Once you have a load balancer in place, you have the ability to configure request routing. What is the idea behind it? You can have multiple backend sets for a given load balancer. Why would you want to do that? In case you have multiple applications available, you may have multiple compute instances, or backend servers that are available, and you can thereby create a separate backend set for each application as such. Wherein, I'm going to group this group and this group as two different backends sets. I call this as Backend Set 1. I call this as Backend Set 2, and maybe Backend Set 1 gives me my ERP application, maybe Backend set 2 has the CRM application from the application standpoint. When we provision our load balancer, we choose only one backend set as part of the provisioning of the load balancer. We have configured a listener through which it will listen and respond. But once the load balancer is provisioned, we see that there is the ability to go to the load balancer page and add backend sets. When you are adding a backend set, the whole idea is, given a new backend set, choose the SSL, what type of communication you want, what load balancing policy you want, whether you want session persistence health check. This health check is specific to this backends set, and as a result, when you have one load balancer, you can actually use it to frontend the different applications, each coming from its own backend set. In fact, you can also attach an instance pool as a backend set, so that a collection of nodes which offer the same applications from the compute instance pool feature that we have, which you can create from Compute Instance Pools, can be attached to a load balancer so that that application hosted in the Instance Pool can be front-ended by the load balancer. Backend sets gives you the ability to create different set of nodes for different applications. Now, when we come into load balancing routing, when we have multiple applications, how will the load balancer know which application is there and which backend set? For which you have two choices. You can use virtual hostnames or path route rules. When we talk about virtual hostnames, you assign a virtual hostname with a listener, and you use that to be listening for a particular application. From the DNS, when the IP addresses result, you have only one IP address for your load balancer. But when the DNS resolution comes through a particular virtual hostname, it will be route it to the appropriate backend server, which means when you go to your load balancer's page, you have the ability to configure additional hostnames. When you go and add additional listeners, you will have the ability to choose the backend set for which this listener will listen, and how do you want to route it with the hostnames. That is one choice that you have. The other choice that is available is, so you define the exact virtual hostnames and include it in the listener. You have the ability to use wildcards either exact match or a wildcard in front or a wildcard at the end. You can use the way you want the hostname resolution to route traffic. Or you may use path route rules. Depending on the URL in the application URL that you have. For example, if I go back to the example that I gave earlier, wherein you will have multiple applications in different backend sets grouped together. Maybe you have your Backend Set 1 with the ERP application, Backend Set 2 with the CRM application, and you have a common load balancer that is set up. The way you are going to make your clients access it is from a single URL. Let's say app.com is the URL. Now, you are not going to have different virtual hostnames and listeners for each application, instead, if somebody is accessing the ERP application, maybe they give app.com/erp. Maybe when people are accessing the CRM application, they give the URL as app.com/crm. Now, when the request comes for a particular path in the URL from your clients, you will want them to be routed to the corresponding backend set, which is exactly what you achieved by using path route rules. If you go to your load balancer's page, you'll have path route rule sets, wherein you can choose what URL matches you want to do. There are various options available to specify, and based on the URL string, you can get it routed to a particular backend set. That's about using load balancing request routing with either the ability to use virtual hostnames or path route rules, so that you can distribute your load based on the application logic to specific backend sets.
