Middleware: a model for distributed system services

he computing facilities of largescale enterprises are evolving into a utility, much like power and telecommunications. In the vision of an information utility, each knowledge worker has a desktop appliance that connects to the utility. The desktop appliance is a computer or computer-like device, such as a terminal, personal computer, workstation, word processor, or stock trader’s station. The utility itself is an enterprise-wide network of information services, including applications and databases, on the localarea and wide-area networks. Servers on the local-area network (LAN) typically support files and file-based applications, such as electronic mail, bulletin boards, document preparation, and printing. Local-area servers also support a directory service, to help a desktop user find other users and find and connect to services of interest. Servers on the wide-area network (WAN) typically support access to databases, such as corporate directories and electronic libraries, or transaction processing applications, such as purchasing, billing, and inventory control. Some servers are gateways to services offered outside the enterprise, such as travel or information retrieval services, news feeds (e.g., weather, stock prices), and electronic document interchange with business partners. In response to such connectivity, some businesses are redefining their business processes to use the utility to bridge formerly isolated component activities. In the long term, the utility should provide the information that people need when, where, and how they need it. Today’s enterprise computing facilities are only an approximation of the vision of an information utility. Most organizations have a wide variety of heterogeneous hardware systems, including personal computers, workstations, minicomputers, and mainframes. These systems run different operating systems (OSs) and rely on different network architectures. As a result, integration is difficult and its achievement uneven. For example, local-area servers are often isolated from the WAN. An appliance can access files and printers on its local server, but often not those on the servers of other LANs. Sometimes an application available on one local area server is not available on other servers, because other departments use servers