10.3. Open Grid Service Architecture (OSGA)

OGSA draws on the same infrastructure as used in Web services: XML, SOAP, WSDL, and WSIL. However, there are some important conceptual and practical extensions that arise from the need to address a dynamic grid environment providing mechanisms to create and discover customized service instances with controlled, fault-resilient, and secure management of distributed atomic or collective services, often with a long-lived state.

Four important concepts in OGSA are:

• Naming: Each grid service instance is globally, uniquely, and for all time named by a Grid Service Handle (GSH).

• Factories: Create new grid service instances and maintain a group of service data elements that can be queried. Factories play the role of a gatekeeper or xinet daemons for grid services. A factory will also have an associated registry to keep track of these instances and enable discovery. The OGSA defines registries as places to store various kinds of information about grid resources.

• Instances: The GSH is just a minimal name in the form of a URI and does not carry enough information to allow a client to communicate directly with the service instance. Instead, a GSH must be mapped to a Grid Service Reference (GSR) via the registry.

• Stateful: A grid service instance has a state. A process can be initiated via a method call on a service port type and its state checked at a later time using the GSR.

A GSR contains all information that a client may require to communicate with the service via one or more protocols. While a GSH is valid for the entire lifetime of the grid service instance, a GSR may become invalid, therefore requiring a client to use the mapping service to acquire a new GSR appropriate to a particular binding using the GSH.

A GSR is encoded using WSDL, so that a WSDL document should be the minimal information required to fully describe how to reach the particular grid service instance.

Globus Toolkit V3 is roughly the Globus Toolkit 2 that uses OGSA architecture to make its components available via Web services.