How to configure multiple EC2 with an Application load balancer

Step 1: Create a Simple VPC.

Search VPC and create VPC as the following VPC setting:
- Resources to create: VPC Only
- Name tag - optional: myVPC
- IPv4 CIDR: 10.0.0.0/16
- Leave all default and Create VPC
Select subnets and create subnets as:
- VPC ID: Select myVPC
- Subnet Settings: (for subnet1)
  - Subnet name: subnet1
  - Availability Zone: Select 1st zone
  - IPv4 CIDR block: 10.0.0.0/24
- Add new subnet
- Subnet Settings: (for subnet2)
  - Subnet name: subnet2
  - Availability Zone: Select 2nd zone
  - IPv4 CIDR block: 10.0.1.0/24

( Note: you can add more than 2 subnets also )

Create subnet

Create an internet gateway and attach it to VPC as:
- Internet gateway settings
  - Name tag: myIG
    
    Create internet gateway
- Attach to a VPC and select myVPC
  
  Attach internet gateway
Select the route table and Create the route table as:
- Route table settings
  - Name - optional: route
  - VPC: Select myVPC

Create route table

Select route ID and then go to Routes -> Edit routes -> Add route
- Destination: 0.0.0.0/0
- Target: Select internet gateway and select that we created
  
  and Save changes
Similarly, Select route ID and then go to Subnet associations -> Edit Subnet associations
- Select all subnets and Save associations

Step 2: Create a security group for ELB:

Search VPC, scroll down its left sidebar and create a security group as:

Security group name: ELBsecurity
Description: Elastic load balancer use it
VPC: remove existing and select myVPC
Inbound rules:
- Add rule
  1. Type: SSH, Source: 0.0.0.0/0
  2. Type: HTTP, Source: 0.0.0.0/0
Outbound rules:
- Leave default i.e. Type: All traffic, Source: 0.0.0.0/0
Create security group

Step 3: Create two EC2 instances.

Search EC2 and Launch EC2 instance as:
For the first EC2 instance
- Name: server1
- Create a key-pair or choose any existing key.
- Network Setting > Edit
  - VPC: Select myVPC
  - Subnet: Select subnet1
  - Auto-assign public IP: enable
  - Firewall: Check select existing security group and select ELBsecurity that we created
- Leave all default and Expand Advanced details
  - Go directly to the user data field at last and copy-paste this:
```
  #!/bin/bash
  sudo yum update -y
  sudo yum install -y httpd
  sudo systemctl start httpd
  echo "<html><body><h1> Hello! I am server1 </h1></body></html>" > /var/www/html/index.html
```
    Launch instance
For the second EC2 instance
- Name: server2
- Create a key pair or choose any existing key.
- Network Setting > Edit
  - VPC: Select myVPC
  - Subnet: Select subnet2
  - Auto-assign public IP: enable
  - Firewall: Check select existing security group and select ELBsecurity that we created
- Leave all default and Expand Advanced details
  - Go directly to the user data field at last and copy-paste this:
```
  #!/bin/bash
  sudo yum update -y
  sudo yum install -y httpd
  sudo systemctl start httpd
  echo "<html><body><h1> Hello! I am server2 </h1></body></html>" > /var/www/html/index.html
```
    Launch instance
Also, you can check after the 2/2 check passed whether your instances are publically accessible or not by rendering their public IP in a new tab.

For example, server2 public IP gives:

Step 4: Create an Application load balancer.

In the left bar of EC2 scroll down and click on Load balancers
Click Create load balancer -> Application load balancer Create
Create Application Load Balancer as:
Basic configuration
- Load balancer name: balanceLoad
- Leave all default
Network mapping
- VPC: Select myVPC
- Mappings: Select at least two availability zones for now we have only 2 subnets of different availability zones so select both
Security groups
- Security groups: Select ELBsecurity and remove the default one
Listeners and routing
- Listener HTTP:80
  
  Default action: Select a target group but we don't have a target group. Our priority is to create a target group, So select Create target group.
  
  [you can also create a target group first and then associate at that point.]
- It renders directly to the Create target group or you can select manually at the left bar of the EC2 instance just below the load balancers

{

Create target group as:
- Specify group details:
  - Target group name: targetELB
  - VPC: myVPC
  - Protocol version: leave as HTTP1
  - Leave all default and Next
- Register targets
  - Select both available instances and Click on Ports for the selected instances: 80 and
    
    Include as pending below
  - Then Create target group

}

Go back to Listeners and routing, refresh, and select targetELB

Leaving all default and Create load balancer

Step 5: Render the DNS of an Application load balancer.

Go to load balancer and copy the DNS name of balanceLoad that we created.
Open a new tab of any browser and paste it.
Now, observe by refreshing the tab how the load is distributed.

Application load balancer Demonstration