Interacting with Data Service Resources

OGSA-DAI supports interaction with data service resources via a document-oriented interface. The client does not speak directly to the data service resource but instead sends a perform document to a data service. The data service then forwards the document to a data service resource which represents the actual data resource. The data service resource interprets the perform document and performs the actions described within it. These actions may involve interacting in some way with the underlying data resource, for instance, by executing an SQL query statement. A response document describing the results of the request is then composed by the data service resource and returned to the client via the data service. An example of this process is illustrated in the figure below.

Interaction via a document-oriented interface.

Data service resources are able to process multiple requests concurrently. When the concurrency limit has been reached, the data service resource begins queueing further requests. The concurrency limit and maximum queue size are specified when a data service is deployed and can be adjusted later through properties of the web service deployment descriptor.

The pages that follow introduce the main components of the OGSA-DAI interaction model:

• Activities - the operations that a data service resource can perform, including data resource manipulation, data transformation and data delivery operations.
• Sessions - objects for storing state in the data service resource across multiple requests.
• Perform Documents - the documents used by clients to describe the activities that they wish a data service resource to execute.
• Response Documents - the documents that describe to clients the results of the processing of perform documents by data service resources.