Azure application gateways

Azure load balancer helps us to enable solutions at the infrastructure level. However, there are times when advance services and features are required from the load balancer itself. These advance services include SSL termination, sticky sessions, advanced security, and more. Azure application gateways are built on top on Azure load balancers to provide these additional features. The Azure application gateway is a level 7 load balancer that works with the application and session payload in a TCP OSI stack. Application gateways have more information compared to the Azure load balancer to take decisions on request routing and load balancing between servers. Application gateways are managed by Azure and are highly available.

An application gateway sits in between the users and virtual machines, as shown in the following figure:

Application gateways are internally implemented using virtual machines. Internet information service (IIS) is installed and configured with Application Request Routing (ARR) on these virtual machines. These gateways can be installed on multiple virtual machines providing High Availability for the gateways themselves. Although not visible, Azure load balancers distribute loads among multiple application gateway servers. Creating an application gateway needs an internal or public IP address and that is used by users to send requests to it. This public IP or internal IP is provided by the Azure load balancer working at the transport level (TCP/UDP) and having all incoming network traffic being load balanced to the application gateway worker instances. The application gateway then routes the HTTP/HTTPS traffic based on its configuration whether it's a virtual machine, cloud service, internal, or an external IP address.

An application gateway is similar to Azure load balancer from a configuration perspective with additional constructs and features. It provides frontend IP, protocol, certificate and port configuration, backend pool, port, session affinity, and protocol configuration.