Coordination Patterns in Mobile Object Spaces

Mobility is a mechanism to manage accessibility dependencies. As such, it is a coordination mechanism in the interaction of agents, users, and information. The Lindalike coordination language Mobile Object Spaces is used to express coordination of entities. Its primitives support the basic operations for coordination by mobility. Higher order coordination structures can be captured as coordination patterns that describe reoccuring building blocks. We present a template for the description of such patterns and describe some very simple coordination patterns in the area of mobility and information systems. 1 Mobility in Information Systems Mobility is one aspect of agent and object systems that has attracted high interest recently. The widespread availability of enabling technology for mobile agents and objects – the Internet and Java – has led to a large number of prototypes for mobile agent systems. It is hoped that applications in the areas of data mining, electronic commerce, and system administration – especially in the field of telecommunication networks – will demonstrate the usefulness of the mobility approach ([10]). However, mobile agents and objects today are not widely accepted and there is no standard environment available and in use aside of research prototypes. Aside from exploring the technical fascination of being able to migrate alive agents and objects, we believe that understanding mobility as a mechanism of coordination could provide a clearer view on possible applications. The emerging field of coordination theory is characterized as an interdisciplinary enterprise to study how coordination can occur in different kinds of systems. Disciplines involved include computer science, organization theory, management sciences, economics, linguistics, and psychology ([9]). The effort of establishing a separate field is motivated by the observation that the phenomenon coordination is apparent in all of these disciplines. It is hoped that the transfer of ideas on coordination amongst them can lead to a better understanding of coordination fundamentals and to new ideas in the respective disciplines. At the center of coordination theory is the description of coordination as the “management of dependencies between activities” ([9]). It is hoped that the new discipline can be fostered by “characterizing different kinds of dependencies and identifying the coordination processes that can be used to manage them”. Work in coordination theory has led to a number of studies which aim at identifying categories of such processes and management options ([2]). In [4] “accessibility dependency” is identified as a dependency in resource flows, namely that produced resources must be made accessible to users. The placement or movement of activities, and the transport of resources is identified as the basic coordination mechanisms, i.e. the alternatives to manage accessibility dependency. Of specific interest in the field of mobile agents and objects in open systems is the movement of activities at runtime as a coordination mechanism. In the following we will use a coordination language from the Linda family ([8]) to express such mobility. Coordination languages are unique wrt. the dependencies managed. Aside from transporting data and activities, they also allow for the synchronization on the availability of some data. In coordination theory, this dependency is call “prerequisite” dependency. Our specific interest with this paper, however, is on higher level operations wrt. mobility. We consider the migration of data or agents from a source to a destination as a primitive which can be found in virtually all proposed systems in the field of mobile agents. When aiming at useful applications, the question is how to combine such primitives in a useful way. In order to facilitate easy construction of applications with mobile agents, it seems necessary to record such useful combinations as reusable higher order structures. This need has been recognized in the field of objectoriented design as patterns ([7]). The work there has spread