While creating a load balancer, you need to configure several items such as the front-end and backend pools, load balancing rules, session persistence, and health probes. In Exercise 5.1, you will create an availability set and configure a load balancer; however, before that, let’s learn the terminologies related to the Azure load balancer configuration. Let’s start with backend pools.
Backend Pools
When you configure a load balancer, you need to specify the backend pool. The backend pool contains the IP address of the network interface cards that are attached to the set of virtual machines or virtual machine scale set. In Figure 5.4, you can see that the backend pool is represented using a set of VMs. Depending on the type of SKU you are choosing, the type of backend pool that can be associated will be different. The Standard SKU supports an availability set, a single virtual machine, or a virtual machine scale set.
In the Standard SKU, you can have up to 1,000 instances, and the Basic SKU can have up to 100 instances in the backend pool.
Health Probes
The purpose of a health probe is to let the load balancer know the status of the application. The health probe will be constantly checking the status of the application using an HTTP or TCP probe. If the application is not responding after a set of consecutive failures, the load balancer will mark the virtual machine as unhealthy. Incoming requests will not be routed to the unhealthy virtual machines. The load balancer will start routing the traffic once the health probe is able to identify that the application is working and is responding to the probe.
In the HTTP probe, the endpoint will be probed every 15 seconds (default value), and if the response is HTTP 200, then it means that the application is healthy. If the application is returning a non-2xx response within the timeout period, the virtual machine will be marked unhealthy. While configuring the HTTP probe, you can configure the port number, path to probe, interval, and timeout period.
Using the TCP probe will help you verify if the load balancer is able to establish a TCP connection on the specified port. If the connection is successful, then the endpoint is healthy. If the connection is refused, the probe fails, and the virtual machine will be marked as unhealthy. For the TCP probe, you configure the port, interval, and unhealthy threshold.
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