This article reports our reflection on software architecture modelling for multi-user systems (or groupware). First, we introduce the notion of software architecture and make explicit the design steps that most software designers in HCI tend to blend in a fuzzy way. Building on general concepts and practice from main stream software engineering, we then present a comparative analysis of the most significant architecture models developed for singleand multi-user systems. We close with the presentation of PAC*, a new architectural framework for modelling and designing the software architecture of multi-user systems. PAC* is a motivated combination of existing architectural models selected for the complementarity of their "good properties". These include operational heuristics such as rules for deriving agents in accordance to the task model or criteria for reasoning about replication, as well as properties such as support for style heterogeneity, portability, and reusability. Keywords ...

... advent of new technologies and user-centred concerns, the user interface portion of interactive systems is becoming increasingly large and complex. In this article, we discuss agent-based architectural styles and show how sound tradeoffs between conflicting requirements and properties can be done using the PAC-Amodeus conceptual model.

A number of architectural models, such as PAC, are available for the software design of interactive systems. These design abstractions, however, are not always clearly articulated nor do they explicitly exploit the foundational concepts developed recently in main-stream software architecture engineering. Similarly, technical solutions from main-stream software engineering may improve portability and reusability at the code level while hindering the quality of the resulting user interfaces. This article is an attempt to undertake an explicit bridging effort between software engineering and the specific domain of user interface software design using PAC as the running example. We present a brief evolution of the architectural models for single-user systems that motivated PAC. We then unfold PAC into PAC* for designing the conceptual architecture of multi-user systems. Keywords : Software architecture modelling, multi-agent modelling, PAC, PAC-Amodeus, PAC*. 1 Introduction Software arch...

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