Middleware in System Integration
Integrating diverse software applications, whether on a single processor or across a network, demands a deep understanding of the operating systems on the connected machines and the communication protocols governing data packet transmission and verification. However, the complexities introduced by programming at the operating system or network level are often undesirable for control engineers. As an alternative, middleware, depicted in Figure 1, serves as an intermediary layer between the operating system and application software.
Application of Middleware
Middleware plays a major role in setting up communication protocols and interfacing with the operating systems of networked machines. This enables various software applications, running on different or the same machines, to communicate seamlessly. Middleware acts as a buffer, shielding application users from lower-level operations. A notable advantage of middleware is its ability to facilitate interactions between different application software programs, allowing functionalities like cutting and pasting across diverse application windows.
When used as the foundation for application integration, middleware can be viewed as a software substrate. For optimal effectiveness and user-friendliness, middleware should possess specific properties. Firstly, it must feature a standard interface, facilitating easy integration of all application software programs adhering to the standard. Secondly, middleware should support heterogeneous interoperability, enabling the integration of application software written in different languages or running on various processor platforms. The ideal middleware for control systems should exhibit plug-and-play capabilities and support runtime reconfiguration of software components, allowing online updates or changes to algorithms in real time.
While the control engineering field has yet to fully leverage middleware for plug-and-play and runtime reconfiguration, advancements are being made. The latter section of this chapter delves into available products. Before exploring these, fundamental concepts in software engineering are introduced.
Object-oriented middleware, a key approach for distributed communication channels, employs the publish/subscribe communication method. Treating connected software and hardware components as objects, object-oriented middleware employs remote procedure calls (RPCs) to make method requests transparent to calling objects on a distributed communication network. The standardized interface and mediated coupling through the communication channel simplify the addition or removal of objects from the system.
Three alternatives for object-oriented middleware—DCOM, CORBA, and Java-based technologies, including Jini technology—are considered for their unique contributions in enabling seamless communication and integration within distributed systems.