ing and maintaining complex systems.

The key observation was that programmers, who were very expensive, had to rewrite large amounts of application software manually whenever the content or physical organization of a database changed. Because the application generally knew in detail how its data was stored, including its on-disk layout, reorganizing databases or adding new information to existing databases forced wholesale changes to the code accessing those databases.

ILLUS TRATION B Y CELIA JOHNSON

Relational databases solved this problem in two ways. First, they hid the physical organization of the database from the application and provided only a logical view of the data. Second, they used a declarative language to describe the data of interest in a particular query, rather than forcing the programmer to write a collection of function calls

to fetch the data. These two changes allowed programmers to describe the information they wanted and to leave the details of optimization and access to the database management system. This transformation relieved programmers of the burden of rewriting application code whenever the database layout or organization changed.

Relational databases enjoyed tremendous success in the IT shops and data centers of the world. Businesses with large quantities of data to manage and sophisticated applications using that data adopted the new technology quickly. Demand for relational products created a market worth billions of dollars in licensing revenue per year. Several RDBMS vendors arose in the 1980s to compete for this lucrative business.

In the 20 years that followed, two

related trends emerged. First, the RDBMS vendors increased functionality to provide market differentiators and to address each new market niche as it arose. Second, few applications need all the features available in today’s RDBMSs, so as the feature set size increased, each application used a decreasing fraction of that feature set.

This drive toward increasing DBMS functionality has been accompanied by increasing complexity, and most deployments now require a specialist, trained in database administration, to keep the systems and applications running. Since these systems are developed and sold as monolithic entities, even though applications may require only a small subset of the system’s functionality, each installation pays the price of the total overall complexity. Surely, there must be a better way.