Amazon ELB allows you to make your applications highly available by using health checks and distributing traffic across a number of instances. Consider that you have a WordPress blog which is running on a single t2-micro EC2 instance. Now you publish an article, it goes viral and your site gets hundreds of thousands of requests. Since you are using a single t2-micro, your website will probably crash. So, what can you do to avoid this? You may decide to launch a larger instance like an m5-large in place of t2-micro. This is called vertical scaling when you replace an instance with a more powerful instance. But vertical scaling isn’t always economical. Another approach can be to use a bunch of smaller instances like t2-micros and distribute the website traffic between them. And Elastic Load Balancer allows you to do just that. It distributes incoming application or network traffic across multiple targets, such as Amazon EC2 instances, containers, and IP addresses, in multiple Availability Zones. Credits — aws.amazon.com It uses health checks to detect which instances are healthy and directs traffic only across those instances. Types of Elastic Load Balancers There are three types of load balancers available. You can use the one that best fits your use case. 1. Classic Load Balancer (CLB) This is the previous generation load balancer that was used for EC2-classic instances. It operates on both the request level and the connection level. But it doesn’t support features like host-based routing or path-based routing. Once configured, it distributes the load across all the registered instances regardless of what is present on the servers. Hence, it can only be used to distribute traffic to a single URL. 2. Application Load Balancer (ALB) This load balancer is specially designed for web applications with HTTP and HTTPS traffic. There is a networking model called the OSI Model (Open Systems Interconnection) that is used to explain how computer networks work. This model has 7 layers and the top layer is the Application Layer. This load balancer works at this Application Layer, hence the name. It also provides advanced routing features such as host-based and path-based routing and also works with containers and microservices. **Host-based Routing**Suppose you have two websites and . Each website is hosted on two EC2 instances for high availability and you want to distribute the incoming web traffic between them. medium.com admin.medium.com If you were using the CLB you would have to create two load balancers, one for each website. But you can do the same thing using a single ALB! Hence you will be saving money as you will only be paying for a single ALB instead of two CLBs. **Path-based Routing**Suppose the website of your company is and the company’s blog is hosted on . The operations team has decided to host the main website and the blog on different instances. payzello.com payzello.com/blog Using ALB you can route traffic based on the path of the requested URL. So again a single ALB is enough to handle this for you. 3. Network Load Balancer (NLB) This load balancer operates at the Network layer of the OSI model, hence the name. Suppose your company’s website is running on four m4-xlarge instances and you are using an ALB to distribute the traffic among them. Now your company launched a new product today which got viral and your website starts to get millions of requests per second. In this case, the ALB may not be able to handle the sudden spike in traffic. This is where the NLB really shines. It has the capability to handle a sudden spike in traffic since it works at the connection level. It also provides support for static IPs. I hope by this time you have got a rough idea about load balancers. Now, enough talking, let’s go practical. Creating an Application Load Balancer We will handle a case of path-based routing. We will be handling two paths here, “/” and “/blog”. We will launch two instances, one for handling each path. Let’s get started! 1. Launch two EC2 instances To learn how to launch an EC2 instance, you can read my article on . Launching an Amazon EC2 instance When launching, give a Name tag to your instances. For the first instance, give a tag with as key and as the value. For the second instance, give a tag with as key and as the value. This will help us in distinguishing between them. Name Main Name Blog After launching the two instances, your dashboard should look like this. **2. Install Apache server on instances**Now SSH into the first instance (with name Main) and run the following commands to install and start the apache server. sudo yum update -y   sudo yum install -y httpd   sudo service httpd start   sudo chkconfig httpd on   cd /var/www/html   sudo su   echo "This is the Main Website" > index.html Now paste the IP address of the instance in the browser and hit Enter. You should see something as shown in the picture below. Now SSH into the second instance (with name Blog) and run the same commands except the last command. Instead, run the following command. echo “This is the Blog Website” > blog Paste the IP address of this instance with as the suffix in the browser and hit Enter. You should see something like below. /blog 3. Create Target Groups A target group allows you to tell the load balancer which protocol and port will receive the traffic on the registered instances. 1. In the left navigation bar, scroll down and click on **Target Groups.**Now click on at the top. Create target group 2. Give your target group a name and click button. Main Create 3. Now, navigate to the tab at the bottom, click on select the Main instance, click and click Targets Edit, Add to registered Save. Create another target group with the name Blog and add the Blog instance to it as we did above. 4. Creating and configuring the Application Load Balancer Now, in the left navigation scroll down and click on Click on the button at the top. Load Balancers. Create Load Balancer 1. Choose the Application Load Balancer. 2. Give a name to your load balancer and select at least two availability zones for high availability and click on the button. Next: Configure Security Settings 3. You may see a warning message but that is because we are only listening for HTTP traffic which is fine for our case, so click on the button again. Next: Configure Security Groups 4. Here select the existing group option and select the same security group that you assigned to the instances you launched. Once done click on button. Next: Configure Routing 5. In Target groups, select the existing target group. In the name select and click Main Next. 6. Click Next again, review and click Create. Congratulations, you have just created an Application Load Balancer! But we still have to configure our Blog instance so let’s continue. Take a note of the of the Load balancer here. We will need it at the end. DNS name 7. Select the tab and Click on . Listeners View/edit rules 8. Click the + sign at the top to add a rule. In Add Condition select “ ” and type . Path is /blog 9. Then in Add Action select and select and then click Forward to Blog Save. Now, we can use the of our load balancer to visit the two different paths and see the results. DNS name For / For /blog I hope you learned something from this article. If you have any questions, please feel free to comment below. 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