Controllled flexibility in system design

Abstract

1 Challenge Historically, applications have been distributed in order to access more resources than a lone machine could supply, such as CPU power, disks, or the computers themselves (for fault tolerance). We are increasingly seeing applications which are distributed because they are meant to be used by multiple people, and multiple people generally have multiple machines. These applications are distributed by definition. A good example of this is a distributed multimedia application such as multicast video, email, or the World Wide Web. It is well understood how the interaction of software with people.makes the software more complicated; we are entering the realm of software which exists purely to glue people together. The popular focus on multimedia is on video and audio applications. While these certainly challenge current computers and profit companies, they are merely a shadow of the possibilities. This potential is not the ability of a computer to display one medium, but the ability to gracefully and efficiently handle a wide variety of data flows with differing requirements, many of which did not exist when that computer or its operating system was designed. The variation of types and topology of such flows is limited only by the creativity and whims of the human users. What sort of information flows will computers serve to us? What kinds of hardware and software will these machines need and have? How will these systems interact? We do not know. Nobody does. And we have to design our software to face this environment. This is the challenge of multimedia, and it parallels the general challenges facing these diffusely distributed applications in the near future. The important issue is not that software needs to be flexible and easily reconfigurable. We need to be able to control, predict, and modify the behavior of our software, and this requires that we have a firm understanding of and control over how software is configurable. We should not strive to make endlessly extendable systems-rather we should strive to make systems which are exactly as extendable as required. It is in this context that we are designing the Scout[6] system. Software designers have always been challenged to keep pace with the increasing demands placed on computers. Software, once written, is released into a dynamic world. Therefore, a fundamental property of successful software is that it can be changed. Most of the attention to this has been placed on static changes …