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May 3, 1999

Excelon Stores And Delivers XML Data

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First Look

Excelon Studio is a client application that lets you graphically define the structure and relationship of XML documents. Excelon saves the structure in a Document Content Description format to create a schema, which it uses to generate HTML forms and Java server extensions.

The Excelon Management Console is a Microsoft Management Console snap-in application similar to SQL Server 7.0's Enterprise Manager. The Management Console is Excelon's configuration and management tool for configuring servers, creating XMLStores, configuring user access, and setting deployment options.

Other tools include Visual Query Builder, Visual XML Editor, the Backup and Restore utility, and a visual development environment integrated with Explorer.

Accessing XML Data
Excelon supports several ways to access the data it manages. Its Explorer and Management Console contains basic tools for viewing and changing data directly. But the full benefit of using Excelon is through programmatic access, for which it provides plenty of options. A client application can access data via an application programming interface using the proposed XQL standard.

Access to the data store is through COM objects, and Excelon supports an adequate set of API functions for this function. Using XML data is similar to the steps involved in using relational data: open a data store, load a document, execute a query, and work with the data. The API returns result sets via Microsoft's ActiveX Data Objects 2.1, which supports XML as a native format.

Excelon accesses back-end data sources through OLE DB, Open Database Connectivity, and other XML adapters that can be customized and plugged into Excelon.

Object Design has provided plenty of hooks into Excelon that facilitate extending the capabilities of the data server. One option is to develop server extensions as Java code that runs in an XML cache after an Excelon client application invokes it. Server extensions manipulate the contents of an XMLStore through the DOM and return XML or any other Internet media type.

First Generation
This release is the first iteration of a new product in a new product category; I expect that as Excelon is put to extensive real-world uses, Object Design will learn how to enhance the product to make it more effective. It has a few fairly minor limitations, such as some query facilities and Java DOM API methods that are not implemented, and an encoding specification that is case-sensitive but shouldn't be.

Working with the product in a fairly light workload environment, I've found it to be stable and reliable. Probably the biggest problem for some companies will be that Excelon can't write data to a back-end data source, such as a relational database. This effectively makes Excelon a read-only view into non-XML databases.

But the larger question is whether an XML data server on the middle application tier is the right solution to serving up XML data efficiently. Duplicated data always makes me nervous, but Object Design's Cache-Forward seems to work well as a compromise.

Some observers say that an object database is not an appropriate storage option for XML because of the complex conceptual foundation that such database systems involve and the specialized object-oriented development techniques XML requires. The tables and rows of a relational database don't fit XML well, whereas an object database has an intelligence about data contained within it.

As a first attempt at bringing XML data into enterprise applications, Excelon deserves a close look.