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May 3, 1999

Excelon Stores And Delivers XML Data

Object Design's data store provides a single, unified view of application data

By Don Kiely

First Look

bject Design's recently released Excelon Extensible Markup Language server is a data store based on the company's successful ObjectStore database. Excelon stores, caches, and delivers XML data across the middle tier of distributed applications, similar to conventional relational databases.

Excelon provides a single, unified view of data, whether you use it as an application cache for existing data stores or as a complete data management system for XML applications. This flexibility gives applications centralized access to multiple dissimilar data sources, such as relational record sets, documents, and multimedia files.

As an application cache, Excelon integrates data between application servers and data sources, connecting disparate data sources into a single virtual data store or importing it into a physical data store. Either way, it caches data as needed to make it available to applications, keeping it up-to-date and maintaining transactional consistency.

As a physical data store, Excelon copies data from any data source, parsing and storing it in an XMLStore as discrete XML elements. As a virtual data store, Excelon acts as an interface between the client application and the data services layer, storing data only as needed within its own store and translating on the fly.

The XMLStore is organized as a hierarchy of file types, including XML documents, multimedia and other binary files, Extensible Query Language queries, and Java extensions. The XMLStore is indexed and manipulated using the World Wide Web Consortium's Document Object Model, soon to be a new standard, and queried using XQL, a proposed standard.

This structure is certainly flexible, but it inserts a potential bottleneck between an application and the data it needs, raising consistency and performance issues. Object Design has built Excelon on its object database, which handles maintenance chores such as transactional consistency and load balancing.

Object Design maintains that XML and object databases were made for each other, breathing new life into an underutilized technology. On the surface, object databases seem well-suited to storing hierarchical data such as XML. Like XML, they inherently support both data and metadata--information about data--in a flexible, hierarchical structure. XML's unlimited, user-defined tags fit better into an object database than the row-and-table metaphor of a relational system.

But the fact that data is being stored in two places at once, in the database and the XMLStore, creates a potential problem. Any time data is duplicated this way, there is the risk that the two stores will no longer contain the same data. A single update operation requires reliable application of the changes to the data in both locations.

Object Design uses its Cache-Forward architecture from its ObjectStore database to overcome these consistency and performance problems. Most databases cache data only at the database server, where multiple applications can potentially overwhelm it. Such applications must contend with other processes for server resources, which can burden the network with excessive traffic. Cache-Forward establishes distributed data caches for all components that connect to an Excelon server. Applications then access data from their local cache at in-memory speed instead of connecting to the server for each database request.

This scheme uses a callback locking technology to ensure the integrity and transaction consistency of the locally cached data while maintaining good system performance. Because the cache is moved forward to distributed servers, it's easy to add servers to cache data for additional application loads. This also optimizes performance by moving data closer to client applications and reducing the layers through which data must be passed.

Full-Service Data
Excelon provides all the tools necessary to set up, design, distribute, and use XML data stores. On the server side, it includes XML Parser, XQL support, Java, and DOM support for manipulating all parts of the data store, as well as Web Server Extensions.

The XML server includes several visual tools for administering and maintaining data stores. Excelon Explore is an Excelon client application that provides a visual interface for storing data in diverse ways, such as by dragging and dropping simple XML files into an XMLStore or by importing data from an OLE DB data source. Once data is stored in an XMLStore, you can use Excelon Explorer to browse, query, and update it.