Reverse Engineering

So, you think that models might provide some useful insights and lead to better-designed databases. But what if you're faced with legacy databases created over time by different development teams for different applications, and n o models exist? Most modeling tools provide just what's needed to atone for these past sins: reverse engineering.

When you reverse-engineer a database, the modeling tool inputs schema descriptions from a relational DBMS and produces a model for the database. It does this by creating an entity for each table an d creating relationships by using the connections shown by keys that appear in multiple tables. You can then use the model to clarify the semantics in the database that guide you to modifications or integrate this database with another (see the figure "Backward and Forward" ). Reverse engineering also allows you to compare two different databases by creating a model for each and comparing them -- or, in a development environment, to compare two versions of the same database to identify differences, additions, and deletions.

Some tools support a dynamic connection between the physical database and a model representation (e.g., ERWin's Server FRE [for Forward-and-Reverse Engineering]). The model becomes a live representation used to monitor the physical database. You can then use the model to implement changes (forward) or to observe them (reverse). This provides a database administrator with an easy way to manage and control the database. Using graphical representations of tables and t he relationships among them (rather than database-description-language [DDL] code) simplifies the task of maintaining the database.

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Backward and Forward

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Here's how a databse-modeling tool might reverse-engineer a database.