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Service-oriented architecture

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In computing, the term service-oriented architecture (SOA ) expresses a perspective of software architecture that defines the use of loosely coupled software services to support the requirements of the business processes and software users. In an SOA environment, resources on a network are made available as independent services that can be accessed without knowledge of their underlying platform implementation

A service-oriented architecture is not tied to a specific technology and may be implemented using a wide range of interoperability standards including RPC, DCOM, ORB or WSDL.

In fact, SOA can be implemented without any of these very useful protocols, and could just as appropriately, for example, use a file system mechanism to communicate data conforming to a defined interface specification between processes conforming to the SOA concept. The key is independent services with defined interfaces which can be called to perform their tasks in a standard way, without the service having pre-knowledge of the calling application, and without the application having or needing knowledge of how the service actually performs its tasks.

SOA can also be regarded as a style of information systems architecture that enables the creation of applications that are built by combining loosely coupled and interoperable services. These services inter-operate based on a formal definition (or contract, e.g., WSDL) which is independent of the underlying platform and programming language. The interface definition hides the implementation of the language-specific service. SOA-compliant systems can therefore be independent of development technologies and platforms (such as Java, .NET etc). For example, services written in C# running on .Net platforms and services written in Java running on Java EE platforms can both be consumed by a common composite application. In addition, applications running on either platform can consume services running on the other as Web services, which facilitates reuse.

SOA can support integration and consolidation activities within complex enterprise systems, but SOA does not specify or provide a methodology or framework for documenting capabilities or services.

High-level languages such as BPEL and specifications such as WS-Coordination extend the service concept further by providing a method of defining and supporting orchestration of fine grained services into coarser grained business services, which in turn can be incorporated into workflows and business processes implemented in composite applications or portals.

Elements of SOA, by Dirk Krafzig, Karl Banke, and Dirk Slama. Enterprise SOA. Prentice Hall, 2005

SOA definitions

SOA is a design for linking computational resources (principally, applications and data) on demand to achieve the desired results for service consumers (which can be end users or other services). OASIS (the Organization for the Advancement of Structured Information Standards) defines SOA as the following:

A paradigm for organizing and utilizing distributed capabilities that may be under the control of different ownership domains. It provides a uniform means to offer, discover, interact with and use capabilities to produce desired effects consistent with measurable preconditions and expectations.

There are multiple definitions of SOA but currently only the OASIS group has created a formal definition with depth which can be applied to both the technology and business domains.

Though many definitions of SOA limit themselves to technology or just web services, this is predominantly pushed by technology vendors; in 2003 they talked just of web services, while in 2006 the talk is of events and process engines.

Why SOA?

Enterprise architects believe that SOA can help businesses respond more quickly and cost-effectively to the changing market conditions . This style of architecture promotes reuse at the macro (service) level rather than micro levels (eg. objects). It can also simplify interconnection to and usage of existing IT (legacy) assets.

SOA Practitioners Guide: Why Services-Oriented Architecture? provides a high-level summary on SOA.

In some respects, SOA can be considered an evolution in architecture, not a revolution. It captures many of best practices or actual use of the architectures that came before it. In communications systems, for example, there has been little development in recent years of solutions that use truly static bindings to talk to other equipment in the network, but by formally embracing a SOA approach, solutions are better positioned to stress the importance of well-defined, highly interoperable interfaces.

Some have questioned whether SOA is just a revival of modular programming (1970s), event-oriented design (1980s) or interface/component-based design (1990s). SOA promotes the goal of separating users (consumers) from the service implementations. Services can therefore be run on various distributed platforms and be accessed across networks. This can also maximise reuse of services.

SOA principles

The following guiding principles define the ground rules for development, maintenance, and usage of the SOA

  • Reuse, granularity, modularity, composability, componentization, and interoperability
  • Compliance to standards (both common and industry-specific)
  • Services identification and categorization, provisioning and delivery, and monitoring and tracking

The following specific architectural principles for design and service definition focus on specific themes that influence the intrinsic behaviour of a system and the style of its design:

  • Service Encapsulation
  • Service Loose coupling - Services maintain a relationship that minimizes dependencies and only requires that they maintain an awareness of each other
  • Service contract - Services adhere to a communications agreement, as defined collectively by one or more service description documents
  • Service abstraction - Beyond what is described in the service contract, services hide logic from the outside world
  • Service reusability - Logic is divided into services with the intention of promoting reuse
  • Service composability - Collections of services can be coordinated and assembled to form composite services
  • Service autonomy – Services have control over the logic they encapsulate
  • Service statelessness – Services minimize retaining information specific to an activity
  • Service discoverability – Services are designed to be outwardly descriptive so that they can be found and assessed via available discovery mechanisms

In addition, the following factors should also be taken into account when defining an SOA implementation:

  • SOA Reference Architecture SOA Practitioners Guide Part 2: SOA Reference Architecture covers the SOA Reference Architecture, which provides a worked design of an enterprise-wide SOA implementation with detailed architecture diagrams, component descriptions, detailed requirements, design patterns, opinions about standards, patterns on regulation compliance, standards templates etc.
  • Life cycle management SOA Practitioners Guide Part 3: Introduction to Services Lifecycle introduces the Services Lifecycle and provides a detailed process for services management though the service lifecycle, from inception through to retirement or repurposing of the services. It also contains an appendix that includes organization and governance best practices, templates, comments on key SOA standards, and recommended links for more information.
  • Efficient use of system resources
  • Service maturity and performance
  • EAI Enterprise Application Integration

Service-oriented design and development

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The modelling and design methodology for SOA applications has become known by the terms service-oriented analysis and design and SODA.

Service contract

A service contract needs to have the following components:

  • Header
    • Name - Name of the service. Should indicate in general terms what it does, but not be the only definition
    • Version - The version of this service contract
    • Owner - The person/team in charge of the service
    • RACI
      • Responsible - The role is the person/team responsible for the deliverables of this contract/service. All versions of the contract
      • Accountable - Ultimate Decision Maker in terms of this contract/service
      • Consulted - Who must be consulted before action is taken on this contract/service. This is 2-way communication. These people have an impact on the decision and/or the execution of that decision.
      • Informed - Who must be informed that a decision or action is being taken. This is a 1-way communication. These people are impacted by the decision or execution of that decision, but have no control over the action.
    • Type - This is the type of service to help distinguish the layer it resides.
      • Data
      • Process
      • Functionality
      • Presentation
  • Functional
    • Functional Requirement (From Requirements Document) - Indicates the functionality in specific bulleted items what exactly this service accomplishes. The language should be such that it allows test cases to prove the functionality is accomplished.
    • Service Operations - Methods, actions etc. Must be defined in terms of what part of the Functionality it provides.
    • Invocation - Indicates the invocation means of the service. This includes the URL, interface, etc. There may be multiple Invocation paths for the same service. We may have the same functionality for an internal and external clients each with a different invocation means and interface. Examples:
      • SOAP
      • REST
      • Events Triggers
  • Non-Functional
    • Security Constraints - Defines who can execute this service in terms of roles or individual partners, etc. and which invocation mechanism they can invoke.
    • Quality of Service - Determines the allowable failure rate
    • Transactional - Is this capable of acting as part of a larger transaction and if so, how do we control that?
    • Service Level Agreement - Determines the amount of latency the service is allowed to have to perform its actions
    • Semantics - Dictates or defines the meaning of terms used in the description and interfaces of the service
    • Process - Describes the process, if any, of the contracted service

SOA and web service protocols

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SOA may be built on Web services standards (e.g., using SOAP or REST) that have gained broad industry acceptance. These standards (also referred to as web service specifications) also provide greater interoperability and some protection from lock-in to proprietary vendor software. However, one can implement SOA using any service-based technology, such as Jini.

Service-oriented architecture is often defined as services exposed using the Web Services Protocol Stack . The base level of web services standards relevant to SOA includes the following:

  • XML - a markup language for describing data in message payloads in a document format
  • HTTP (or HTTPS) - request/response protocol between clients and servers used to transfer or convey information
  • SOAP - a protocol for exchanging XML-based messages over a computer network, normally using HTTP
  • Web Services Description Language (WSDL) - XML-based service description that describes the public interface, protocol bindings and message formats required to interact with a web service
  • Universal Description, Discovery, and Integration (UDDI) - An XML-based registry to publish service descriptions (WSDL) and allow their discovery

Note, however, that a system does not necessarily need to use any or all of these standards to be "service-oriented." For example, some service oriented systems have been implemented using Corba and Jini.

SOA, Web 2.0, and mashups

Web 2.0 refers to a "second generation" of web sites, primarily distinguished by the ability of visitors to contribute information for collaboration and sharing. Web 2.0 applications use Web services and may include Ajax program interfaces, Web syndication, blogs, and wikis. While there are no set standards for Web 2.0, it is characterised by building on the existing web server architecture and using services. Web 2.0 can therefore be regarded as displaying some SOA characteristics.

Mashups are also regarded by some as Web 2.0 applications. The term "enterprise mashup" has been coined to describe Web applications that combine content from more than one source into an integrated experience, which share many of the characteristics of service-oriented business applications (SOBAs), which are applications composed of services in a declarative manner. There is ongoing debate about "the collision of Web 2.0, mashups, and SOA", with some stating that Web 2.0 applications are a realisation of SOA composite and business applications.

SOA 2.0 or Advanced SOA

Oracle is talking up SOA 2.0 as "the next-generation version of SOA" combining service-oriented architecture and Event Driven Architecture, and categorizing the first iteration of SOA as client-server driven . Even though Oracle indicates that Gartner is coining a new term, Gartner analysts indicate that they call this advanced SOA and it is 'whimsically' referred to as SOA 2.0. Most of the pure-play middleware vendors (e.g., webMethods and TIBCO) have had SOA 2.0 attributes for years. SOA 2.0 can therefore be regarded as marketing and evangelism rather than a new "way of doing things".

However, other industry commentators have criticized attaching a version number to an architecture, while others have stated that the "next generation" should apply to the evolution of SOA techniques from IT optimization to business development .

What are the challenges faced in SOA adoption?

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One obvious and common challenge faced is managing services metadata. SOA-based environments can include many services which exchange messages to perform tasks. Depending on the design, a single application may generate millions of messages. Managing and providing information on how services interact is a complicated task.

Another challenge is providing appropriate levels of security. Applications which consume services, particularly those external to company firewalls, are more visible to external parties than traditional monolithic proprietary applications. The flexibility and reach of SOA can compromise security; the WS-Security suite of specifications is being developed to provide appropriate security .

As SOA and the WS-* specifications are constantly being expanded, updated and refined, there is a shortage of skilled people to work on SOA based systems, including the integration of services and construction of services infrastructure.

There is significant vendor hype concerning SOA that can create expectations that may not be fulfilled. SOA does not guarantee reduced IT costs, improved systems agility or faster time to market. Successful SOA implementations may realise some or all of these benefits depending on the quality and relevance of the system architecture and design . See also: WS-MetadataExchange OWL-S

Criticisms of SOA

One of the criticisms of SOA is that the addition of XML layers introduces XML parsing and composition. In the absence of native or binary forms of Remote Procedure Calls (RPCs) applications may run slower and require more processing power, which increases costs.

SOA and Business Architecture

One area where SOA has been gaining ground is in its power as a mechanism for defining business services and operating models and thus provide a structure for IT to deliver against the actual business requirements and adapt in a similar way to the business. The purpose of using SOA as a business mapping tool is to ensure that the services created properly represent the business view and are not just what technologists think the business services should be. At the heart of SOA planning is the process of defining architectures for the use of information in support of the business, and the plan for implementing those architectures (Enterprise Architecture Planning by Steven Spewak and Steven Hill). Enterprise Business Architecture should always represent the highest and most dominant architecture. Every service should be created with the intent to bring value to the business in some way and must be traceable back to the business architecture.

Within this area, SOMA (Service-Oriented Modelling and Architecture) was announced by IBM as the first SOA-related methodology in 2004. Since then, efforts have been made to move towards greater standardization and the involvement of business objectives, particularly within the OASIS standards group and specifically the SOA Adoption Blueprints group. All of these approaches take a fundamentally structured approach to SOA, focusing on the Services and Architecture elements and leaving implementation to the more technically focused standards.

SOA and network management architecture

The principles of SOA are currently being applied to the field of network management. Examples of Service-Oriented network management architectures are TS 188 001 NGN Management OSS Architecture from ETSI, and the recently published M.3060 Principles for the Management Of Next Generation Networks recommendation from the ITU-T.

Jargon

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SOA is an architectural style rather than a product. Several vendors offer products which can form the basis of, or enable, SOA--particularly Enterprise Service Bus (ESB) products. ESBs provide infrastructure that can be purchased, implemented and leveraged for SOA-based systems. SOA relies heavily on metadata design and management. Metadata design and management products are also critical to implementing SOA architectures. See the list of SOA related products for an overview and ideas.

Literature

  • Erl, Thomas (2005). Service-Oriented Architecture: Concepts, Technology, and Design. Upper Saddle River: Prentice Hall PTR. ISBN 0-13-185858-0.
  • Pulier, Eric (2005). Understanding Enterprise SOA. Greenwich: Manning Publications. ISBN 1-93239-459-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Barry, Douglas K. (2003). Web Services and Service-Oriented Architectures: The Savvy Manager's Guide. San Francisco: Morgan Kaufmann Publishers. ISBN 1-55860-906-7.
  • Bieberstein, Norbert (2006). Service-Oriented Architecture Compass - Business Value, Planning and Enterprise Roadmap. Upper Saddle River: Pearson. ISBN 0-13-187002-5. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Krafzig, Dirk (2004). Enterprise SOA Service Oriented Architecture Best Practices. Upper Saddle River: Prentice Hall PTR. ISBN 0-13-146575-9. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Erl, Thomas (2004). Service-Oriented Architecture: A Field Guide to Integrating XML and Web Services. Upper Saddle River: Prentice Hall PTR. ISBN 0-13-142898-5.
  • Hurwitz, Judith (2006). Service Oriented Architecture for Dummies. Hoboken: Wiley. ISBN 0-470-05435-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

References

  1. Channabasavaiah, Holley and Tuggle, Migrating to a service-oriented architecture, IBM DeveloperWorks, 16 Dec 2003
  2. SOA Reference Model definition
  3. Christopher Koch A New Blueprint For The Enterprise, CIO Magazine, Mar 1 2005
  4. Yvonne Balzer Improve your SOA project plans, IBM, 16 July 2004
  5. Thomas Erl Serviceorientation.org - About the Principles, 2005-2006
  6. Dion Hinchcliffe Is Web 2.0 The Global SOA?, SOA Web Services Journal, 28 October 2005
  7. Jason Bloomberg Mashups and SOBAs: Which is the Tail and Which is the Dog?, Zapthink
  8. Paul Krill Make way for SOA 2.0, InfoWorld , May 17, 2006
  9. Yefim Natis & Roy Schulte Advanced SOA for Advanced Enterprise Projects, Gartner, July 13, 2006
  10. Joe McKendrick Anti-SOA 2.0 petition nears 400, ZDNet.com, June 29, 2006
  11. Is There Real Business Value Behind the Hype of SOA?, Computerworld, June 19, 2006

See also

Footnotes

  • An alternative view, particularly after initial deployments, is that SOAs properly ought not dictate physical implementation, so the formal definition should not include "network." High performance SOAs may not be viable deployed to distributed nodes on a network, and separate nodes for every (or most) services could be prohibitively expensive.

External links

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