Related Work

2.5.1. Virtual Organizations and NESSI business models

The concept of Virtual Organizations (VO) was born from Grid Computing [60]. Ian Foster et al [61] describe exible, secure, coordinated resource sharing among dynamic collections of individuals, institutions, and resources, what they refer to as Virtual Organizations (VOs). Nurmela [99] denes Virtual Organizations as loosely-coupled, inter-enterprize col- laboration, and points out the need of common facilities for managing contract-governed collaborations and the autonomous business services between which those collaborations are formed. He further highlights the signicance of Service Level Agreements (SLAs ) in the formation of such business collaborations. Nurmela provides a detailed description of Service Level Agreement Management in Federated Virtual Organizations by comparing the Non Functional Aspects of some of the popular SLA languages. NESSI (Networked European Software and Services Initiative) [10] is a consortium of over 300 ICT indus- trial partners. According to NESSI-Grid's Strategic Research Agenda (SRA) [10], Virtual Enterprize Organizations (VEOs) form when two or more administrative domains overlap and share resources. NESSI considers VEOs as a business model and lists various business requirements for it. NESSI denes Business Value Networks as ways in which organizations interact with each other forming complex chains including multiple providers/administra- tive domains in order to drive increased business value. NESSI in its Strategic Research Agenda (SRA) has highlighted the importance of Business Value Networks [10] as a vi- able business model in the emerging service oriented ICT infrastructures. In addition to the notion of Business Value Networks, NESSI has pointed out various other possibili- ties for similar inter-organizational business models; Hierarchical Enterprizes, Extended Enterprizes, Dynamic Outsourcing, and Mergers to name a few.

2.5.2. Optimization against QoS constraints

There are a number of Workow Management Systems available for the Grid [47] [120] [142] [26]. Workow QoS constraints are an essential part of the workow design and play an important role in their scheduling[139, 74, 35, 51, 117]. These have been dened [138] to consist of ve components which are Time, Cost, Fidelity, Reliability and Security. These constraints can be dened at the workow level or at the task level [139] with in a workow. Traditional workow systems lack the ability to be dynamically modeled and scheduled.

Mapping abstract workows on to service enriched environments such as Grid has been a challenging research area [47] [142]. QoS constraints are an essential part of the workow design and play an important role in service selection and scheduling [139] [38]. They can be dened at the workow level or at the task level [139] within a workow. Binder et al. [30] extract the resource requirements of the services from the OWL-S [91] descriptions that allow dening nonfunctional properties of the components. A mathematical model then computes the execution cost of the workow and afterwards a genetic algorithm is used to optimize the workow execution. This approach very successfully maps the resources on workow tasks but does not discuss dynamically changing conditions. Ambrosi et al.

[54] propose to optimize workow scheduling through reactive and proactive actions. They have proposed a concept of optimization by assessing the current situation and forecasting the best possibilities for the future. Their approach lacks concrete examples. Huang et al. [69] present a very good approach to workow optimization by dynamic web service selection. An optimal service is selected based on historical data and real-time data. Their approach does not discuss the case of adapting to user-dened QoS constraints. Jia Yu et al. [139] propose a QoS-based workow management system and scheduling algorithm for the service Grid that minimizes the execution cost and yet meets the time constraints imposed by the user. The QoS-level constraints can be dened at the task level as well as at the workow level.

Tao Yu et al. [141] [140] have made an extensive study on service selection algorithms for composing web services with multiple QoS constraints. They have devised two kinds of models to address this problem: the combinatorial model and the graph model. Their com- binatorial model describes this problem as the Multidimensional Multi-choice Knapsack Problem (MMKP) and the graph model denes it as the Multi-Constraint Optimal Path (MCOP) problem. They have evaluated dierent heuristic and non-heuristic algorithms after sequential executions and found the Branch and Bound algorithm to be optimal but slow. The research conducted as part of this thesis studied their approach and have gone a step ahead by implementing a parallel version of the Branch and Bound algorithm to retain the optimality while coupling it with a sequential heuristic algorithm to ensure the eciency.

Blackboards [42] are a mechanism to solve complex problems and have been successfully employed in various elds. Following [133], the author of this thesis, has contributed

in [115] in terms of the development of a blackboard [42] approach coupled with an A∗

algorithm to automatically construct and optimize the Grid-based workows.

2.5.3. Views and e-Contracts in Workows

The concept of Workow Views is used to maintain the balance between trust and security among business partners [37, 48, 52]. Schulz et al [78] have introduced the concept of view based cross-organizational workows and call it as coalition workows. Liu et al have contributed in the process-view model [48, 52]. Chiu et al [40] present a meta model of workow views and their semantics based on supply chain e-service but their model lacks an integrated cross-organizational perspective. Other authors [116, 85], however, do propose a global view or a decomposition process based on the views. But none of them have focussed on the dynamic workows in their approach. Static and dynamic verication of temporal constraints [38, 39] is very crucial in workows to avoid any temporal violations during the workow life cycle. Eder et al [53] employ the concept of views to calculate the temporal consistency of interorganizational workows by using abstraction and aggregation operators of views but their approach is also limited to static or predened workows. Chebbi et al [37] provide a very comprehensive approach that is view based, web services focused and is applicable to dynamic inter-organizational workow cooperation. This means that the cooperation across organizations is described through views without specifying the internal

structure of participating workows. This concept of contracts is similar to that of SLA although SLAs are more dynamic due to negotiation, renegotiation and fault tolerance features. There is a very relevant work done by Chiu et al. [41] in terms of a contract model based on workow views. They demonstrate how management of contracts can be facilitated. They start with an example, highlight domains of dierent participating organizations and then develop a model to identify the corresponding workow views. They go on further to develop an e-contract model that denes e-contracts in plain text format. This is an old paper and the modern form of e-contracts are Service Level Agreements which are XML based, more complex and more dynamic due to features such as negotiation, renegotiation, and fault tolerance, etc. Furthermore their approach starts with dening views in an inter-organizational workow and then describing e-contracts to enforce the obligatory communication links in the views. The model presented in this thesis allows SLAs to maintain their individual identity. Therefore views are dened directly on the SLA aggregation structure rather than on workows. Moreover the proposed approach also provides a formal description of hierarchical SLAs and their aggregation.

2.5.4. Distributed Trust and Security

In the Trust-EC project, Jones [76] denes trust as the property of a business relationship, such that reliance can be placed on the business partners and the business transactions developed with them. Transport Layer Security (TLS) is a popular authentication protocol in VOs [50]. It is derived from the Secure Sockets Layer (SSL) protocol [63] and uses X.509 public key certicate [68], which binds a Distinguished Name (DN) to a public key. The binding is attested to by a Certication Authority (CA) [70]. Kerberos [98] is an authentication system designed to allow a single sign-on to many machines within a single administrative domain. The Grid Security Infrastructure (GSI) and the security modules of middle-ware, provide a set of security protocols for achieving mutual entity authentication between a user (actually a user's proxy) and resource providers [144]. Each party has a public-key based cryptographic credential in the formulation of a certicate. GSI uses X.509 proxy certicates (PCs) to enable Single sign-on and Delegation [86]. PCs can be created on the y without requiring any intervention from conventional CAs. In the cross-CA Hierarchical trust model [86][144], the top most CA is called the root CA that provides certicates to its subordinate CAs. These subordinates can further issue CA to other CAs (subordinates), services or users. Community Authorization Service (CAS) [111, 64] allows the expression of policies regarding resources distributed across a number of sites. Similarly, the Virtual Organization Membership Service (VOMS) [21] also gives the capability to provide authorization information by a secure server that the local site has chosen to trust.