All OGSA-DAI presentation layers back onto an OGSA-DAI server which manages a number of OGSA-DAI resources. This section outlines the OGSA-DAI web serviced-based presentation layers that OGSA-DAI currently supports.
As there are six types of OGSA-DAI resource, there are six types of OGSA-DAI web service that can be envisaged as being supported by a standard OGSA-DAI web service-based presentation layer.
Data Request Execution Service (DRES)
Data Resource Information Service (DRIS)
Data Sink Service
Data Source Service.
Session Management Service
Request Management Service
Each service provides access to all resources of the corresponding type that reside in the OGSA-DAI server. Operations of the service map directly onto the operations of the corresponding resources. The individual implementations of web service-based presentation layers determine:
How the resource of interest is specified to the service.
How service operation names and arguments are mapped to resource operation names and arguments.
How resource return values and exceptions are mapped to service return values and faults.
An important criterion for OGSA-DAI web services, or indeed any OGSA-DAI presentation layer, is that they should not implement any OGSA-DAI functionality themselves - they should just pass arguments and results to and from the OGSA-DAI server.
The default OGSA-DAI web service-based presentation layers are based upon the WSRF specifications.
Both clients and servers will typically manipulate OGSA-DAI resource IDs which are simple server-unique names of resources that exist on an OGSA-DAI server.
However, there may be clients that expect WS-EPRs rather than resource IDs (e.g. third party applications). When interacting with OGSA-DAI WSRF-compliant presentation layers there may be a need to obtain WS-EPRs corresponding to these resource IDs.
As WS-EPRs are meant to be opaque to clients OGSA-DAI WSRF-compliant web services need to manage the mapping between resource IDs and associated WS-EPRs.
The WSRF extension to WS-Addressing - the notion of WS-resource qualified endpoint references - is used to identify to OGSA-DAI web services the resource at which a specific operation is targeted.
For functionality relating to lifetime management and the access of resource properties OGSA-DAI web services implement operations conformant with the WS-ResourceProperties and WS-ResourceLifetime specifications.
In addition, OGSA-DAI port types (groups of related Web service operations) are defined for:
DataRequestExecutionPortType - to support exposure of the Execute operation of DRERs.
DataSinkPortType - to support exposure of the PutBlock, PutNBlocks and PutFully operations of data sinks.
DataSourcePortType - to support exposure of the GetBlock, GetNBlocks and GetFully operations of data sources.
WSEPRResolverPortType - to support the conversion of OGSA-DAI resource IDs into WSRF-specific WS-EPRs.
IntrinsicsPortType - to support access of generic OGSA-DAI server information, e.g. server version and resources known to a specific service.
The WSRF and OGSA-DAI-specific portTypes are aggregated into services. Each of the six types OGSA-DAI service support operations from the following portTypes:
IntrinsicsPortType
WSEPRResolverPortType
WS-ResourceProperties portTypes
In addition each service supports operations from the following portTypes:
Data Request Execution Service - DataRequestExecutionPortType
Data Resource Information Service - WS-ResourceLifetime portTypes
Data Sink Service - DataSinkPortType and WS-ResourceLifetime portTypes
Data Source Service - DataSourcePortType and WS-ResourceLifetime portTypes
Session Management Service - WS-ResourceLifetime portTypes
Request Management Service - WS-ResourceLifetime portTypes