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By nature, all large systems are heterogeneous, i.e. they lack uniformity. Their components were initially developed to address various purposes and evolved towards accretions of different platforms, programming languages, and even middleware. The SOA paradigm enables dealing with such heterogeneous systems in a decentralized way as much as possible. Decentralization helps to obtain loose coupling, one of SOA’s key technical concepts along with services and interoperability. We briefly describe these three concepts below.
Although several definitions exist, in short, a service is an information technology (IT) representation of self-contained business functionality.
Loose coupling minimizes dependencies and thus helps scalability, flexibility and fault tolerance. When dependencies are reduced, modifications have minimized effects and the systems still run when part of them are down. When problems occur, it is important to decrease their effects and consequences. Josuttis (2007) elaborates on several strategies to apply loose coupling.
The ISO terminology recommendation (ISO/IEC JTC-1 (ISO) 1993) describes interoperability as the capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units. Thus, interoperability enables systems to communicate, understand each other and exchange information. Syntactic and structural interoperability is already set up with transformations, for instance, using standards like XML and XML Schema and associated tools. Syntactic and structural transformations are used to convert schema representations into a target format. However, approaches that target at enhancing interoperability based on structure and on syntax can only produce improvements when a certain conceptual homogeneity between graphs to compare exists. Solving mismatches on the semantic level, i.e. to come up to semantic interoperability, is a complex accomplishment. An increasing number of semantic resources is available, namely in the Web, that portray many different cognitive viewpoints over application domains.