The challenge of interoperating distributed systems, in particular database systems, has existed for a long time and has been extensively researched. Many approaches have been proposed to allow integration and interoperability of distributed systems developed in an independent way. These approaches have been proposed as outcomes of research work in both academia and industry.
We can divide the existing approaches into two main groups [34]. In the first group of approaches a global schema is used as another layer on the top of existing schemas which gives the users and applications the illusion of a single, centralized database system. Examples of these approaches include systems like DATAPLEX [9], DDTS [13], MULTIBASE [29], and PEGASUS [3]. However, the construction of a global schema is not a simple task, does not guarantee the autonomy of the participating database systems, and does not allow easy evolution of the system in terms of adding and removing of participating databases.
In order to overcome the problem of constructing a global integrated schema the second group of approaches has been proposed, in which ‘partial’ or ‘no integration’ is performed. Examples of these approaches include the federated architecture [19], five-level schema architecture [28], multidatabase architecture [22][23], the Jupiter system [18], and [33]. Within the approaches that do not use a global schema some of them proposed the use of mediators and wrappers. In these approaches data sources are encapsulated to make it usable in a more convenient manner by hiding or exposing the internal interface of the data sources, reformat data, and translate queries. Examples of systems that use wrappers and mediators are DIOM [24], DISCO [31], Garlic [27], and TSIMMIS [16].
In any of the above approaches and existing technologies the problems related to how to format data to be exchanged and how to transmit the data are still open problems. Regarding data format, there are almost no tools that can automate the process of translating data in different formats. Many systems use ASCII-based text files to represent their data. However, there is no standard way of formatting or describing the values in the files. The different systems exchanging data in ASCII format must have custom-built loading software to handle different file formats. Other systems exchange data via a specified file format, which does not scale well (e.g. Microsoft Excel).
On the other hand, data transmission has also been difficult to implement. The use of the File Transfer Protocol (FTP) facilitates file transfer, but this is not a tight, object-oriented approach to exchanging data. Electronic Data Interchange (EDI) has also been used for exchanging data. However, EDI is rigid, complex, and expensive to implement. More recently some technologies have been proposed to allow a more object-oriented and less expensive approach, based on Remote Procedure Calls. Examples of these approaches are DCOM [12] and CORBA/IIOP [10]. The problems with these technologies are that they are platform specific, do not easily integrate, and pose network security risks due to the requirement of having open ports to accommodate messages.
The existing approaches have contributed to alleviate the problems of sharing data between autonomous and heterogeneous data sources. However, the development of Web services [17], SOAP [8] and XML technologies support the problems of e-business by allowing the ability of representing data structures and describing these structures in an easy way to implement and maintain. In the next section we describe an approach that uses Web services.
