1. An Introduction to Web Services
Sriram Krishnan, Ph.D.

2. Web Services: Definition
Many different definitions are available
IBM (Gottschalk, et al): A web service is an interface that describes a collection of operations that are network accessible through standardized XML messaging.
Microsoft (on MSDN): A programmable application logic accessible using standard Internet protocols.
Another Web Service definition:
A network service that provides a programmatic interface to remote clients
Point to note is that Web services are for programmatic access, not human consumption. Unlike Websites

3. Web Services: Requirements
Accessible through standard Web protocols
Interoperability is important
Standard description language
Publishable to a registry of services
Discoverable via standard mechanisms
Web Services != web sites/servers

4. Web Services: Features
Independent of programming language and OS
Not bound to any single platform or specific protocol
All information required to contact a service is captured by the Web Service Description
Web Services Description encapsulates an interface definition, data types being used, and the protocol information
Web Services description are platform neutral

5. Web Services Architecture
Registry
Lookup
Publish
Service
Requestor
Service
Provider
Interact

6. Need to know three things about WS
What does the service do?
How is the service accessed?
Where is the service located?

7. Web Services Definition Language (WSDL)
Standard to describe the invocation syntax of a Web Service
Authors: IBM, Microsoft and others
WSDL is an XML document that describes
Interface types for each port
Content of messages it receives and sends
Bindings of interfaces to protocols
SOAP is default, others possible
Describes the access points (host/port) for the protocol

8. Web Services Stack Service Discovery Layer Service Description Layer
Service Messaging Layer
Service Transport Layer
Framework Layer:.NET, Sun’s Open Net Env, provides hooks to the application to other web service layers
Discovery: UDDI, web services publish and discovered
Description: interface, transport
Messaging: process of marshalling and unmarshalling of data
Service Transport: technology for messages between applications: HTTP, SMTP, FTP

9. SoapStruct echoStruct(SoapStruct ss)
WSDL Elements
PortTypes
Message
Types
Binding
Port
Service
SoapStruct echoStruct(SoapStruct ss)

10. <schema xmlns="http://www.w3.org/2001/XMLSchema">
WSDL: Types
Types: collection of all data types used in the Web service (any schema language can be used, typically XML Schemas)
referenced by messages
<types>
<schema xmlns="
<complexType name="SOAPStruct">
<all>
<element name="varString" type="string"/>
<element name="varInt" type="int"/>
</all>
</complexType>
</schema>
</types>

11. Message: abstract, typed definition of the data being sent
WSDL: Message
Message: abstract, typed definition of the data being sent
<message name="echoStructRequest">
<part name="inputStruct“ type="s:SOAPStruct"/>
</message>
<message name="echoStructResponse">
<part name="return" type="s:SOAPStruct"/>

12. <portType name="TestPortType">
WSDL: PortType
portType: an abstract set of operations supported by one or more endpoints
<portType name="TestPortType">
<operation name="echoStruct">
<input message="tns:echoStructRequest"/>
<output message="tns:echoStructResponse"/>
</operation>
</portType>

13. WSDL: Binding
Protocol Binding: details of how elements in PortType are converted into concrete representations
<binding name="TestSoapBinding" type="tns:TestPortType">
<soap:binding style="rpc“ transport="
<operation name="echoStruct">
<soap:operation soapAction="
<input>
<soap:body use="encoded" namespace="
encodingStyle="
</input>
<output>
</output>
</operation>
</binding>

14. Service: a named collection of ports
WSDL: Service and Port
Service: a named collection of ports
Port: how a binding is deployed to a particular endpoint
<service name="TestService">
<port binding="tns:TestSoapBinding“
name="echo">
<soap:address
location="
</port>
</service>

15. Three properties of a Web service
PortType: What does the service do?
Binding: How is the service accessed?
Service: Where is the service located?

16. Web Services Stack Service Discovery Layer: WSIL, UDDI
Service Description Layer: WSDL, RDF
Service Messaging Layer: SOAP
Service Transport Layer: HTTP, SMTP
Framework Layer:.NET, Sun’s Open Net Env, provides hooks to the application to other web service layers
Discovery: UDDI, web services publish and discovered
Description: interface, transport
Messaging: process of marshalling and unmarshalling of data
Service Transport: technology for messages between applications: HTTP, SMTP, FTP

17. WSDL Information Model
Separation between
Abstract specification
Concrete implementation of specification
In WSDL
Abstract: portType + messages
Concrete: service + port + binding

18. SOAP = HTTP + XML XML HTTP Interoperability:
De facto standard for representing data in a platform independent way
HTTP
Simple universally supported protocol
Interoperability:
Easily understood Network Protocol: HTTP
Common Data Format: XML
Lightweight mechanism for exchanging structured information in a decentralized environment using XML
SOAP just talks about encoding style for parameters
Does not defined application semantics like a programming model

19. Characteristics of SOAP
XML based
Internet-based
Independent of
Programming language
Transport mechanism
Can be used with protocols other than HTTP
SMTP: Simple Mail Transfer Protocol
JMS: Java Message Service

20. Example of SOAP request
POST /serv/echo HTTP/1.1
Host:
Content-Type: text/xml
Content-Length: 357
SoapAction:”
<SOAP-ENV:Envelope
xmlns:SOAP-ENV =
SOAP-ENV:encodingStyle=“
<SOAP-ENV:Body>
<m:echoStructRequest xmlns:m=“
<inputStruct>
<varString>Stay Classy, San Diego!</varString>
<varInt>2006</varInt>
</inputStruct>
</m:echoStructRequest>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>
SOAP: 1 envelope: what is in a message, who should deal with it
Encoding rules: serialization mechanism
SOAP RPC: defines a convention that can be used to represent RPC
SoapAction: used to direct the message to the appropriate handler
Essentially one way transmission: but can be combined to implement a request/response pattern
Body is what is meant for the ultimate recipient
Encoding style: is to indicate the serialization rules in SOAP

21. Example of SOAP response
HTTP/ OK
Content-Type: text/xml
Content-Length: 343
<SOAP-ENV:Envelope
xmlns:SOAP-ENV=“
SOAP-ENV:encoding=“
<SOAP-ENV:Body>
<m:echoStructResponse xmlns=“
<return>
<varString>I am Ron Burgundy?</varString>
<varInt>2006</varInt>
</return>
</m:echoStructResponse>
</SOAP-ENV:Body>
</SOAP-Env:Envelope>

22. Apache Axis: http://ws.apache.org/axis/
Web Service Toolkits
Apache Axis:
One of the most popular toolkits
User by several projects here like NBCR, GEON, CAMERA, GLEON
Microsoft .NET:
SUN:
Several other lightweight implementations: XSUL, ZSI (Python), SOAP::Lite (Perl)

23. Writing a Web Service: Apache Axis
Define a Web service interface
Use WSDL to define port types, messages, and data types
Use a stub compiler to generate Java sources
WSDL2Java generates client and server side bindings from the WSDL
Also generates Web Services Deployment Descriptors (WSDD)
WSDD contains information about the services like service name, implementation class, type mappings, etc

24. Writing a Web Service: Apache Axis (contd.)
Implement the service implementation (server-side)
Deploy the service into a Container
Jakarta Tomcat:
Hosting environment - provides desirable features such as reliability, scalability, security, etc
Write custom clients based on the generated stubs
Can also write clients in other languages, viz. Python, Javascript, Perl, etc
WSDD contains information about the services like service name, implementation class, type mappings, etc

25. Hopefully, we have a better idea of what Web services are
Summary
Hopefully, we have a better idea of what Web services are
What? How? Where?
Next, we will see how Web services are applicable to the scientific community