Jeff Magee

Specifying Distributed Software Architectures

There is a real need for clear and sound design specifications of distributed systems at the architectural level. This is the level of the design which deals with the high-level organisation of computational elements and the interactions between those elements. The paper presents the Darwin notation for specifying this high-level organisation. Darwin is in essence a declarative binding language which can be used to define hierarchic compositions of interconnected components. Distribution is dealt with orthogonally to system structuring. The language supports the specification of both static structures and dynamic structures which may evolve during execution. The central abstractions managed by Darwin are components and services. Services are the means by which components interact.

The Koala component model for consumer electronics software

Most consumer electronics today contain embedded software. In the early days, developing CE software presented relatively minor challenges, but in the past several years three significant problems have arisen: size and complexity of the software in individual products; the increasing diversity of products and their software; and the need for decreased development time. The question of handling diversity and complexity in embedded software at an increasing production speed becomes an urgent one. The authors present their belief that the answer lies not in hiring more software engineers. They are not readily available, and even if they were, experience shows that larger projects induce larger lead times and often result in greater complexity. Instead, they believe that the answer lies in the use and reuse of software components that work within an explicit software architecture. The Koala model, a component-oriented approach detailed in this article, is their way of handling the diversity of software in consumer electronics. Used for embedded software in TV sets, it allows late binding of reusable components with no additional overhead.

Self-Managed Systems: an Architectural Challenge

Self-management is put forward as one of the means by which we could provide systems that are scalable, support dynamic composition and rigorous analysis, and are flexible and robust in the presence of change. In this paper, we focus on architectural approaches to self-management, not because the language-level or network-level approaches are uninteresting or less promising, but because we believe that the architectural level seems to provide the required level of abstraction and generality to deal with the challenges posed. A self-managed software architecture is one in which components automatically configure their interaction in a way that is compatible with an overall architectural specification and achieves the goals of the system. The objective is to minimise the degree of explicit management necessary for construction and subsequent evolution whilst preserving the architectural properties implied by its specification. This paper discusses some of the current promising work and presents an outline three-layer reference model as a context in which to articulate some of the main outstanding research challenges.

Dynamic structure in software architectures

Much of the recent work on Architecture Description Languages (ADL) has concentrated on specifying organisations of components and connectors which are static. When the ADL specification is used to drive system construction, then the structure of the resulting system in terms of its component instances and their interconnection is fixed. This paper examines ADL features which permit the description of dynamic software architectures in which the organisation of components and connectors may change during system execution.The paper outlines examples of language features which support dynamic structure. These examples are taken from Darwin, a language used to describe distributed system structure. An operational semantics for these features is presented in the π-calculus, together with a discussion of their advantages and limitations. The paper discusses some general approaches to dynamic architecture description suggested by these examples.

Model-based verification of Web service compositions

In this paper, we discuss a model-based approach to verifying Web service compositions for Web service implementations. The approach supports verification against specification models and assigns semantics to the behavior of implementation model so as to confirm expected results for both the designer and implementer. Specifications of the design are modeled in UML (Unified Modeling Language), in the form of message sequence charts (MSC), and mechanically compiled into the finite state process notation (FSP) to concisely describe and reason about the concurrent programs. Implementations are mechanically translated to FSP to allow a trace equivalence verification process to be performed. By providing early design verification, the implementation, testing, and deployment of Web service compositions can be eased through the understanding of the differences, limitations and undesirable traces allowed by the composition. The approach is supported by a suite of cooperating tools for specification, formal modeling and trace animation of the composition workflow.

Software Engineering for Self-Adaptive Systems: A Second Research Roadmap

The goal of this roadmap paper is to summarize the state-of-the-art and identify research challenges when developing, deploying and managing self-adaptive software systems. Instead of dealing with a wide range of topics associated with the field, we focus on four essential topics of self-adaptation: design space for self-adaptive solutions, software engineering processes for self-adaptive systems, from centralized to decentralized control, and practical run-time verification & validation for self-adaptive systems. For each topic, we present an overview, suggest future directions, and focus on selected challenges. This paper complements and extends a previous roadmap on software engineering for self-adaptive systems published in 2009 covering a different set of topics, and reflecting in part on the previous paper. This roadmap is one of the many results of the Dagstuhl Seminar 10431 on Software Engineering for Self-Adaptive Systems, which took place in October 2010.

Constructing distributed systems in Conic

The Conic environment provides a language-based approach to the building of distributed systems which combines the simplicity and safety of a language approach with the flexibility and accessibility of an operating systems approach. It provides a comprehensive set of tools for program compilation, configuration, debugging, and execution in a distributed environment. A separate configuration language is used to specify the configuration of software components into logical nodes. This provides a concise configuration description and facilitates the reuse of program components in different configurations. Applications are constructed as sets of one or more interconnected logical nodes. Arbitrary, incremental change is supported by dynamic configuration. In addition, the system provides user-transparent datatype transformation between heterogeneous processors. Applications may be run on a mixed set of interconnected computers running the Unix operating system and on base target machines with no resident operating system. The basic principles adopted in the construction of the Conic environment are outlined and the configuration and run-time facilities provided are described. >

Dynamic Configuration for Distributed Systems

Dynamic system configuration is the ability to modify and extend a system while it is running. The facility is a requirement in large distributed systems where it may not be possible or economic to stop the entire system to allow modification to part of its hardware or software. It is also useful during production of the system to aid incremental integration of component parts, and during operation to aid system evolution. The paper introduces a model of the configuration process which permits dynamic incremental modification and extension. Using this model we determine the properties required by languages and their execution environments to support dynamic configuration. CONIC, the distributed system which has been developed at Imperial College with the specific objective of supporting dynamic configuration, is described to illustrate the feasibility of the model.

Synthesis of behavioral models from scenarios

Scenario-based specifications such as Message Sequence Charts (MSCs) are useful as part of a requirements specification. A scenario is a partial story, describing how system components, the environment, and users work concurrently and interact in order to provide system level functionality. Scenarios need to be combined to provide a more complete description of system behavior. Consequently, scenario synthesis is central to the effective use of scenario descriptions. How should a set of scenarios be interpreted? How do they relate to one another? What is the underlying semantics? What assumptions are made when synthesizing behavior models from multiple scenarios? In this paper, we present an approach to scenario synthesis based on a clear sound semantics, which can support and integrate many of the existing approaches to scenario synthesis. The contributions of the paper are threefold. We first define an MSC language with sound abstract semantics in terms of labeled transition systems and parallel composition. The language integrates existing approaches based on scenario composition by using high-level MSCs (hMSCs) and those based on state identification by introducing explicit component state labeling. This combination allows stakeholders to break up scenario specifications into manageable parts and reuse scenarios using hMCSs; it also allows them to introduce additional domain-specific information and general assumptions explicitly into the scenario specification using state labels. Second, we provide a sound synthesis algorithm which translates scenarios into a behavioral specification in the form of Finite Sequential Processes. This specification can be analyzed with the Labeled Transition System Analyzer using model checking and animation. Finally, we demonstrate how many of the assumptions embedded in existing synthesis approaches can be made explicit and modeled in our approach. Thus, we provide the basis for a common approach to scenario-based specification, synthesis, and analysis.

Self-organising software architectures for distributed systems

A self-organising software architecture is one in which components automatically configure their interaction in a way that is compatible with an overall architectural specification. The objective is to minimise the degree of explicit management necessary for construction and subsequent evolution whilst preserving the architectural properties implied by its specification. This paper examines the feasibility of using architectural constraints as the basis for the specification, design and implementation of self-organising architectures for distributed systems. Although we focus on organising the structure of systems, we show how component state can influence reconfiguration via interface attributes.