OGC Capabilities Documents In The ROA

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June 2008
TC Meeting: Architecture DWG Presentation

66th OGC Technical Committee

Potsdam, Germany
Arnulf Christl
June 02, 2008
Copyright © 2008, Open Geospatial Consortium, Inc., All Rights Reserved.

Agenda

• Scope of this presentation
• RESTful OGC Web Services?
• Introducing to the ROA (Resource Oriented Architecture)
• OGC WMS in the ROA
• Recap WMS Capabilities
• Reading the Capabilities but not getting Maps
• How to do Better: LegendURL
• Making Capabilities findable
Copyright: WhereGroup GmbH & Co. KG.
Licensed under GNU FDL http://www.gnu.org/licenses/fdl.txt
and CreativeCommons 3.0 ShareAlike

Helping the World to Communicate
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Scope of this presentation

• This presentation shows that an OGC WMS Capabilities
document is the representation of a resource (even if it is
the result of the operation of a software).
• The idea is NOT to start a
discussion whether SOAP will
clean the REST of the web.
• The idea is NOT to present REST
as the solution to all problems.
• The idea is to start thinking about
how REST can be put to use.
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RESTful OGC Web Services?

• According to Wikipedia OGC Services have been designed
around the REST paradigm [1].
• This is funny because 10 years ago the term REST and
RESTful services was yet to be born.
• Maybe the OGC was more avantgarde than we might
have expected?
• Probably the OGC design simply followed standard best
practices to serve the emerging WAN architecture.
• As an example we look into the standard OGC WMS.
[1] http://en.wikipedia.org/wiki/Open_Geospatial_Consortium

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Introducing the ROA

• The Resource Oriented Architecture is about enabling the
programmable web. It is the home for RESTful web
services.
• Instead of calling remote procedures at one single address
it uses distributed resources across the web.
• Many implementations of OGC WMS standards are based
on a software's perspective of the architecture.
• The ROA is the architecture's perspective of a software that
runs as a service.

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The ROA Concepts and Properties

Four concepts: Resources
Their names (URL)
Their representations
The links between them

and four properties Addressability
Statelessness
Connectedness
A uniform Interface

From: RESTful Web Services, Richardson & Ruby, page 105

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OGC WMS in the ROA

• The OGC WMS standard is not all that interesting from a
programmable web's perspective because it only returns
pretty but dumb images.
• But the OGC WMS Capabilities document is a good start to
look what could be done better
• The Capabilities already contains (almost) everything that
is required to make clients get maps in the ROA.
• Clients must know how to formulate a GET request with
scoping parameters, some of which have explicit values.

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The OGC WMS Capabilities Document

• The OGC WMS capabilities document contains information
about "maps" (the resource) and how to access them (GET
a representation).
• The Capabilities document is one possible representation
(the meta data) of the resource "maps".
• It contains hints on how to GET (GetMap) more
representations (images) of the named resource (maps).
• Although it contains redundant information about WHERE
the resource is, the client still must have implicit knowledge
of HOW to GET the representation.
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Getting OGC WMS Capabilities
[1]

• OGC WMS capabilities documents have a location (URL)
http://wms.oregonexplorer.info/ecwp/ecw_wms.dll?

• Then the resource has to be scoped with a set of defined
parameters to actually get the document.
?REQUEST=GetCapabilties&SERVICE=WMS&VERSION=1.1.1

• This scoping information (in practice) never changes.
http://wms.oregonexplorer.info/ecwp/capabilites

...would be just fine, maybe add the version of the service:
http://wms111.oregonexplorer.info/ecwp/capabilites

• The Capabilties document is just a representation of the
resource "maps" that exposes its meta data.
[1] http://wms.oregonexplorer.info/ (OGC web site of the month 03/2008)

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Reading the Capabilities Documents

• The "Capabilities" contains information WHERE to get
<Request>
<GetCapabilities>
maps but not HOW to get them:
<Format>application/vnd.ogc.wms_xml</Format>
<DCPType>
<HTTP>
<Get>
<OnlineResource xlink:href="http://wms.oregonexplorer.info/ecwp/ecw_wms.dll?"/>
</Get>
</HTTP>
</DCPType>
</GetCapabilities>
<GetMap>
<Format>image/png</Format>
<Format>image/jpeg</Format>
<DCPType>
<HTTP>
<Get>
<OnlineResource xlink:href="http://wms.oregonexplorer.info/ecwp/ecw_wms.dll?"/>
</Get>
[...]

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Not Getting the Map

• To get a map representation of the OGC WMS resource
the client has to scope it with parameters.
http://wms.oregonexplorer.info/ecwp/ecw_wms.dll?
REQUEST=GetMap
SERVICE=WMS
VERSION=1.1.1
LAYERS=2005ORTHOIMAGERY.ECW
FORMAT=image/jpeg
STYLES=
BGCOLOR=0xFFFFFF
TRANSPARENT=TRUE
SRS=EPSG:4326
BBOX=122.168515234362,42.8926048497946,122.039660890008,42.9934387129544
WIDTH=835
HEIGHT=705

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Getting the Map

• Some of these parameters are irrelevant, some should be
part of the URL, some I don't know what to do about either.
http://wms111.oregonexplorer.info/ecwp/ecw_map?
REQUEST=GetMap
SERVICE=WMS
VERSION=1.1.1
? LAYERS=2005ORTHOIMAGERY.ECW
FORMAT=image/jpeg
STYLES=
BGCOLOR=0xFFFFFF
TRANSPARENT=TRUE
? SRS=EPSG:4326
BBOX=122.168515234362,42.8926048497946,122.039660890008,42.9934387129544
WIDTH=835
HEIGHT=705

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LegendURL does it better [1]

• This OnlineResource has all the WHERE and the HOW:
[...]
    <Layer queryable="0" opaque="0" cascaded="0">
        <Name>railroad_line</Name>
        <Title>Railroad lines</Title>
        <SRS>epsg:4326</SRS>
        <LatLonBoundingBox minx="5.98259" miny="47.4064" maxx="15" maxy="54.9045" />
        <BoundingBox SRS="EPSG:4326"
                    minx="5.98259" miny="47.4064" maxx="15" maxy="54.9045" />
        <Style>
          <Name>default</Name>
          <Title>default</Title>
          <LegendURL width="20" height="10">
             <OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:
               href="http://wms.wheregroup.com/cgibin/germany?version=1.1.1&service=WMS&
               amp;request=GetLegendGraphic&layer=railroad_line&format=image/png"/>
          </LegendURL>
        </Style>
    </Layer>
[1] http://wms.wheregroup.com/cgibin/germany
[...]

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Modifying Capabilities Documents

• Capabilities with WHERE and HOW to get maps:
<Request>
<EPSG4326>
<DCPType>
<HTTP>
<Get>
<OnlineResource xlink:href="http://wms.oregonexplorer.info/ecwp/4326/ecw_map?"/>
</Get>
</HTTP>
</DCPType>
</GetCapabilities>
<EPSG:26910>
<DCPType>
<HTTP>
<Get>
<OnlineResource xlink:href="http://wms.oregonexplorer.info/ecwp/26910/ecw_map?"/>
</Get>

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The ROA Rules!

Four concepts:
in              up Resources capabilities
/   left  <[MAP]>  right Their names (URL) maps
out           down      Their representations legends
capabilities   Legend The links between them ...

and four properties Addressability
Statelessness
Connectedness
A uniform Interface
From: RESTful Web Services, Richardson & Ruby, page 105

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Making Capabilities Findable

• Expose OGC WMS Capabilities documents as really
beautiful documents not as the result of a remotely
invoked operation of a software.
• Give the Capabilities Document an extra home (URL).
• The Capabilities Document is the vcard of the service and
must contain all information that is needed to access the
resource and get a result.
• Explore how to use XHTML as a format that is readable by
machines and humans alike.

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Discussion

Thank you for your attention.

Copyright: WhereGroup GmbH & Co. KG.
Author: Arnulf Christl
This presentation is dual licensed under GNU FDL and
WhereGroup GmbH & Co. KG
Creative Commons 3.0 Share Alike. Choose the one that fits
Siemensstr. 8
your needs best. The presentation master, first page and last
53121 Bonn page (this one) are what the GNU FDL refers to as invariant
Germany sections. Please do not modify these pages without getting
http://www.wheregroup.com/ written permission by the author. Find the full text of the GNU
FDL at: http://www.gnu.org/licenses/fdl.txt

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OGC Capabilities Documents In The ROA

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