Microsoft Application Request Routing (ARR) is a proxy-based routing module that forwards HTTP requests to application servers based on HTTP headers, server variables, and load balance algorithms. With ARR you can increase application availability and scalability by better utilization of content server resources with lower management cost by creating opportunities for shared hosting environments.
ARR relies on the URL rewrite module to inspect incoming HTTP requests to make the routing decisions. Therefore, the URL rewrite module is required to enable ARR features. Using ARR in a shared hosting environment introduces a new deployment architecture that provides additional benefits and opportunities
for shared hosting. This scenario is enabled by a feature called host name affinity. The host name affinity feature in Application Request Routing enables shared hosting to rethink how sites are deployed. Application Request Routing affinitizes the requests, regardless of whether they are made from one client or multiple clients, to one server behind ARR, ensuring that a given site is consuming resources only on one of the servers.
For e.g. in the diagram the ARR load balances the request to one of the server and affinitizes the request for a DNS to the same server for...(Read whole news on source site)