Now, I will go to each of those two instances and will start tomcat server. After some minutes (or seconds) I should see my deployed web application is up and running. Which means, I can navigate to these URLs and able to see Log-In screen (initial page of my web app).
All of the above steps which I described so far, have nothing to do with Elastic Load Balancing (ELB). Just like everyone, I just deployed a web app in tomcat server. Before I start showing steps for ELB, I'm assuming your web application is also up and running and you can navigate through URLs separately.
Create Load Balancer
Step#1: On AWS EC2 console, click on the Load Balancer option under "Network & Security" section. If you do not have any ELB yet, you will see an empty list. Click on "Create Load Balancer" button.
Step#2: Write a name of your Load Balancer, this name will be used when it will create a default link. I'm also creating this Load Balancer inside my Virtual Private Cloud (VPC) that's why I'm selecting a specific VPC Id. By default, you might see only port 80 in the listerner configuration list, I have added port 8080 as my web app is running on port 8080. Add appropriate port based on your web application and click "Continue".
Step#3: This screen is dedicated to Health check configuration. Based on configuration, ELB will ping that path with that port to check the health condition and if it fails it will automatically remove your instances from the load balancer.
Since, Log-In is the default screen of my application (welcome page), so I'm using the path of Login screen as my ping path.
Step#4: Choose your Subnet id based on where you want to use your Load Balancer. For my case, subnet-2e961843 is my expected Subnet id.
Step#5: Next screen will ask you to select your security groups. I already have a security group for my VPC and I'm using it here too.
Step#6: In the "Add EC2 Instances" section, add the instances in where you already deployed Tomcat and your web application.
Step#7: This screen is for review purpose. Once you review it you can finally create your load balancer by clicking on the "Create" button.
Step#8: Once you create your balancer, it will redirect you to Load Balancer list and now you will see your newly created load balancer in the list. DNS Name column shows newly created DNS Name for your load balancer and you should be able to navigate it with proper port.
So for my case, I can navigate my load balancer by using:
Since you are using Tomcat with load balancer, it's pretty obvious that you might want to enable sticky session with session replication in Tomcat. My web application is a Spring MVC application and it uses Spring Security for all type of authorizations and authentications. If I directly go to the Log-In screen of my load balancer and try to authenticate, it might not work. It's expected as Tomcat gets confused when sending request and response in multiple instances. If I enable sticky session I will not face this issue.
You can do it with the help of AWS EC2 console. Open the Load Balancer screen and select your newly created load balancer.
If you look carefully at the port configuration part, you will see "Stickiness: Disabled" for all of your ports. By default, stickiness is disabled for all the ports you select for load balancer. Now click on the "edit" button of the port on where you want to enable stickiness. For my case, it will be port 8080. Once you click on the "edit" button, it will ask you how you want to enable session stickiness. You can either choose Load Balancer Generated Cookie Stickiness or Application Generated Cookie Stickiness. For my simple application, I have selected "Load Balancer Generated Cookie Stickiness" and I entered 86400 as my cookie expiration period which is a day in seconds.
Some considerations: Sometimes you might see your load balancer is down or the link is not working or shows no page. In that case, best way to quickly test is to check each of the instance where tomcat is running and check whether you can access them individually (e.g. http://ip.address.instance-1:8080/HelloWorld/login.jsp). If you find that each of the instance is up and running, you can try removing them from your load balancer and add them again. Remember, "Status" section under "Description" tab of your load balancer does not get updated instantly. It takes some time and it waits for the result of the next health check. So wait few minutes until you see "Status: N of N instances in service".
That's pretty much it! This is the very basic AWS Load Balancer example with minimum configuration of Tomcat + Session Stickiness. Once its working for you, you can try other options (highly encouraged) and see how it works for you.
Note: For privacy purpose, I had to modify several lines on this post from my original post. So if you find something is not working or facing any issues, please do not hesitate to contact me :)