Open Geospatial Consortium Catalogue Services

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Open Geospatial Consortium Catalogue Services

• History and activities
• Douglas Nebert, FGDC

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Overview

• Inception of Catalogue Services V1.0
• Geospatial Profile Implementation
• Catalogue Interoperability Profile (CIP)
• CORBA Implementation
• Catalogue Services 2.0 Concepts
• Protocol Bindings and Profiles
• Interest in Web Services

3
OGC Implementation Specs

• Abstract Specification Topic documents called
  for the specification of interfaces for discovery
  of geospatial resources
• Request For Proposals published to assess
  industry interest in catalog services
• Responses guided primarily through sponsored
  integrators and interests

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Objectives of Catalog Services

• To define interfaces to support the discovery of
  and handles to geospatial information (feature
  collections, datasets) and services.
• To integrate with or complement existing
  technologies and OGC specifications wherever
  possible

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Catalog-based architecture
Client
Description (Metadata)
Resource
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Catalog Services Version 1.0

• Response to standardization interests from the
  (vector) geospatial, earth observation, and
  defense communities for interoperable search
  across distributed catalogues
• Published in 1999 as an OGC Specification
• One abstract model (UML) and three Implementation
  Profile sections

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Search abstraction

• Catalogue services were designed around an
  abstract query model defined by the ISO23950/ANSI
  Z39.50 standard
• Well-known fields are identified for
  cross-catalogue search (e.g. title) that are
  independent of native document structure
• Well-known response messages include Brief,
  Summary, and Full designations
• Format of response syntax was also defined, e.g.
  XML, text, HTML

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Query Language Premise

• Common Query Language predicate language (e.g.
  WHERE clause) that can be transformable to/from
  CQL without loss
• Z39.50 RPN query (Type-101)
• OGC Filter language (POST)
• OGC Common Query Language (GET)

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Version 1.0 design

• Implementation Profiles in the specification
• CORBA
• OLE/COM, OLEDB
• Z39.50
• Interoperability bridges were demonstrated at the
  OGC Southampton meeting in 1999 to pass search
  between CORBA and Z39.50

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Catalog Services Response
Fine UML Model
Coarse UML Model
CORBA
OLEDB
CORBA
WWW/ Z39.50
Normative
Informative
Catalog Services Specification 1.0 passed August
1999
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Details

• Coarse-grain model is more message-oriented,
  supportive of and mapped to key Z39.50 concepts,
  ideal for general solutions, Relatively simple.
• Fine-grain model is more object-oriented (many
  objects), asynchronous, ideal for enterprise
  solutions. Very complex.
• Both approaches session based with a balloon
  proviso for Stateless capabilities

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Features

• A catalog can reference data or services through
  indexed, structured metadata
• Multiple query languages supported
• Common Query Language (CQL) is described using
  BNF as a reasonable subset of the SQL WHERE
  statement against abstract target fields
• Capabilities function intended to share service
  details (like explain)
• Element set concept preserved (Brief, Full)
• Response format preserved (XML, HTML, etc)

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Packages in Catalog
Catalog Service
Discovery
Management
Access (Order)
required
optional
optional
Discovery is the most common set of
functions supported by the specification,
management and access/order are not mandatory.
Details exist in CIP, for example, for order
functionality
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Version 1.0 Profiles

• Community-defined profiles were agreed-upon,
  generally outside of OGC, for specific contexts
  and payloads
• Geospatial Profile GEO
• Catalogue Interoperability Profile (CIP)
• CORBA Profile (part of specification)
• Identify the context of application, special
  metadata requirements, and more rigorous
  opportunity for conformance testing

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Version 1.0 Profiles

• Z39.50 Geospatial Profile GEO designed to
  provide search facility against heterogeneous
  metadata formats (400 catalogues)
• Z39.50 Catalog Interoperability Profile (CIP)
  developed through CEOS members for earth
  observation catalogues (10 catalogues)
• CORBA Profile implemented by Geodan in the
  Netherlands (5 catalogues)
• OLEDB Profile never commercially implemented

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Evolving Interests

• Original Web Mapping Testbed identified a need
  for a simpler HTTP-based interface for catalogs
  to parallel GET-style request/response pairs of
  emerging Web Services Model
• Multiple services in testbed also showed a
  requirement for a services metadata model and
  service registry as a catalog implementation
• Interest in elaborating optional management and
  access interfaces within Catalog Services spec

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Issues with WMT approach

• HTTP approach well-liked, simple
• Crawler method to populate services catalog also
  popular
• Services registry used model tuned to discovery
  of WMS layers, not clear if adaptable to other
  services
• Approach not consistent with emerging Web Service
  Registries (UDDI, ebXML)
• Poor description of data (layers), no links to
  data metadata

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Stateful/Stateless

• Message-based request-response model is
  preferred by many for all types of Web Services
  inside and outside of OGC. Also referred to as
  stateless.
• Stateless and stateful (session) approaches for
  catalogs can be used equally well for data and
  for services
• Invocation can be styled after the GET approach
  of WMS and the POST approach of WFS following

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Catalogue Services 2.0

• Implementation profiles removed from
  specification
• Abstract model plus Protocol Binding chapters to
  describe specific implementation context
• Application Profile documents now required to
  provide selection of Protocol Binding, specific
  use context, restricted/extended operations, and
  metadata schema(s) used in response

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Catalogue Service Protocol Bindings

• CORBA Binding carrying forward previous
  capabilities
• Z39.50 Binding specifies three implementation
  environments
• Search and Retrieval Web, SRW (SOAP)
• Search and Retrieval URL, SRU (Get)
• Z39.50 over TCP/IP (traditional)
• HTTP Protocol Binding using GET POST, known as
  Catalogue Service for the Web, CS-W

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CS-W over HTTP

• CS-W is the primary Web/HTTP environment for
  stateless access to a catalogue
• Two draft Application Profiles have been created
  for CS-W
• ebRIM Registry Information Model Profile
• ISO19115/19119 Profile

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CS-W profiles

• Profile services should interoperate by
• Adopting basic interfaces (getCapabilities,
  describeRecordType, getRecord)
• Supporting neutral query entry points (e.g.
  title creator
• Support GET and POST methods
• Respond with common Brief record as XML

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ISO19115/19119 App Profile

• Manages two metadata resource types
• ISO 19115 Data descriptions
• ISO 19119 Service descriptions
• All properties in metadata may be searchable
• Assertions can be traversed that define data and
  service associations
• Originated in NRW Germany SDI and in deployment
  testing in various parts of Europe
• Permits SOAP access, WSDL declaration
• Recommended for Implementation Specification
  status, November 2005

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ebXML Registry Information Model Application
Profile

• General purpose registry that can be applied to
  describe data, services, schemas, documents, etc.
• Adopts ebXML RIM metadata model for query on
  small set of common properties
• Objects being described are extrinsic objects
  and are not fully queryable
• May reference service instance WSDL
• Recommended for Implementation Specification
  status, November 2005

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Alternatives

• Formal ebRIM provides SQL and SOAP interfaces and
  free implementation outside OGC
• SRU/SRW deployed in the library community on
  Dublin Core-style metadata and library catalogues
• UDDI well-suited to discovery of services, but
  not to specific content