WS Information Services: Key Concepts

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

Queries

Data discovery often requires instance-specific, sometimes dynamic information.  So far in this document, we've seen that each Grid service instance has a particular set of service data associated with it.  This service data offers a general solution to the issues of discovery, in that every service must support some common service data, and may support any additional service data desired, such as metadata and instance state data.

FindServiceData

A primary function of the Index Service is to provide an interface for querying aggregate views of service data collected from other services.  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.

NotificationSource

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.

Pull/Push (synchronous/asynchronous)

Both subscription and FindServiceData are forms of query against the service data of an instance.  FindServiceData is a simple synchronous query (pull), while a subscription notification is a persistent query, with asynchronous response (push).

Registry

The Registry interface supports discovery by returning the GSHs of a set of Grid services.  The 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.