Implementing Metcast in Scheme

Abstract

This paper describes the experience of implementing a core component of an operationally deployed large distributed system in Scheme. Metcast is a request-reply and subscription system for the dissemination of real-time weather information. The system stores vast amounts of weather observation reports, forecasts, gridded data produced by weather models, and satellite imagery. A Metcast server delivers a subset of these data in response to a query formulated in a domain-specific declarative language. Decoders of World Meteorological Organization's data feed, the Metcast application server, XML encoders and decoders, auxiliary and monitoring CGI scripts are all written in a mostly pure functional subset of Scheme. This paper describes three examples that demonstrate the benefits of our choice of the implementation language: parsing of the data feed; XML transformations and Web services; a modular interpreter for the extensible and expressive request language. We also discuss general-purpose extensions to Scheme developed in the project.