What is a Web Service

1
What is a Web Service
or some relevant web services

• (for) Developing Applications for
• Real-time Environmental Data
• Bryan Lawrence
• Head, NCAS British Atmospheric Data Centre
• P.I. The NERC DataGrid.

2
Outline

• Question What is a Web Service?
• Answer Acronym Soup!
• Fundamentals (XML)
• SOAP and REST
• WSDL
• OGC Web Services
• WFS, WCS, WFS
• UNIDATA
• LDM
• NDG Web Services

3
What is a web service?

• Googling gives
• Answers.Com Web-based applications that
  dynamically interact with other Web applications
  using open standards that include XML, UDDI and
  SOAP.
• (Yuck UDDI?)
• emergingtech.ittoolbox.com
• A Service-Oriented Architecture (SOA) is a
  collection of services or software agents that
  communicate freely with each other.
• Sub-topic definition Web Services protocols and
  standards are the technology that promote the
  sharing and distribution of information and
  business data. A protocol is a standard method
  for transmitting data through a network. There
  are many different specialized protocols to
  accommodate the many kinds of data that might be
  transmitted.

4
SOAP

• Simple Object Access Protocol

Horrible looking XML specification, but lots of
SOAP pacakges available which are (generally easy
to use), eg
from SOAPpy import SOAPproxy urlhttp//services.x
methods.net80/soap/servlet/rpcrouter nurnxmet
hods-Temperature serverSOAPProxy(url,namespacen
) temperatureserver.getTemp(27502) Returns the
temperature at U.S. zipcode 27502 (from Dive Into
Python, Mark Pilgrim)
(One knew a priori that a method named getTEmp
would return a float given an argument of a zip
code, provided one used the right namespace
How?)
5
WSDL Web Services Description Language
ABSTRACT PROPERTIES
6
RESTful Web Services

• REST (representational state transfer) is an
  approach for getting information content from a
  Web site by reading a designated (ok, you need a
  URI) Web page that contains an XML (Extensible
  Markup Language) file that describes and includes
  the desired content.
• Thats it! You use HTTP (get,put,) to a URI,
  with XML as the payload

7
Web Services the confusion

• (Tim Ewald) Today, I see three camps
• 1) It's all about SOAP messages ,
• 2) It's all about WSDL,
• 3) It's all about XML/HTTP.
• Microsoft is definitely in the first camp,
  focusing on doing things with SOAP whether or not
  they are ultimately represented as XML or a
  binary data rep.
• IBM and Iona are in the second camp, focusing on
  describing in WSDL, whether or not they are
  ultimately represented as SOAP or a binary
  protocol.
• Lots of developers and some big companies like
  Amazon are in the XML/HTTP camp, focusing on
  getting data from one place to another in a way
  they can consume it.
• Tim Ewald http//pluralsight.com/blogs/tewald/ar
  chive/2004/10/18/2875.aspx

8
Standards

• Dimensions of interoperability for earth science
  data

ISO / OGC
9
OGC web services

• Data access services, e.g.
• Live Access Server, NDG DataExtractor
• GADS
• DODS/OPeNDAP
• OGC web services
• OGC web services
• Web Map Service
• Web Feature Service
• Web Coverage Service
• Open Geospatial Consortium (OGC) International
  consortium of nearly 300 companies, government
  agencies and universities participating in a
  consensus process to develop publicly available
  geoprocessing specifications

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OGC web services

• Web Map Service (WMS)
• Three operations
• GetCapabilities (required)
• GetMap (required)
• GetFeatureInfo (optional)
• Typically support HTTP GET binding, SOAP being
  considered
• GetCapabilities request
• ltServicegt metadata for service as a whole (ISO
  19115 compliant)
• ltCapabilitygt metadata describes request bindings,
  exceptions, and
• ltLayergts title, name, SRS, bounding box,
  keywords, style nested layers inherit from parent

eg, GLOBE program WMS server GetCapabilities htt
p//viz.globe.gov/viz-bin/wmt.cgi?REQUESTGetCapab
ilitiesSERVICEWMS
11
OGC web services

• e.g. ERA40 re-analysis surface air temperature,
  2001-04-27
• deegree open-source WMS modified with netCDF
  connector

12
OGC web services

• Web Coverage Service (WCS) cf DODS/OPeNDAP/GADS
• A geographic coverage is raw (typically gridded)
  data
• WCS similar to WMS, but provides data, not
  images(cf DODS/OPeNDAP, GADS)
• Operations
• GetCapabilities
• DescribeCoverage
• GetCoverage
• Coverages supplied in well-known-binary format
• GeoTIFF, HDF-EOS, DTED, NITF, GML, netCDF???

13
OGC web services

• Web Feature Service (WFS)
• Provides access to geographic feature instances
• Features are defined by application schema
  compliant with the Geography Markup Language
  (GML)
• Operations
• DescribeFeatureType (required) returns XML
  schema for the feature
• GetFeature (required) allows retrieval of
  features in XML compliant to the feature schema.
  Selected features may be constrained through
  Xpath expressions
• Transaction and LockFeature (optional) for
  inserting, updating and deleting feature
  instances
• GetCapabilities describes capabilities of WFS
  (as for WMS)
• Typically used for vector data (eg cruise
  tracks), but can be any application schema

14
OGC web services

• FOSS implementations
• server
• deegree
• geoserver
• mapserver
• client
• quickWMS
• www.wmsviewer.com
• List maintained on NDG wiki at http//ndg.nerc.ac.
  uk/kwiki/ndg/index.cgi?OtherProjects

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OGC web services

• Summary
• Web Map Service (WMS, ISO 19128)
• GetCapabilities
• GetMap
• Web Feature Service (WFS)
• GetCapabilities
• DescribeFeatureType
• GetFeature
• Transaction, LockFeature
• Web Coverage Service (WCS)
• GetCapabilities
• DescribeCoverage
• GetCoverage
• Catalog Service for the Web (CSW) very new!
• REST, not SOAP (currently)

16
NDG CSML

• Climate Science Modelling Language (CSML, a GML
  application schema)
• http//ndg.nerc.ac.uk/csml
• Defines seven features of interest!

17
CSML
18
NERC DataGrid web services coming soon
Discovery Service (here now) Data Extractor
(next week) Vocab Service (next month) Attribute
Authority (next couple of months) Data Provider
(next couple of months) DeliveryBroker (later
this year) Browse Service (a long way off) All
Open Source, documented, clear APIs
19
NERC DataGrid Discovery Service
20
NDG dataExtractor and GeoSplat
21
UNIDATA

• Unidata (www.unidata.ucar.edu) provide data feeds
  using their Local Data Manager (LDM) technology
  to push data around the U.S. (and further afield)
  in the Internet Data Distribution system (IDD).
• Users to specify in advance which data should be
  delivered to their local systems. The IDD then
  delivers the data as soon as they are available.
• The IDD is a data subscription service,
  implemented in such a way that delivery (and
  often processing) are triggered by external
  events.
• WMO considering the use of LDM for some data
  movement.

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Unidata LDM

• LDM Delivery
• Scalable
• Hierarchical fan out
• Relying on relay sites with appropriate resources
• Flexible
• New data products can be introduced from any node
  in the system

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UNIDATA IDS Topology
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Summary

• Web services not just about SOAP and WSDL
• We should use
• web service standards,
• WS mechanisms optimised for environmental
  applications
• Currently need to extend GML (CSML)
• Options for data push if UK community want it
  based on LDM (or similar) technologies
• NDG can (and is) building a range of
  environmental web services