introduction to web services protocols 1

7/30/2019 Introduction to Web Services Protocols 1

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Introduction to Web Services

Protocols


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Communication and standards

Efficient (or indeed any)

communication is dependent on ashared vocabulary and grammar.

Because web services deals withinter-organisation communicationthese must be universal standards.


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Underlying standards

The basic standards for web services are:

XML (Extensible Markup Language)

SOAP (simple object access protocol)

WSDL (web services description language)

UDDI (universal description, discovery andintegration)


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The state of standards

XML 1.0 fairly stable, although Schema are in the processof replacing DTDs (currently Schema 1.1 being worked on).

SOAP 1.2

WSDL 2.0 (coming out, 1.2 current)

UDDI version 3 (Aug 2003)

BPEL 1.1 (Business Process Execution Language)choreography description language (web services workflows)

started January 2003.

Standards are still volatile and in the process of development.


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Web Services Architecture

Web Services involve three majorroles

Service Provider

Service Registry

Service Consumer

Three major operations surround webservices

Publishing making a service available

Finding locating web services

Binding using web services


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Making a service available (1)

In order for someone to use your servicethey have to know about it.

To allow users to discover a service it is

published to a registry (UDDI).

To allow users to interact with a service youmust publish a description of its interface

(methods & arguments).

This is done using WSDL.


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Making a service available (2)

Once you have published a description of yourservice you must have a host set up to serve it.

A web server is often used to deliver services(although custom application application

communication is also possible).

This is functionality which has to be added to theweb server. In the case of the apache web server acontainer application (Tomcat) can be used to make

the application (servlet) available to apache(deploying).


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The old transfer protocols arestill there.

Like the grid architecture webservices is layered on top of existing,mature transfer protocols.

HTTP, SMTP are still used over TCP/IPto pass the messages.

Web services, like grids, can be seenas a functionality enhancement to theexisting technologies.


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XML

All Web Services documents arewritten in XML

XML Schema are used to define theelements used in Web Servicescommunication


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SOAP

Actually used to communicate with the WebService

Both the request and the response are SOAP

messages

The body of the message (whose grammaris defined by the WSDL) is contained within

a SOAP envelope

Binds the client to the web service


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WSDL

Describes the Web Service anddefines the functions that areexposed in the Web Service

Defines the XML grammar to be usedin the messages

Uses the W3C Schema language


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UDDI

UDDI is used to register and look upservices with a central registry

Service Providers can publish information

about their business and the services thatthey offer

Service consumers can look up services that

are available by Business

Service category

Specific service


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Section

XML


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What is XML

XML stands for extensible markup language

It is a hierarchical data description language

It is a sub set of SGML a general documentmarkup language designed for the Americanmillitary.

It is defined by w3c.


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How does XML differ fromHTML?

HTML is a presentation markup language provides no information about content.

There is only one standard definition of all of the

tags used in HTML.

XML can define both presentation style and giveinformation about content.

XML relies on custom documents defining themeaning of tags.


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What is a Schema?

A schema is the definition of the meaning ofeach of the tags within a XML document.

Analogy: A HTML style sheet can be seen as

a limited schema which only specifies thepresentational style of HTML which refers toit.

Example: in HTML the tag pre-defined. In XML you would need to definethis in the context of your document.


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A minimal XML document

Jim

A tag

An attribute

value

Closing tag


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Valid and well formedA correct XML document must be both valid

and well formed.

Well formed means that the syntax must becorrect and all tags must close correctly (eg

).

Valid means that the document mustconform to some XML definition ( a DTD or

Schema).

(Otherwise there can be no definition of whatthe tags mean)


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Using namespaces in XML

To fully qualify a namespace in XML write thenamespace:tag name. eg.

In a globally declared single namespace the qualifier

may be omitted.

More than one namespace:

can co-exist if correctly qualified.


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Namespaces in programminglanguages

In C/C++ defined by #includes and classes(eg. myclass::variable).

In PERL defined by package namespace,

$local and $my (eg. myPackage::variable).

In JAVA defined by includes and packagenamespace (eg. java.lang.Object)

Defines the scope of variables


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Schema

sam smith

sam smith

Simple schema

saved as order.xsd

XML document

derived from

schema.


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Section

SOAP


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Request Response WebServices

Currently the most common implementationof Web Services

Work in a very simple request response

paradigm

For Example: A Weather Service simple request for weather in an area, simple

response with the weather report

An Airline special offers service travel agents would simply makerequests for latest offers and would receive the offers as aresponse


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SOAP messagesSOAP provides a standard envelope within

which a message can be delivered.

SOAP is mechanism (protocol) fortransferring information (messages)

between applications which may be widelydistributed.

SOAP says nothing about the content of the

message the sender and the receiver mustunderstand the message for themselves.

SOAP is part of a communication stack.


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SOAP Structure(1)

Each SOAP messagewill have:

An Envelope

A Header (optional)

A Body

The Body may containa Fault element

SOAP BODY

SOAP ENVELOPE

FAULT

SOAP HEADER

Transport protocol

MIME header


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SOAP Structure(2)

The envelope wraps the entire soap document

The header contains allows additional information tobe passed as well as the body of the document e.g.authentication

The body element contains the core of the SOAPdocument this will contain either the RPC call orthe XML message itself

The fault information will contain any exceptioninformation


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Anatomy of a SOAP message


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SOAP protocol binding

SOAPAction = urn:soaphttpclient-action-uri

Host = localhost

Content-Type = text/xml; charset=utf-8

Content-Length = 701


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SOAP RPC

SOAP RPC messages contain XML thatrepresents a method call or methodresponse

The SOAP XML will be converted intoa method call on the server and theresponse will be encoded into SOAP

XML to be returned to the client


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SOAP Faults

SOAP errors are handled using aspecialised envelope known as a FaultEnvelope

A SOAP Fault is a special elementwhich must appear as an immediatechild of the body element

and arerequired.


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A SOAP fault

SOAP-ENV:Server

Test fault/soap/servlet/rpcrouter

..


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SOAP Attachment

Large quantities orbinary data may not fitwell into a XML SOAPmessage.

In which case it can besent out of band byattaching it to a SOAP

message

Analogy : emailattachments.

SOAP BODY

SOAP ENVELOPE

FAULT

SOAP HEADER

Transport protocol

MIME header

Attachment


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Attaching a file to a SOAPmessage

To add a file to a SOAP message a tagis added within the body of themessage.

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