WEB SERVICES Introduction

Grid Computing, B. Wilkinson, 2004 3a.1

WEB SERVICES

Introduction


Web Services

• Software components designed to provide specific operations (“services”) accessible using standard Internet technology.

• For machine interaction over a network.

• Usually through SOAP (simple Object Access Protocol) messages carrying XML documents, and a HTTP transport protocol.


Basic client-server model


• Client needs to:– Identify location of the required service– Know how to communicate with the service

to get it to provide the actions required.

• Uses service registry - a third party.


Service-Oriented ArchitectureSteps:

• Services “published” in a Service registry.

• Service requestor asks Service Registry to locate service.

• Service requestor “binds” with service provider to invoke service.


2. Find

3. Bind

1. Publish

Service-Oriented Architecture

Service requester

Service registry

Service provider


Key aspects

Has similarities with RMI and other distributed object technologies (CORBA etc.) but::

• Web Services are platform independent– They use XML within a SOAP message).– Most use HTTP to transmit message.


XML-based Web Services

• XML provides a flexible basis for storing and retrieving service information on web services.

• Web services use data-centric XML documents to communicate information.


Web Services “Stack”

• HTTP transport

• SOAP message carrying XML documents

• WSDL (Web Services Description Language used to describe message syntax for invoking a service and its response.

• UDDI (Universal Description, Discovery and Integration) used as web service discovery mechanism.


Web Services “Stack”

+ XML


Web Services

From http://www.globus.org

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23

45

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Simple Object Access Protocol (SOAP)

A communication protocol for passing XML documents. Provides mechanisms for:

– Defining communication unit - a SOAP message– Error handling– Extensions– Data representation– Remote Procedure Calls (RPC’s)– Document-centric approach for business transactions– Binding to HTTP


SOAP Envelope<SOAP-ENV:Envelope

xmlns=“http://schemas.xmlsoap.org/soap/envelope/”>

<SOAP-ENV:header>.

.

.

</SOAP-ENV:Header>.

.

.

<SOAP-ENV:Body>.

.

.

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

namespace,see later


What goes down the Wire

HTTP packet containing:– Stuff about context, transactions, routing,

reliability, security– SOAP message– Attachments

XML/SOAP standardization body, World Wide Web Consortium (W3C) covers SOAP and attachments.


Structure of an XML document

• Optional Prolog

• Root Element


Prolog

• Includes processing instruction (<? … ?>) to specify how to process document..

• Includes meta-information about document, and comments.

• One PI identifies document as a XML document, e.g.

<?xml version=“1.0” encoding=“UTF-8”?>

• Comments, same form as HTML:


Root element

• Root element contains contents of document.

• Other elements are within root element and can be nested.


XML Tags

• Not predefined as in HTML.

• Must define your own tags using names as names in a programming languages

• As in programming languages, restrictions. Case sensitive. Start with a letter.

• “Elements” have start and end tags.

• Start tags can have attributes as in HTML.


Namespace Mechanism

• If XML documents combined, can be problem if different documents use the same tag names to mean different things.

• With namespace mechanism, tags given additional namespace identifier to qualify it.


Qualifying names

• Qualified name given by namespace identifier and name used in document:

Qualified name = namespace identifier + local name


Namespace identifier

• Uses URI’s (Uniform Resource Identifiers) - web naming mechanism.

• URLs are a subset of URI, and would typically be used, e.g.:

http://www.cs.wcu.edu/~abw/ns


URIs also include email addresses, i.e.

mailto:[email protected]

and

Uniform Resource Names (URNs) which are globally unique and persistent. UDDI uses URNs.


Associating namespace identifier with local name

• Names in document given a prefix, i.e.:<mypo:street>

• Namespace identifier associated with prefix in root element::

xmlns:po=“http://www.cs.wcu.edu/~abw/ns”


Namespace Example

<mypo:po xmlns:mypo=“http://www.cs.wcu.edu/~abw/ns”>

<mypo:description>

Computer, Pentium IV, 2.8 Ghz, 4 Gbytes main memory

</mypo:description>

</mypo:po>

prefix


Can apply namespace to every tag without a prefix automatically if that is required:

<mypo:po xmlns=“http://www.cs.wcu.edu/~abw/ns”>

<description>

Computer, Pentium IV, 2.8 Ghz, 4 Gbytes main memory

</description>

</mypo:po>


Defining Legal XML Tags

• Legal tags in a document defined optionally using either:

– Document type definitions (DTD) within document. <!DOCTYPE …. > (old, not allowed with SOAP).

or– XML schema


XML Schema

• Flexible way of handing legal element names. Expressed in XML.

• Schema is an XML document with required definitions.

• Handles namespaces.

• Has notation of data types


XML schema

Document

xsi:schemaLocation=“ .. “


XML Schema StructureExample

<?xml version=“1.0” encoding=“UTF-8”?>

<xsd:schema xmlns=“http://www.skatestown.com/ns/po”

xmlns:xsd=“http://www.w3.org/2001/XMLSchema”

targetNamespace=http://www.skatestown.com/ns/po”>

<xsd:annotations>

<xsd:documentation xml:lang=“en”>

Purchase order schema for SkatesTown.

</xsd:documentation>

</xsd:annotation>.

.

.

</xsd:schema>From: “Building Web Services with Java, making sense of XML, SOAP, WSDL, and UDDI, 2nd ed”

by S. Graham et al, SAMS publishing, 2004, p 54.


Associating schema to documentExample

<?xml version=“1.0” encoding=UTF-8”?>

<po:po xmlns:po=“http://www.skatestown.com/ns/po”

xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance”

xsi:schemaLocation=“http://www.skatestown.com/ns/po

http://www.skatestown.com/schema/po.xsd”

id=“43871” submitted=“2001-10-05”>.

.

.

</po:po>

From: “Building Web Services with Java, making sense of XML, SOAP, WSDL, and UDDI, 2nd ed” by S. Graham et al, SAMS publishing, 2004, p 54.


Additional XML materialsOn-line materials

• W3C consortium home page:

http://www.w3.org/XML/

• W3Schools XML Tutorial :

http://www.w3schools.com/xml/


Books

Several books on XML, e.g.:

“Building Web Services with Java: Making sense of XML, SOAP, WSDL, and UDDI, 2nd edition”

by S. Graham et al, SAMS publishing, 2004

Very good but 792 pages!!


Additional SOAP materials

See:

http://www.w3c.org/TR/soap


Hosting Environments for Web Services

• Microsoft .NET

• IBM Websphere

• Apache Axis - we will be using this for assignment 1


More information on Axis

http://xml.apache.org/axis

WEB SERVICES Introduction

Documents

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