The Windows Azure AppFabric Service Bus (or just service bus for short) is arguably the most accessible, powerful, and needed piece of the new Windows Azure Cloud Computing initiative. In its first release, the service bus has several manifestations: as a relay service, as an events hub, and as a message buffer. While the service bus is designed to address some tough connectivity issues, it also provides an attractive solution for scalability, availability, and security issues by lowering the technology entrance barrier and making what used to be an advanced communication scenario mainstream and mundane. As a relay service, the service bus addresses the challenge of Internet connectivity. Truth be told, Internet connectivity is difficult. Often, the service is located behind firewalls (both software and hardware firewalls) and behind a load balancer, and its address is dynamic and can be resolved only on the local network and cannot be translated to outside addressing. Virtualization adds a new dimension to connectivity along with maintaining transport sessions across machines. This situation is depicted in Figure 11-1.

Figure 11-1. Internet connectivity challenges

Moreover, since it is typically inadvisable to open the intranet to callers from the Internet, businesses often resort to using DMZs, but in turn incur the increased complexity of deployment, managing multiple sets of client credentials, and the unwanted traffic of unauthorized calls. Commonplace solutions for Internet connectivity from dynamic DNS to static IPs are often cumbersome, do not reflect changes in real time, have severe scalability and throughout implications, and are potentially insecure unless done by experts.

When it comes to having the service call back to the client, the problem is compounded by all the connectivity issues surrounding the client, virtually a mirror image of Figure 11-1. And yet, callbacks, events, and peer-to-peer calls are often an integral part of many applications, ranging from consumer machines such as non-real-time games, interactive communication, and media sharing to roaming business machines using ad hoc connections and full-fledged cross-intranet business-to-business applications.