In collaborative systems, a set of organizations shares their computing resources, such as compute cycles, storage space or on-line services, in order to establish Virtual Organizations (VOs) aimed at achieving common tasks. The formation and operation of Virtual Organizations involve establishing trust among their members and reputation is one measure by which such trust can be quantified and reasoned about. In this paper, we contribute to research in the area of trust for collaborative computing systems along two directions: first, we provide a survey on the main reputation-based systems that fulfil the trust requirements for collaborative systems, including reputation systems designed for e-commerce, agent-based environments, and Peer-to-Peer computing and Grid-based systems. Second, we present a model for reputation management for Grid Virtual Organizations that is based on utility computing and that can be used to rate users according to their resource usage and resources and their providers according to the quality of service they deliver. We also demonstrate, through Grid simulations, how the model can be used in improving completion and welfare in Virtual Organizations.as Grid-based supply chains [2], among others. In such collaborative systems, trust management is a fundamental problem as resource owners must share their resources with unknown organizations as well as ensuring that all users abide by the VO agreement to which the resources have been allocated. This paper investigates how to exploit reputation systems in the management of VOs. Reputation is one measure by which trust among different members of a In this section, we develop a general utility-based reputation model for VOs, which will be used later to manage reputation in service-oriented VOs. Our reputation model is based on the model described in Reference [3]. The model was initially devised for service-oriented computing in grid systems and improves the models presented in Subsection 2.5. Central to our model is the notion of an organization. The set of all organizations is denoted by Org. We keep REPUTATION MANAGEMENT IN COLLABORATIVE COMPUTING SYSTEMS 553 Aggregating the reputation of an entity over all its consumers within a VO produces the reputation of the entity in the VO with respect to a particular issue of interest.Srv rep eic == rep eic [Time, VOUser, Srv, {QoS}] Srv rep ei == rep ei [Time, Srv, {QoS}] Srv rep e == rep ei [Time, Srv] Srv rep == rep [Time, Srv] REPUTATION MANAGEMENT IN COLLABORATIVE COMPUTING SYSTEMS 557 User rep eic == rep eic [Time, Srv, VOUser, {Usage}] User rep ei == rep ei [Time, VOUser, {Usage}] User rep e == rep ei [Time, VOUser] User rep == rep [Time, VOUser]
