For information systems, it is increasingly difficult to draw a line around an application system and say that you own and control it. For example, as value chains extend beyond enterprises, supplier and customer systems become part of each other's information architectures. Furthermore, in many application areas, data is distributed over a multitude of heterogeneous, often autonomous information systems, and an exchange of data among them is not easy. Figure 1 illustrates such a vertical fragmentation of organizational units. Each unit may be structured within three architectural layers, as described in the following.
The business architecture layer defines the organizational structure and the workflows for business rules and processes. It is a conceptual level expressed in terms meaningful to actual users of application systems.
The application architecture layer defines the actual implementation of the business concepts in terms of enterprise applications. At this layer, it is the central goal to provide the "glue" between the application domain described in the business architecture and the technical solutions described in the technology architecture. Research in information systems aims at filling the gap between business and technology, which requires interdisciplinary cooperation between the application domain and information technology.
The technology architecture layer defines the information and communication infrastructure. At this layer, IT is challenged to achieve the business requirements.
It is important to realize that Figure 1 does not adequately reflect the reality. In practice, the business architectures of the individual organizational units cannot be treated in isolation: the business processes of cooperating units are highly interrelated and should be handled as such. Figure 2 illustrates this situation. Certain kinds of interactions among computer systems resemble interactions among people; thus, it is important to consider all levels when integrating those systems. A horizontal integration of the layers is required to support the business processes effectively, as indicated here.
Interorganizational processes. At this layer, business engineering  seeks to organize a commercial undertaking in a competitive way, whereby business processes cut horizontally through the traditional organization structure. Business process reengineering aims at continuously improving those processes.
To support the intraorganizational business processes within organizations effectively, the existing information systems must be integrated. This is already a nontrivial task, particularly if heterogeneous information systems exist (legacy systems). To allow, for instance, for electronic supply chains, interorganizational processes have to be supported, whereby the involved information systems are highly autonomous, making the integration process an even more challenging task.
Enterprise application integration. The goal is to integrate independent enterprise resource planning (ERP) systems at this layer. This is usually achieved by means of some kind of messaging services. Even the SAP R/3 approach, which aims at enterprise integration via one single database (no borders between enterprise units), acknowledges the fact that messaging services are required for integrating autonomous ERP systems, both within and across enterprises . TSI Software's Mercator product (www.mercator.com), for instance, specializes in pre-built application adapters, data transformations, and messaging services among the ERP systems SAP R/3 and PeopleSoft.
The deployment of ERP systems often requires reengineering the business processes to align with the ERP system. However, it is usually unacceptable to require the business to change to the applications' functionality; instead the information architecture should align with the business organization. SI and componentization aim at supporting the business processes, while preserving the investments in (legacy) systems.
Applications need to understand the data provided by other applications; for instance, a common understanding is required of what a person's bank account is. Standardization of message formats and message content plays an important role in this context. Meanwhile, XML (www.w3.org) is emerging as the standard for defining the syntax of data structures to be transferred over the Internet. In order to provide interoperability across implementations, the concrete syntax and the semantics of standardized messages must be defined. Traditional EDI (Electronic Data Interchange) is often being reexamined to define the meaning of the transferred data, and XML is employed as the practical foundation used to structure this information.
Middleware integration. At this layer, the techniques for building componentized information systems with state-of-the-art infrastructures such as CORBA, database gateways, and transaction monitors, are employed. Middleware integration addresses the syntactical level ("plumbing" and "wiring") while Enterprise Application Integration also addresses a semantic level.
The borderline between Enterprise Application and middleware integration cannot always be pinpointed precisely. For instance, the Object Management Architecture of the OMG defines the Object Request Broker, which can be deployed for middleware integration, and also high-level services (such as business objects) that address Enterprise Application Integration.
The integration of heterogeneous systems is a research topic for different disciplines in IT, and includes the consideration of the requirements of the specific application domains involved. Therefore, the study of SI is highly interdisciplinary, as discussed in the sidebar "IT Disciplines Involved in Information SI." Despite the differences among the various disciplines involved, the work on SI focuses to a great extent on three issues: autonomy, heterogeneity, and distribution, as discussed in the sidebar "Dimensions of Information SI."
There is often no time and justification to replace legacy systems. New functionality must be integrated with other packages, existing applications, and data sources. Therefore, SI aims at building applications that are adaptable to business and technology changes while retaining legacy applications and legacy technology as reasonably as possible. The speed of business and technology change does not allow time for total replacement, therefore, evolution and migration of legacy and new application systems is required. Migration and evolution aim at protecting existing investments and enabling rapid response to the changing user requirements. For managing the evolution of those complex systems, it is necessary to deal with change on the organizational level, group collaboration level, and system level in a coherent manner .
SI plays an important role in such application areas as health care, digital libraries, e-commerce, telecommunications, Web applications, and data warehousing, to name just a few. This special section presents the various problems and solutions for SI from different perspectives, with discussion of the specific requirements of selected application domains. The articles in this special section can be categorized according to the three-layer architecture of information SI, namely the business, application, and technology layers.
Yang and Papazoglou address the business architecture layer in their article, discussing the integration at the level of the business architecture making reference to business-to-business e-commerce. To make e-commerce possible, it is necessary to let the information systems of dissimilar organizations cooperate. The article is a survey in which relevant problems are presented and their possible solutions are discussed in the context of a layered strawman reference architecture for interoperation support in e-commerce. The Business Information Systems point of view to SI is presented, with achievement of business goals an important issue.
The article by Grimson et al. focuses on the application architecture, discussing the problem of integrating electronic patient records. For health care information systems, integration is a decisive factor to successfully support the work within hospitals as well as for the cooperation among the various health care providers. The article describes several standards that have been defined and are being defined, and their actual use in practice. Since this article addresses the application layer, the integration among ERP packages and other applications is discussed in the context of Enterprise Application Integration. The Health Informatics point of view to SI is presented; to achieve semantic interoperability, health care-specific standards play an important role.
SI aims at building applications that are adaptable to business and technology changes while retaining legacy applications and legacy technology as reasonably as possible.
The articles by Rundensteiner et al. and Adam et al. address the technology architecture. Rundensteiner et al. discuss the technology architecture, making reference to data warehouses offering a rich view of the field. To build data warehouses that aim at supporting decision support systems, it is a basic requirement to integrate the data from operational information systems; thus the emphasis is on coping with the dynamics in the operational information sources.
The article by Adam et al. focuses on integration of digital libraries and presents a survey of the field, addressing CORBA, mediators, and agent architectures. For digital libraries, the integration of information from different sources is a central problem to be solved. The integration of multimedia objects is an important issue in that context. The similarities of the two technology articles are evident when comparing their figures that illustrates the respective system architectures.
It should be noted that within this special section not all possible facets of information SI can be covered, but I hope you will enjoy reading the articles in this section and derive some conceptions beneficial to your own work.
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