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< Contents ERCIM News No. 54, July 2003
SPECIAL THEME: Applications and Service Platforms for the Mobile User

A Mobile Schema Browser for Integration Specialists

by Gerald O’Connor and Mark Roantree

Middleware developed at Dublin City University allows a mobile user to query metadata on an object-relational database and automatically display its structure. This is particularly useful for example for displaying and browsing complex schema information on a Portable Digital Assistant (PDA).

Integration Engineers are often faced with a requirement to display and analyse the complex schemas of information systems to be merged. As these systems can be dispersed over a wide geographic area, a Portable Digital Assistant (PDA) provides a flexible means of viewing and displaying schema information. However the browsing process, which is often complex and problematic on a workstation screen, becomes more difficult on the smaller PDA. Using our metadata middleware, a mobile user can query metadata on an object-relational database and automatically display its structure. To demonstrate this, an application was developed which exploits our interface to the extended object Object-Relational (O-R) schema repository, to manipulate complex metadata information.

The O-R model is a complex structure combining features of relational and object models. The relational model consists of tables, triggers, constraints, views and procedures, while the object model consists of types, associations, aggregations, encapsulation, inheritance and other complex structures. This combination of features provides a powerful environment for representing data, yet if this data is not fully specified and manageable through a metadata interface, its reuse and sharing becomes a time consuming and expensive task. The Efficient Global Transactions for Video media research project (EGTV) has defined a complete metadata interface to O-R metadata and developed a tool to demonstrate and evaluate its effectiveness.

Mobile Object-Relational Schema Browser.
Mobile Object-Relational Schema Browser.

Mobile Layer
The Figure illustrates the deployment architecture for O-R metadata access. In the Mobile Layer, a PDA uses the metadata middleware in a specific application. The Schema Browser queries the schema using the Extended-SQL metadata query language, developed by the Interoperable Systems Group (ISG). This includes metadata query options for any O-R schema, role views and multimedia objects. The extended-SQL queries are wrapped in XML to provide a robust, non-proprietary, persistent and verifiable file format for the storage and transmission of data. Using the XML version of the language, the Schema Browser automatically queries and displays the complex metadata required to display the O-R schema model.

XML Translation Layer
The XML Translation Layer resides at the database server. Its purpose is to provide a standard interface to the Extended-SQL query language. When receiving a query it is unwrapped to form an Extended-SQL query, which is subsequently passed to the SQL translation layer. After execution, results received from the SQL Translation Layer are XML-wrapped using a basic rule set, and then returned to the application.

SQL Translation Layer
The SQL Translation Layer is where most of the metadata processing takes place. Current approaches to interfacing metadata for O-R databases were examined before this layer was specified. It was found that relationships between types very often complex and difficult to retrieve using SQL:99. In fact, SQL:99 is not powerful enough to extract some metadata (nested table associations) and programming language extensions were specified to overcome this. The SQL Translation Layer accepts an Extended-SQL query, which is parsed to invoke a sequence of actions against the schema repository. The results may be comprised of conventional O-R metadata, role metadata and/or multimedia metadata. After the results are restructured to an O-R format, they are passed back to the XML Translation Layer.

Database Layer
The O-R schema repository was extended by other EGTV researchers to provide new interfaces to role and multimedia metadata. Role metadata was added because it adds to the expressiveness of O-R databases. Roles provide temporal aspects to entities, a feature that is missing in conventional models. Without roles, a new object must be created each time the structure of an object evolves and many complex issues are involved in maintaining such an operation. The current popular O-R repositories fail to manage multimedia data adequately, as it is often represented as a BLOB with no further metadata to explore its application or use. This was addressed with the metamodel extensions. All of this metadata can be accessed using this framework and the Extended-SQL:99 language. Thus, it becomes accessible to integration engineers using mobile devices.

The mobile display tool can automatically display schema, view, role, and multimedia information in just a few selection buttons. This provides the mobile integration engineer with a fast completion of the first phase in the integration process. It also illustrates that the mobile device can offer a viable platform for some CASE tools. Current research is focused on providing update facilities through the mobile device, in order to make structural (metadata) changes to underlying O-R schemas, and to define new role views.


Please contact:
Gerald O’Connor and Mark Roantree
Dublin City University, Ireland
E-mail: {goconnor, mark}