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Grid Architecture Blueprint Builds On Web Services

The Open Grid Services Architecture (OGSA) described in the new Globus-IBM paper, "The Physiology of the Grid," builds on the widely adopted Globus Toolkit and Web services technologies.

The toolkit components most relevant to OGSA are the Grid Resource Allocation and Management (GRAM) protocol and its "gatekeeper" service, which provides for secure, reliable, service creation and management; the Meta Directory Service (MDS-2), which provides for information discovery through soft state registration, data modeling, and a local registry ("GRAM reporter"); and the Grid Security Infrastructure (GSI), which supports single sign-on, restricted delegation, and credential mapping.

Web services standards addressed by the paper include SOAP, WSDL, and WS-Inspection. The Simple Object Access Protocol (SOAP) provides a means of messaging between a service provider and a service requestor. The Web Services Description Language (WSDL) is an XML document for describing Web services as a set of endpoints operating on messages containing either document-oriented (messaging) or RPC payloads. WS-Inspection comprises a simple XML language and related conventions for locating service descriptions published by a service provider.

The Web services framework offers two advantages for the Open Grid Services Architecture, the paper said.

First, the need to support the dynamic discovery and composition of services in heterogeneous environments requires mechanisms for registering and discovering interface definitions and endpoint implementation descriptions and for dynamically generating proxies based on bindings for specific interfaces, the authors wrote. WSDL supports this requirement by providing a standard mechanism for defining interface definitions separately from their embodiment within a particular binding (transport protocol and data encoding format).

Second, the widespread adoption of Web services mechanisms means that a framework based on Web services can exploit numerous tools and extant services, such as WSDL processors that can generate language bindings for a variety of languages (such as Web Services Invocation Framework, or WSIF), workflow systems that sit on top of WSDL, and hosting environments for Web services (such as Microsoft .NET and Apache Axis), the authors wrote.

Services At Core Of OGSA

The Open Grid Services Architecture (OGSA) described in the paper supports the creation, maintenance, and application of ensembles of services maintained by virtual organizations (VOs). A service is a network-enabled entity that provides some capability, such as computational resources, storage resources, networks, programs and databases.

The paper defines a Grid service as a Web service that provides a set of well-defined interfaces and follows specific conventions. The interfaces address discovery, dynamic service creation, lifetime management, notification, and manageability; the conventions address naming and upgradeability. The authors also expect to address authorization and concurrency control as OGSA evolves. Two other issues, authentication and reliable invocation, are viewed as service protocol bindings and are thus external to the core OGSA definition.

The interfaces and conventions that define a Grid service are concerned, in particular, with behaviors related to the management of "transient service instances," the paper said. VO participants typically maintain not only a static set of persistent services that handle complex activity requests from clients, but often need to "instantiate" new transient service instances dynamically, which then handle the management and interactions associated with the state of particular requested activities. When the activity's state is no longer needed, the service can be destroyed.

For example, in a videoconferencing system, the establishment of a videoconferencing session might involve the creation of service instances at intermediate points to manage end-to-end data flows according to QoS constraints. Or, in a Web serving environment, service instances might be instantiated dynamically to provide for consistent user response time by managing application workload through dynamically added capacity.

Other examples of transient service instances might be a query against a database, a data mining operation, a network bandwidth allocation, a running data transfer, and an advance reservation for processing capability.

First of two pages.