Introduction toWeb Services

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Web Services A Web service is a method of

communication between two electronic devices over the World Wide Web

In other words, a web service helps to convert your application into a web-based application.

Why web services? Your application can publish its function

or message to the rest of the world.

Department of Information Science Engg

WEB Services

REST Services

SOAP and WSDL

REST Services Relying on URIs and HTTP verbs Usage of 5 big HTTP verbs

POST, HEAD, GET, PUT, DELETENothing more than CRUD concept of the Web

Ex: Delicious APIsDelicious is a leading social bookmarking serviceVisit: http://www.peej.co.uk/articles/restfully-delicious.html

Data exchange formatXML, JASON or both

SOAP based services Simple Object Access Protocol Exclusively use XML as the data format to

exchange info over HTTP. A service that needs to be used by another

service needs to specify its usage through a “Service Description”.

In this case, we use WSDL – Web Services Description Language

Ex: Apache Axis, Apache CXF SOAP has nothing to do with SOA – Service

Oriented Architecture

continued…...

Continued……. • A service is hosted on a “Discovery Service” in the internet.• A client which wants to use this service will have to “discover” this service (similar to RMI) using the ‘Discovery Service’.• Once the service is “discovered”, the client asks the service how it should be invoked. The service replies in WSDL format.

• The service is now invoked using SOAP. The client issues a ‘SOAP Request’.

• The service responds with a ‘SOAP Response’, which includes the result.

A SOAP message is an ordinary XML document containing the following elements:• Envelope - identifies the XML document as a SOAP

messageo Header - contains information about the request. o Body

Message data - contains request and response information itself.

Fault (optional) - containing errors and status information.

<?xml version="1.0"?><soap:Envelopexmlns:soap="http://www.w3.org/2001/12/soap-envelope"soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock">  <m:GetStockPrice>    <m:StockName>IBM</m:StockName>  </m:GetStockPrice></soap:Body>

</soap:Envelope>

A SOAP request:

<?xml version="1.0"?><soap:Envelopexmlns:soap="http://www.w3.org/2001/12/soap-envelope"soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock">  <m:GetStockPriceResponse>    <m:Price>34.5</m:Price>  </m:GetStockPriceResponse></soap:Body>

</soap:Envelope>

The SOAP response:

Continued……

WSDL WSDL stands for Web Services

Description Language It  is an XML-based language that is used for

describing the functionality offered by a Web service.

WSDL file contains info abouto How the service can be calledo What parameter it expectso What data structure it returnso Which port the application useso Which protocol the web service uses (like https)

Need for stubs (similar to RMI)

What are stubs? A stub is a small program routine that

substitutes for a longer program, possibly to be loaded later or that is located remotely.

The stub accepts the request and then forwards it (through another program) to the remote procedure.

When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.

ComparisionREST SERVICES SOAP SERVICES Architectural style Simply calls services

via URL path Lightweight – not a

lot of extra xml markup

Easy to build – no toolkits required

XML-based protocol Invokes services by

calling RPC method Rigid – type

checking, adheres to a contract

Development tools – WSDL Soap analyser tool, oXygen XML

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