The Grid Architecture

The Grid architecture offers a collection of fundamental components, and interactions between these components, that collectively meet the requirements of VOs (Foster et al., 2001). In more detail, Foster et al. adopt an hourglass model to specify the layers of the Grid architecture, as shown in Figure 2.2). This architecture classifies the components into a set layers, which help in identifying the general requirements for the components, resulting in an open architecture that allows the creation of solutions that meet VO requirements.

The narrow neck of the hourglass is a set of abstractions and protocols, which are important for two reasons. Firstly, much of the high-level behaviour found at the top of the hourglass can be mapped onto this narrow neck. Secondly, the core abstractions and protocols can themselves be mapped onto many different underlying technologies that enable the overall operation of the Grid. Specifically, the resource and connectivity protocols are designed so that they can be implemented over a range of diverse resources found in the fabric layer. A range of services and behaviours, which are found in the collective layer, can be constructed from the resource and connectivity protocols. Our work is situated in the application layer, which utilises the capabilities offered by the collective layer. More specifically, the trust model will be embedded into an agent-based application within the application layer. We do not work at the lower layers, but a description of all layers is included for completeness of the context.

FREE AGENTS IN THE MULTIAGENT SYSTEM

APPLICATION

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RESOURCE

CONNECTIVITY

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FIGURE2.2: The Grid architecture (adapted from (Foster and Kesselman, 2004)).

The fabric layer consist of the geographically distributed resources, for which the Grid system provides access to protocols in the resource and connectivity layer. Examples of resources in- clude storage systems, network resources, sensors and actuators. The purpose of this layer is

to house the components that implement the local resource-specific operations that a resource is capable of performing. The decision of how much of the functionality of a resource is to be implemented in the fabric component is a trade-off between enabling more complex sharing relationships in VOs and the complexity of deploying the Grid architecture. In order to keep the deployment of the Grid architecture simple, and enable the core operations that users may re- quest from a resource, the fabric components are required to implement at least two main mech- anisms. Firstly, they are required to implement enquiry mechanisms that enable the resource to communicate its structure, state and capabilities. Secondly, they are required to implement resource management mechanisms to allow control over quality of service.

The connectivity layer consists of core communication and authentication protocols for Grid- specific transactions. This layer is responsible for enabling easy and secure communication within the Grid. The communication protocols provide methods of data exchange between fabric layer resources. These protocols are primarily drawn from the internet, transport and the application layers of the internet layered protocol architecture (Baker, 1995). The authentication protocols provide secure mechanisms for verifying the identity of users and resources. These protocols are required to have certain characteristics in order to support VO environments; these are summarised below (from (Butler et al., 2000)):

1. Single sign on — Users should be able to log on only once and gain access to a variety of resources they are authorised to access.

2. Delegation — A user must be able to execute a program that is capable of accessing the resources the user is authorised on. Additionally, the program should be able to delegate subsets of these rights to other programs based on certain conditions.

3. Integration with various local security solutions — Grid-based security solutions must be able to interoperate with various local security solutions, thus they must provide a mapping onto the existing local security infrastructure.

4. User-based trust relationships — The authentication solution should allow the user to access a variety of resources without requiring the security administrators of the resource providers to interact with each other.

The resource layer is a collection of protocols that builds upon the protocols of the connectivity layer. This collection provides the abstraction above the fabric layer functions, and mechanisms for secure negotiation, initiation, control, accounting and payment of operations of resources found in the fabric layer. Each of the protocols can be classified into two main classes:

1. Information protocols – These are used to obtain information about a resource (for ex- ample current loads and configuration).

2. Management protocols – These allow an entity to negotiate access to a shared resource by specifying certain resource requirements, such as minimum quality of service ex- pected. Many of these also support status monitoring and controlling of the operations carried out on the individual resource.

The collective layer is responsible for the coordination of multiple resources. Global protocols, which capture interactions across collective resources, are employed to achieve this. Principally, they implement the wide variety of sharing behaviours to enable the VO life cycle, for example directory services, scheduling services, monitoring services and data replication services. The application layer is the top layer of the Grid architecture. It contains the user-specific applications that are developed and implemented by accessing any of the services defined at the lower layers. The computational model of trust that this research aims to develop will be applicable in this layer of the Grid architecture. It will facilitate and assure interactions between software agents that are part of a particular application, implemented in this layer.