SDI Convergence - Nederlandse Commissie voor Geodesie - KNAW

same implementation of a geospatial grid for NASA, i.e. augmenting the Globus Toolkit’s
Metadata Catalogue Service (MCS) with the profile of the OGC Catalogue Service.
From a grid configuration point of view, there are two relevant services in the Globus
Toolkit, the Monitoring and Discovery System (MDS) (for hosts in Compartimos) and
the Replica Location Service (RLS) (for dataset replicas in Compartimos).
4.3 The AddressDataAccessService
The AddressDataAccessService converts address data from local proprietary format to
an interoperable address data model (described in the information viewpoint of Compartimos,
which is not included in this article), acting as an interpreter for a specific
source of address data and providing a uniform access method to any dataset that is
published in the address data grid. The OGSA data architecture proposes three generic
data access operations for structured data: Create, ExecuteQuery and BulkLoad.
Create creates an association between a data service and an underlying data resource,
which may be created and populated as a result of this operation.
However, in an SDI environment, the main drive for an address data grid is to publish
(as opposed to edit and maintain) address data. The RegisterDataPublication of the
CatalogueService associates a dataset with an AddressDataAccessService (what Create
does for OGSA). The Compartimos model provides for a one-to-many relationship
between a dataset and an access service, thereby increasing scalability and enabling
versioning of the interoperable address data model in the Compartimos catalogue.
ExecuteQuery is represented by the GetAddress operation of the AddressDataAccessService
and BulkLoad is represented by the UploadAddressData operation in
Compartimos, performing more or less the same functionality as their OGSA counterparts,
albeit customised for address data.
The OGC WFS, which returns spatial data in vendor independent Geography Mark-up
Language (GML) is a natural choice for this service. This implementation specification
is currently in the process of being adopted as an ISO standard (ISO 19142 (draft)).
However, functionality over and above normal WFS is required for the conversion to
and from the Compartimos interoperable address data model. Aloisio et al. (2005a), Di
et al. (2008), Goodenough et al. (2007), Wei et al. (2006) and Zhao et al. (2004) report
on grid-enabling OGC web services, such as WFS and the OGC Web Map Service
(WMS).
An alternative technology choice is OGSA-DAI (Data Access and Integration), which is
compatible with the Globus Toolkit. However, OGSA-DAI has been developed for alphanumeric
data and would require some extensions to accommodate spatial data. An
advantage of OGSA-DAI is that its resources are already usable by other Globus Toolkit
services. The choice of OGSA-DAI would influence the technology choice for other
services, such as the CatalogueService and the VirtualAddressDataService.
4.4 The VirtualAddressDataService
The VirtualAddressDataService provides the required consolidation functionality to
make the distributed heterogeneous address datasets appear to be a single virtual address
dataset. This consolidation includes, for example, removing duplicates (due to
multiple address data sources) and resolving ambiguities. The OGSA data architecture
defines a set of operations of a Data Federation service for the logical integration of
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