WS Information Services: Key Concepts

Overview
Uses and Benefits of the Index Service
>Information Model
Queries
User Interface
Security
Related Documents

Information Model

As mentioned above, each Grid service instance has a particular set of service data associated with it. The essence of the Index Service is to provide an interface for operations generating, aggregating, and querying this data.

This section presents a brief overview of OGSA and Globus Toolkit 3.0 concepts and terminology as related to the Index Service. More information on OGSA and GT3.2 can be found in the publications listed in the Related Documents section.

The OGSA model comprises persistent services (typically a few) and transient services (potentially many).  All services adhere to specified Grid service interfaces and behaviors.  The interfaces correspond to WSDL portTypes and are used for managing the Grid service instances.  The GridService portType is used to query and update the service data of a Grid Service instance and control the lifetime of the instance. 

The interfaces and functions most relevant to the Index Service can be summarized as follows:

Factory

Used to create a new Grid service instance via its CreateService operation.  A Factory returns a Grid Service Handle (GSH) as described below, and maintains a set of Service Data Elements that can be queried.

Grid Service Handle (GSH)

Returned by the Factory create operation.  A GSH is a URL used to globally name a Grid service instance for all time.  A GSH must be converted to a Grid Service Reference (see below) in order to use the service. 

Grid Service Reference (GSR)

Describes how a client can communicate with a Grid service instance.  The HandleMap interface allows a client to map from a GSH to a GSR.  While the GSH represents name only, the GSR includes binding information for transport protocol and data encoding format.

Query

Provides extensible query language support.  A Grid service instance maintains a set of Service Data Elements.  The FindServiceData operation from the GridService interface is used to query this information.  This is a standard, extensible query operation against a service's Service Data Elements, which may use either the simple default “by name” query or a more complex language like XPath or XQuery.  The ogsi-find-service-data command (see Querying Service Data ) is the client to the query interface.

Registry

Supports discovery by returning the GSHs of a set of Grid services.  A Registry allows for soft-state registration of Grid services, in that a set of Grid services can periodically register their GSHs into a Registry service to allow for discovery of services in that set.

Notification

The NotificationSource interface is used for client subscription, to register interest in a service.  It sends notification messages about registered topics.  These topics can be linked to arbitrary values (state values) generated by a service at runtime or linked to a service data element.  The NotificationSink interface is used for asynchronous delivery of notification messages; it can be applied to dynamic discovery/registration services, monitoring, application error notification, etc.

To summarize, a Grid service instance is created by the Factory interface, which returns a GSH to identify the service.  The GSH is converted to a GSR (which includes binding information) so that the service can be used.  Service instances can be identified in a Registry, which can be used to support query or other operations on a given service.  The Notification interface provides for subscription to and delivery of notification messages regarding service-related events.