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 url=http://services.xmethods.net:80/soap/servlet/rpcrouter’ n=‘urn:xmethods-Temperature’ server=SOAPProxy(url,namespace=n) temperature=server.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. >>>from SOAPpy import WSDL >>>Server=WDSL.Proxy(‘path/to/WSDL’) >>>server.method.keys() [u’doGoogleSearch’,u’doGetCachedPage’ …] >>>callInfo=server.methods[‘doGoogleSearch’] >>>for arg in callInfo.inparams: . . . print arg.name, arg.type key (u’http://www.w3.org/2001/XMLSchema’,u’string’) . . . Define expected messages for a service, and their (input or output parameters). An interface will group together a number of messages (operations) Bind an Interface via a definition to a specific transport (e.g. HTTP) and messaging (e.g. SOAP) protocol The network location where the service is implemented , e.g. http://localhost:8080 WSDL :Web Services Description Language ABSTRACT PROPERTIES

6. REST’ful 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. • That’s 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/archive/2004/10/18/2875.aspx]

8. discovery access use 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 • OGCweb 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]

10. 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 • <Service> metadata for service as a whole (ISO 19115 compliant) • <Capability> metadata describes request bindings, exceptions, and: • <Layer>s: title, name, SRS, bounding box, keywords, style; nested layers inherit from parent eg, GLOBE program WMS server GetCapabilities: http://viz.globe.gov/viz-bin/wmt.cgi?REQUEST=GetCapabilities&SERVICE=WMS

11. Overlaid with rainfall from globe.digitalearth.gov WMS server 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

15. 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. GridFeature ProfileSeriesFeature ProfileFeature 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.

22. 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

23. UNIDATA IDS Topology

24. 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