Database mix poses a sharing challenge; IS execs build ties when none available; find retrieval easier than distributed update - Client/Server Computing - includes a related article on important client/server issues

Software Magazine, March, 1992 by Barbara Bochenski

Sybase's SQL Server takes a different approach, making calls to a library of APIs, he said. Instead of seeing the actual SQL statements mixed in with Cobol or C code, there would be a CALL to a SQL routine. The SQL activity would be terminated by an END SQL statement. "The two approaches are radically different," he said, "and they are not compatible in any way, shape or form.

"Now imagine one program attempting to access three different databases with three different sets of incompatible interfaces with three different sets of low-level network protocols," he said. This is not easy.

It is certainly easier to share data between relational databases than nonrelational databases when using standard SQL without added features, said Shaku Atre. "But performance and integrity could be issues," she said, adding, "Integrity is not implemented the same way by different vendors."

Retrieval across different databases is much easier than updating across different databases, Atre explained, stressing that updating data in a distributed environment must be addressed.

"Not all data is made equal," said Atre. "Twenty percent of the data is used 80% of the time." Therefore, that 20% is going to be stored in several places. "Replication is bound to happen in every situation," she added.

When the same data is replicated in several locations, this adds to the difficulty of keeping updated data synchronized. Therefore, most sites working with multiple databases in a client/server environment will concentrate on retrieval rather than update, Atre observed.

Many database access tools target a specific database, Vaughn noted. "Other tools can access multiple databases," he added, "but not simultaneously. Only a few tools can access multiple databases simultaneously."

Powerbuilder from Powersoft in Burlington, Mass., is one such tool. It provides a feature called a Data Window. A programmer specifies one database for each Data Window. One application program can have several Data Windows, however, each with a separate database that is open and active at one time.

When front-end vendors say they can talk to a lot of databases, said Vaughn, they do not necessarily mean concurrently. What they may be saying, he continued, is that if a user needs to upgrade the environment in the future, the front-end tool may allow the transition to be made fairly easily.

"So the power of these front-end tools," said Vaughn, "is not in being able to concurrently access several databases, but in being able to change your database vendor and not change the front-end program.

A new verb recently added to SQL lets a developer switch database connections, said IBM's Zagelow. That new verb is CONNECT.

In the DRDA environment an application is connected to one database at a time, while with RDA an application can be connected to more than one database at a time. Zagelow warned, however, that the RDA apporach has not completely sorted out te problems related to updating multiple databases, functions like failures during updating and/or coordinated receoveries.