Achieving Unit Safety in the WebPuff Emergency Management Information System

When IEM was contracted by the Chemical Materials Agency (CMA) to transform their desktop dispersion model, D2-Pufftrade, into a full-fledged browser-based emergency management information system, we resolved to address a weakness common to many scientific applications: lack of an integrated framework for handling of units. By the end of active development on D2-Pufftrade, a significant percentage of both the data model and the source code were devoted to tracking units in a sometimes ad hoc manner. The IEM software developers created a framework for our next generation product, WebPuff, in which a value´s unit is integral and transparent, unit conversions are effortless and accurate, and values stored to a database are guaranteed to be in the correct unit every time. The framework is easily extensible to accommodate custom unit types. Since persistent units are specified in the system configuration rather than in code, they are easy to maintain. We eliminate ambiguity both in integrating modules at design time and in performing calculations by making unit specification a functional configuration item. Through use of Java generics, operations between incompatible unit types will not even compile to an executable module. Two open source projects form the underpinnings of the framework: JScience, the reference implementation for JSR-275, and Hibernate, the open source industry-standard Object Relational Model (ORM) that was the inspiration for the JPA specification. IEM has donated this code to form the basis of a new Open Source project, JScience-JPA.