Web Services

Need for web services

JEE provides a component based approach to development of n-tier applications

Limitations Primarily designed for Java

Does not directly address inter-language integration

Not web centric distribution Uses Java specific mechanisms for inter-component

integration

Not supported by all major vendors (i.e. Microsoft)

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Web Services Two approaches

‘SOAP’ Web Services: The original and most often meant by term Web Services In this module, web services = ‘SOAP’ web services

‘RESTful’ Web Services: Increasing popular due to simplicity of approach

REST versus SOAP is a subject of a continuing ‘religious war’ Caution: Need to check terminology being used when referring

to Web Services

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Web Services: Definitions

W3C: “A software system designed to support interoperable machine-to-machine interaction over a network.” While W3C does refer to both SOAP and REST based web services,

most of their definitions relate to SOAP based web services

SOAP Web Service definitions A Web service is a collection of functions that are packaged as a

single entity and published to the network for use by other programs. Web Service standards form a platform-independent, open

framework for describing services, discovering businesses, and integrating business services.

Web Services in 1 slide

A distributed architecture Does not include component model

Platform neutral Access through platform specific development kits

Based on internet standards XML, http etc

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eXtensible Markup Language

A common data format derived from SGML Language independent, hierarchical and self-describing Uses tag-based structure similar to HTML to define XML

documents<?xml version="1.0" encoding="ISO-8859-1" ?> <bankaccount type=“Current”>  <name>John Smith</name>   <address>

<line 1>1 Grafton Street </line1><line 2>Dublin 2, Ireland </line2></address>

</bankaccount>

Supports arbitrarily complex data formats Schema (defined in xsd files) can be used to define document

formats Fragment defining a BankAccount as a type of financial record

<xsd:element name="purchaseOrder" type="PurchaseOrderType"/>

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eXtensible Markup Language

Independent of IT infrastructure Separates the what from the how

Many industry based schema standards have been developed For example, Financial Services:

ISO15022 (SWIFT, FIX, FpML) XBRL (business reporting)

Any XML based infrastructure can handle multiple and evolving schema standards without excessive cost Due to changes in business relationships Due to regulatory changes (e.g. Basil II) Due to evolution of the schemas themselves

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The Web Services architecture

UDDIRegistry

2. Request contract information

3. Retrieve WSDL definition

4. Exchange SOAP messages

WSDL

1. Register contract information

ServiceConsumer

ServiceProvider

WSDL

Web Services define Service provider: “server” supporting the

interface Service consumer: “client” making

requests Web Services is based on three standards Simple Object Access Protocol (SOAP)

defines the communication mechanism Universal Directory Discovery Interface

(UDDI) defines the registry of interface definitions

Web Service Definition Language (WSDL) defines the actual interfaces

Client code

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Web Services Model

Stub

Server-Sidecode

Consumer-Side Network Provider Sideinvokes

returnresults

connect to remoteservice

invoke

returnresults

returnresults

Stub is auto-generated from the WSDL during developmentThe server side received the call (invocation) through the SOAP runtimelibrary

SOAP runtime library

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SOAP- Simple Object Access Protocol

XML based protocol for exchange of information Encoding rules for datatype

instances Convention for representing

RPC invocations

Used with XML Schema Normally HTTP-based

Uses Request/Response Two different modes

RPC Document

SOAP with Attachments allow arbitrary data to be packaged.

SOAP1.1 MessageStructure

SOAPEnvelope

HeaderEntries

Header

Body

FaultElement

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SOAP Example: Request

Sample SOAP Message embedded in an HTTP Request

POST /StockQuote HTTP/1.1Host: www.stockquoteserver.comContent-Type: text/xml; charset="utf-8"Content-Length: 1024SOAPAction: "http://example.org/2001/06/quotes"

<env:Envelope xmlns:env="http://www.w3.org/2001/06/soap-envelope" ><env:Body><m:GetLastTradePrice env:encodingStyle="http://www.w3.org/2001/06/soap-encoding" xmlns:m="http://example.org/2001/06/quotes"><symbol>IBM</symbol></m:GetLastTradePrice></env:Body></env:Envelope>

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SOAP Example: Response

SOAP message sent by the StockQuote service in the corresponding HTTP response to the request

HTTP/1.1 200 OKContent-Type: text/xml; charset="utf-8"Content-Length: 512

<env:Envelope xmlns:env="http://www.w3.org/2001/06/soap-envelope" ><env:Body><m:GetLastTradePriceResponse env:encodingStyle="http://www.w3.org/2001/06/soap-encoding" xmlns:m="http://example.org/2001/06/quotes"><Price>97.5</Price></m:GetLastTradePriceResponse></env:Body></env:Envelope>

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UDDI• Universal Description, Discovery and Integration

• Platform-independent, open framework for describing services, discovering businesses, and integrating business services.

• A DNS-like model implementation of the directory

• Originally intended to support global registries

• Created during the .com bubble.

• Includes White, Green and Yellow Pages for search purposes

• Now primarily inside organisations if at all

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Web Service Description Language (WSDL)

Interface definition language in XML Conceptually, no different from Enterprise Java Bean or

Common Gateway Interface Based on XML Schema Can appear to be very verbose

An interface description is a contract between the server developers and the client developers WSDL describes the web service Similar to Java method signatures

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Interoperability Challenges

Interoperability remains the chief challenge of web services Complex and imprecise standard Tools generating SOAP, WSDL, UDDI, etc. need to be

interoperable

Web Services Interoperability (WS-I) focus on creating to Interoperability WS-I Basic Profile 1.0 defines an interoperable web services

platform

Current version is WS-I v2.0

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Web Services: Another approach to distributed computing

Transport

Messaging

Description

Discovery

Quality Of Service

Builds on the legacy of CORBA, J2EE etc It is not a new component model, programming

model or programming language. UDDI/WSDL/SOAP provide another distributed

computing technology Standards developing for the Quality of Services

functionality E.g. WS-Security, WS-ReliableMessaging etc

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Web Services Strengths versus JEE

Interoperability Based on open standards, utilizes existing infrastructure Can be accessed from any programming language/model

Ubiquity Communicates through XML/HTTP – Any system that supports

these standards, supports Web Services Support from all major software vendors (IBM and Microsoft)

Low barrier to entry Concepts easy to understand, easy to implement Toolkits allow COM, JEE, and CORBA components to be

exposed as Web Services

RESTful Web Services

REpresentational State Transfer Architecture inherent in all web based system since

1994, not explicitly described as an architecture until later

An architecture - not a set of standard Web Services is both an architecture and a set of standards

Goal: To leverage web based standards to allow inter-application communication as simply as possible Matches the ‘standard’ web interaction model

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The standard Web architecture

We

b S

erv

er

HTTP POST URL 3PO

(HTML)

HTTP GET request URL 1

HTTP responseURL to submitted PO

PartsList

PartData

PO

HTTP responseResponse

(HTML doc)

HTTP responseResponse

(HTML doc)

HTTP GET request URL 2

Copyright © [2005]. Roger L. Costello, Timothy D. Kehoe.

The RESTful architecture

We

b S

erv

er

HTTP POST URL 3PO

(XML)

HTTP GET request URL 1

HTTP responseURL to submitted PO

PartsList

PartData

PO

HTTP responseResponse(XML doc)

HTTP responseResponse(XML doc)

HTTP GET request URL 2

Copyright © [2005]. Roger L. Costello, Timothy D. Kehoe.

REST architecture

Uses HTTP operations: GET = "give me some info" (Retrieve) POST = "here's some update info" (Update) PUT = "here's some new info" (Create) DELETE = "delete some info" (Delete)

Typically exchanges XML documents But supports a wide range of other internet media types

Example of client side REST request: GET /shoppingcart/5873 Server must be able to correctly interpret the client request as there

is no explicitly defined equivalent to an interface definition21

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REST Architecture

Servers are stateless and messages can be interpreted without examining history Messages are self-contained

There is no such thing as a “service”. There are just resources which are accessed through URI

URI = generalisation of URL

Clients navigate through a series of steps towards a goal by following hypertext links (GET) and submitting representations (POST).

REST – strong versus weak

Pure REST should use ‘pure’ URI only E.g. GET /shoppingcart/5873

Many REST implementations also allow parameter passing E.g. GET /shoppingcart/5873?sessionID=123

Allowing parameter passing makes REST a lot more usable but blurs the architectural principle of statelessness

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REST versus web services

REST Strengths No toolkit or infrastructure Independent of programming

language, platform or component model

Easy to learn Easy to get simple systems

working quickly

REST weaknesses No standardisation on use of

URI No standardisation response

document format Yahoo uses XML, Google uses

JSON (JavaScript) Less structured approach

makes it harder to build complex interactions

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