The host infrastructure middleware is closely related with the operating system as well as communication protocols. The main function of recognizing this layer is to create a network environment for higher level layers and applications to communicate. In most literatures, this layer does not exist. Because normally, an implementation in this layer achieves more functionalities than just providing support for distributed applications. A good example is the Sun Java Virtual Machine (JVM). It provides a platform-independent way of executing code by abstracting the differences between various operating systems and CPU architectures. Apparently, JVM provides an environment where other level middlewares can be built upon. By this meaning, JVM is a middleware to help solving heterogeneity problem crossing different platforms. On the other hand, JVM provides more functions than a middleware as an advanced programming environment.
A closer example in sense of providing middleware service in this layer is Microsoft's .NET platform. It is mainly implemented for XML Web services, which is a distributed system in general. It is closely related with DCOM, which belongs to next layer in the architecture. Figure 3 shows the XML Web service interaction from [7].
Figure 3: XML Web service interaction
XML Web service provides a simple model for distributed applications as illustrated in the figure. Some later layer middleware implementation such as SOAP is also evolved. However, just from the .NET platform point of view, it is designed to connect information, devices, and application in a common, yet customizable way to support XML Web service. It thus provides the function of a host infrastructure middleware.