HydroJ: Object-Oriented Pattern Matching for Evolvable Distributed Systems

To appear in Conference on Object-Oriented Programming, Systems, Languages, and Applications, Anaheim, California, October 26-30, 2003.

Keunwoo Lee, Anthony LaMarca, and Craig Chambers,

Electronic copies of conference version: Acrobat, gzip'd PostScript

The accompanying technical report is in preparation.

In an evolving software system, components must be able to change independently while remaining compatible with their peers. One obstacle to independent evolution is the brittle parameter problem: the ability of two components to communicate can depend on a number of inessential details of the types, structure, and/or contents of the values communicated. If these details change, then the components can no longer communicate, even if the essential parts of the message remain unaffected.

We present HydroJ, an extension of Java that addresses this problem. In HydroJ, components communicate using self-describing, semi-structured messages, and programmers use pattern matching to define the handling of messages. This design stems from two central ideas: first, that self-describing messages reduce dependence on inessential message format details; and second, that object-oriented pattern matching naturally focuses on the essential information in a message and is insensitive to inessential information.

We have developed these ideas in the context of Rain, a distributed, heterogeneous messaging system for ubiquitous computing. To evaluate the design, we have constructed a prototype HydroJ compiler, implemented some Rain services in HydroJ, studied the evolution of an existing Rain service over time, and formalized HydroJ's key features in a core language.