How is high availability achieved for our Azure hosted web sites?

To achieve high availability for web site we deploy a copy of each website to a different data centre region. A region is a set of data centres deployed within a latency-defined perimeter and connected through a dedicated regional low-latency network.

Within each region the application is load balanced across at least two web server instances using the Azure App Service platform in-built load balancer. We then use the Azure Front Door Service to load balance across the websites within each region.

Azure Front Door Service is an Application Delivery Network (ADN) as a service, offering various layer 7 load-balancing capabilities for our applications. It provides dynamic site acceleration (DSA) along with global load balancing with near real-time failover. It is a highly available and scalable service, which is fully managed by Azure.

It looks at incoming HTTP requests routes to the closest application backend / region for the specified request. Front Door terminates HTTP requests at the edge of Microsoft's network and actively probes to detect application or infrastructure health or latency changes. Front Door then always routes traffic to the fastest and available (healthy) application backend.