Nelis Boucké

Gradient field-based task assignment in an AGV transportation system

Assigning tasks to agents is complex, especially in highly dynamic environments. Typical protocol-based approaches for task assignment such as Contract Net have proven their value, however, they may not be flexible enough to cope with continuously changing circumstances. In this paper we study and validate the feasibility of a field-based approach for task assignment in a complex problem domain.In particular, we apply the field-based approach for task assignment in an AGV transportation system. In this approach, transports emit fields into the environment that attract idle AGVs. To avoid multiple AGVs driving towards the same transport, AGVs emit repulsive fields. AGVs combine received fields and follow the gradient of the combined fields, that guide them towards pick locations of transports. The AGVs continuously reconsider the situation of the environment and task assignment is delayed until the load is picked, improves the flexibility of the system.Extensive experiments indicate that the field-based approach outperforms the standard Contract Net approach on various performance measures, such as the average wait time of transports and throughput. Limitations of the field-based approach are an unequal distribution of wait times across different transports and a small increase of bandwidth occupation.

Applying the ATAM to an architecture for decentralized control of a transportation system

For two years, we have been involved in a challenging project to develop a new architecture for an industrial transportation system. The motivating quality attributes to develop this innovative architecture were flexibility and openness. Taking these quality attributes into account, we proposed a decentralized architecture using multiagent systems (MASs). A MAS consists of multiple autonomous entities that coordinate with each other to achieve decentralized control. The typical advantages attributed to such decentralized architecture are flexibility and openness, the motivating quality attributes to apply MAS in this case. The Architecture Tradeoff Analysis Method (ATAM) was used to provide insights wether our architecture meets the expected flexibility and openness, and to identify tradeoffs with other quality attributes. Applying the ATAM proved to be a valuable experience. One of the main outcome of applying the ATAM was the identification of a tradeoff between flexibility and communication load that results from the use of a decentralized architecture. This paper describes our experiences in applying the ATAM to a MAS architecture, containing both the main outcomes of the evaluation and a critical reflection on the ATAM itself.

A field-based versus a protocol-based approach for adaptive task assignment

Task assignment in multi-agent systems is a complex coordination problem, in particular in systems that are subject to dynamic and changing operating conditions. To enable agents to deal with dynamism and change, adaptive task assignment approaches are needed. In this paper, we study two approaches for adaptive task assignment that are characteristic for two classical families of task assignment approaches. FiTA is a field-based approach in which tasks emit fields in the environment that guide idle agents to tasks. DynCNET is a protocol-based approach that extends Standard Contract Net (CNET). In DynCNET, agents use explicit negotiation to assign tasks. We compare both approaches in a simulation of an industrial automated transportation system. Our experiences show that: (1) the performance of DynCNET and FiTA are similar, while both outperform CNET; (2) the complexity to engineer DynCNET is similar to FiTA but much more complex than CNET; (3) whereas task assignment with FiTA is an emergent solution, DynCNET specifies the interaction among agents explicitly allowing engineers to reason on the assignment of tasks, (4) FiTA is inherently robust to message loss while DynCNET requires substantial additional support. The tradeoff between (3) and (4) is an important criteria for the selection of an adaptive task assignment approach in practice.

An Architectural Approach to Support Online Updates of Software Product Lines

Despite the successes of software product lines (SPL), managing the evolution of a SPL remains difficult and error-prone. Our focus of evolution is on the concrete tasks integrators have to perform to update deployed SPL products, in particular products that require runtime updates with minimal interruption. The complexity of updating a deployed SPL product is caused by multiple interdependent concerns, including variability, traceability, versioning, availability, and correctness. Existing approaches typically focus on particular concerns while making abstraction of others, thus offering only partial solutions. An integrated approach that takes into account the different stakeholder concerns is lacking. In this paper, we present an architectural approach for updating SPL products that supports multiple concerns. The approach comprises of two complementary parts: (1) an update viewpoint that defines the conventions for constructing and using architecture views to deal with multiple update concerns, and (2) a supporting framework that provides an extensible infrastructure supporting integrators of a SPL. We evaluated the approach for an industrial SPL for logistic systems providing empirical evidence for its benefits and recommendations.

Characterizing Relations between Architectural Views

It is commonly agreed that an architectural description (AD) consists of multiple views. Each view describes the architecture from the perspective of particular stakeholder concerns. Although views are constructed separately, they are related as they describe the same system. A thorough study of the literature reveals that research on relations between views is fragmented and that a comprehensive study is hampered by an absence of common terminology. This has become apparent in the discussion on inter-view relational concepts in the revision of IEEE 1471 as ISO/IEC 42010 (Systems and Software Engineering -- Architectural Description). This paper puts forward a framework that employs a consistent terminology to characterize relations between views. The framework sheds light on the usage, scope and mechanisms for relations, and is illustrated using several representative approaches from the literature. We conclude with a reflection on whether the revision of ISO 42010 aligns with our findings.

Using code analysis tools for architectural conformance checking

Architectural conformance checking verifies whether a system conforms to its intended architecture, which is essential to safeguard the quality attributes of the system. Due to the size of many systems, performing conformance checking by means of manual code inspections is often practically infeasible. Code analysis tools can be used to automatically check architectural conformance. In this paper, we investigate several code analysis tools that offer support for Java and compare them on their usefulness for architectural conformance checking: Architecture Rules, Macker, Lattix DSM, SonarJ, Structure101 and XDepend.

View composition in multiagent architectures

Experience with a multiagent architecture for an industrial Automatic Guided Vehicle Transportation System (AGVTS) reveals a lack of support for relating and composing multiple views in architectural descriptions. This prevents separating concerns while maintaining a good overview of the architecture as a whole, hampering changeability. We argue for explicit support for relating and composing views. Concretely, we propose three types of relations that allow specifying and composing structural views. As validation, we extend xADL with concepts of relations and compositions. The usability and consequences of this extension are assessed in the context of the multiagent architecture for the AGVTS. We learn that the approach improves changeability by offering explicit support for (1) separating concerns in separate views and (2) composing the overall architecture. There is an additional cost of documenting relations. Nevertheless, documenting view relations gives rise to architectural descriptions that are more expressive and less ambiguous.

Relating architectural views with architectural concerns

Architectural views are at the foundation of software architecture and are used to describe the system from different perspectives. However, some architectural concerns crosscut the decomposition of the architecture in views. The drawbacks of crosscutting with respect to architectural views is similar to the drawbacks with respect to code, i.e. hampering reuse, maintenance and evolution of the architecture. This paper investigates the relations between architectural concerns, architectural drivers and views to identify why crosscutting manifests itself. We propose to extend the architectural description with slices and composition mechanisms to prevent this crosscutting and perform an initial exploration of these concepts in an Online Auction system. Within this limited setting the first results look promising to better separate concerns that otherwise would crosscut the views.

DynCNET: A Protocol for Dynamic Task Assignment in Multiagent Systems

Task assignment in multiagent systems is a complex coordination problem, in particular in systems that are subject to dynamic and changing operating conditions. To enable agents to manage dynamism and change, adaptive task assignment approaches are needed. In this paper, we introduce DynCNET, a protocol for dynamic task assignment that extends standard contract net (CNET). DynCNET allows the agents involved in the protocol to switch the assignment of tasks dynamically. We use an industrial automated transportation system as illustration and present results obtained from a real-world test setting that compare DynCNET with standard CNET and afield-based approach for task assignment.

Composing Structural Views in xADL

Experience with building an architecture for an industrial Automatic Guided Vehicle Transportation System (AGVTS) shows that several essential concerns crosscut the architectural views. To cope with this, a stronger separation proved to be necessary, i.e. using different views for different concerns. In practice this was difficult, since the support for relations between views is very limited. This makes separation of concerns in views hard, thereby increasing maintenance overhead and reducing reuse capabilities. Our claim is that specifying compositions of views is as important as specifying the views itself. This paper extends a representative architectural description language (xADL) which support for composing structural views, by introducing three relations, namely refinement, mapping, and unification. Improving separation of concerns in views and their explicit composition enhances architecture understandability and changeability. The feasibility of the relations is assessed by redesigning the AGVTS architecture. Based on a real maintenance scenario, we investigate to what extent these explicit compositions lead (or not) to enhanced architectural changeability for evolving the distribution strategy in the AGVTS system.