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Service Oriented Architecture
Charlie Abela
Department of Artificial Intelligence
[email protected]
Last Lecture
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Web Ontology Language
Problems?
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Lecture Outline
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Service Oriented Architecture
Web Services Architecture
Web Services Standard Technologies
Limitations of Standard Technologies
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Service Oriented Architecture
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Any environment where:
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Services are ubiquitous and organically integrated
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Systems are assembled from a loosely coupled collection of
services, which
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A service is a software building block that is well-defined and selfcontained
Ideally does not depend on the context or state of other services
Have a published interface
Can communicate with each other
Services that utilize Web Services standards (WSDL, SOAP,
UDDI) are the most popular type of services available today
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Elements of SOA
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Loose coupling: focus should be on high-level
contractual relationships
Implementation neutrality: the interface is what should
matter
Flexible configurability: late binding of components
Long lifetime: components should exist long enough to
be discovered, to be relied upon, and to engender trust
in their behaviour
Granularity: interactions and dependencies should
occur at as high level as possible
Teams: computation in open systems should be
conceptualized as business partners working as a team
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Trend
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“By 2008, SOA will be a prevailing software engineering
practice, ending the 40-year domination of monolithic
software architecture (0.7 probability).
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Through 2008, SOA and Web services will be
implemented together in more than 75 percent of new
SOA or Web services projects (0.7 probability).”
David McCoy and Yefim Natis,
Service-Oriented Architecture: Mainstream Straight Ahead
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Benefits of SOA
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… in the long term the values of Web services and SOA will be
evident. Web services deployed in SOA:
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Leverage open standards to represent virtually all software assets as
services including legacy applications, packaged applications, J2EE
components and Web services.
Effect that individual software assets become building blocks that can be
reused in developing other applications.
Provide developers with a standard way of representing and interacting
with software assets without having to spend time working with unique
interfaces and low-level APIs.
Help reducing the complexity, cost, and risk of integration by providing a
single, simple architectural framework in which to build, deploy, and
manage application functionality.
Enable enterprises to easier integrate processes as well as to react on
changing market requirements more quickly.
SOA is useful in many aspects and should be part of the most
forward-looking software projects. Over time, lack of SOA experience
will become a competitive disadvantage for many enterprises.
Siemens - Web Services and Service Oriented Architectures The Impact on
Business Applications
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SOA: Integration layer
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Service-oriented architecture introduces a new logical
layer: the service integration layer
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Establishes a common point of integration within application
tiers and across application boundaries
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SOA: Participant Roles and Interactions
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SOA is based upon the interactions between three roles:
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Provider - the owner of the service
Registry or Broker - manages repositories of information on
providers and their software assets
Requestor - discovers and invokes software assets provided by
one or more providers
There are three fundamentals interactions:
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Publishing - providers publish information (or metadata) about
services to a registry
Finding (service location) - requestors query a public or private
registry for service description
Binding - requestors use the service description to create a
message to be sent to the service provider
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Web Services
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Web services is the integration technology preferred by
organizations implementing SOA
Trigger point – weakness of HTML model for ecommerce solutions
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A new model of distributed computing
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The natural evolution of distributed programming
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Mainly for use by other applications
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Sockets  RPC  Distributed Objects Web Services
To simplify application integration and interoperability
Goal: provide a simple application-to-application interface
Fundamental Issues that had to be addressed
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A common framework for Web service interactions based on
open standards
An agreed set of vocabularies and interactions
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What is a Web Service?
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“A Web service is a software application
identified by a URI, whose interfaces and
binding are capable of being defined, described
and discovered by XML artifacts and supports
direct
interactions
with
other
software
applications using XML based messages via
internet-based protocols” (W3C)
“… a piece of business logic accessible via the
Internet using open standards ..” (Microsoft)
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Component & Web Services: Comparison
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Component-Based Model
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Mainly designed for processes
within the enterprise
Different protocols and
technologies (e.g. EJBs,
DCOM, CORBA)
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Typically, programming
language dependent
Usually bound to a particular
transport
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Web Service Model
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Mainly designed for processes
across enterprises
Uses common protocol and
technologies (e.g. XML,
SOAP, WSDL, …)
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Programming language
independent
Easily bound to different
transport
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Web Services: A New Paradigm?
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What is unique about Web service?
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XML-Based - XML as the data representation layer for all Web
services protocols and technologies
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Loosely-coupled - a consumer of a web service is not tied to that
web service directly
The only contract that have to be agreed upon between
communicating parties is the syntax and semantics of XML
messages.
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Ability to be synchronous or asynchronous
Coarse-grained - a piece of business logic
Will allow:
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No need to agree on object model
No need to agree on programming language,
No need to agree on programming APIs.
On the fly composition of new functionality
Decomposition and distribution of large scale processing tasks
across many devices
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Tight VS Loose Coupling
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Tight coupling
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Very efficient interaction
Very compressed communication
Requires “deep knowledge”
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Problems with changes
Loose coupling
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Less efficient interaction
Not as many subtle details conveyed
Does not require “deep knowledge”
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Why Web Services?
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Interoperable – Connect across heterogeneous
networks using web-based standards
Economical – Recycle components, no installation and
tight integration of software
Automatic – No human intervention required even for
highly complex transactions
Accessible – Legacy assets & internal applications are
exposed and accessible on the web (security issues)
Available – Services on any device, anywhere, anytime
Scalable – No limits on scope of applications and
amount of heterogeneous applications
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Service Composition
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Web services can be interlinked together to provide a
larger set of services
Applications can be developed out of web services
assembled from all over the Internet.
CSA 3210
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Web Services Standards
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Four standards define the critical elements of
Web services:
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Extensible Markup Language (XML + XML Schema)
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Simple Object Access Protocol (SOAP)
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Allows servers to describe services being offered
Universal Description, Discovery, and Integration
(UDDI)
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Describes handshaking with server
Web Service Definition Language (WSDL)
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Describes format of the request and response; data types
Protocol for listing services in a directory
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Relationship between 1st Generation
Standards
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Web Services: Architecture
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Standards: A more detailed view
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Choreography and Orchestration
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Choreography
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Specifies the behavioural interface for service
consumption
A Service User (Client) has to support this for
consuming a Web Service
consists of messages & their order, errors, and
communication
Orchestration
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Specifies how the functionality of a Web Service is
achieved by aggregating other Web Services
Support for Service Provider to specify composed
web services
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Limitations of Standard Technologies
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Standard technologies exhibit little flexibility and
expressiveness and restrict the usability of Web
services to human users rather than software agents.
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UDDI:
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WSDL:
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search mechanism relies on pre-defined categorizations (tModels)
through keywords and does not refer to the semantic content of the
advertisement.
no support for the definition of logical constraints between its input
and output parameters,
has no precondition, postcondition (effect).
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Suggested Reading
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Service Oriented Architecture
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New to SOA
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What is Service Oriented Architecture?
 http://webservices.xml.com/pub/a/ws/2003/09/30/soa.html
Web Services Architecture (W3C)
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http://www-128.ibm.com/developerworks/webservices/newto/
http://www.w3.org/TR/ws-arch/
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Next Lecture
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Business Process Modelling
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BPEL
Semantic Web Services
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CSA 3210
OWL-S Initiative
Service Oriented Architecture ©
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