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In software engineering, a Service-Oriented Architecture (SOA) is a set of principles and methodologies for designing and developing software in the form of interoperable services. These services are well-defined business functionalities that are built as software components (discrete pieces of code and/or data structures) that can be reused for different purposes. SOA design principles are used during the phases of systems development and integration.
SOA also generally provides a way for consumers of services, such as web-based applications, to be aware of available SOA-based services. For example, several disparate departments within a company may develop and deploy SOA services in different implementation languages; their respective clients will benefit from a well-understood, well-defined interface to access them. XML is often used for interfacing with SOA services, though this is not required. JSON is also becoming increasingly common.
SOA defines how to integrate widely disparate applications for a Web-based environment and uses multiple implementation platforms. Rather than defining an API, SOA defines the interface in terms of protocols and functionality. An endpoint is the entry point for such a SOA implementation.
Service-orientation requires loose coupling of services with operating systems, and other technologies that underlie applications. SOA separates functions into distinct units, or services,[1] which developers make accessible over a network in order to allow users to combine and reuse them in the production of applications. These services and their corresponding consumers communicate with each other by passing data in a well-defined, shared format, or by coordinating an activity between two or more services.[2]
SOA can be seen in a continuum, from older concepts of distributed computing[1][3] and modular programming, through SOA, and on to current practices of mashups, SaaS, and cloud computing (which some see as the offspring of SOA)[4].
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The SOA implementations rely on a mesh of software services. Services comprise unassociated, loosely coupled units of functionality that have no calls to each other embedded in them. Each service implements one action, such as filling out an online application for an account, or viewing an online bank statement, or placing an online booking or airline ticket order. Rather than services embedding calls to each other in their source code, they use defined protocols that describe how services pass and parse messages using description metadata.
SOA developers associate individual SOA objects by using orchestration. In the process of orchestration the developer associates software functionality (the services) in a non-hierarchical arrangement using a software tool that contains a complete list of all available services, their characteristics, and the means to build an application utilizing these sources.
Underlying and enabling all of this requires metadata in sufficient detail to describe not only the characteristics of these services, but also the data that drives them. Programmers have made extensive use of XML in SOA to structure data that they wrap in a nearly exhaustive description-container. Analogously, the Web Services Description Language (WSDL) typically describes the services themselves, while the SOAP protocol describes the communications protocols. Whether these description languages are the best possible for the job, and whether they will become/remain the favorites in the future, remain open questions. As of 2008[update] SOA depends on data and services that are described by metadata that should meet the following two criteria:
SOA aims to allow users to string together fairly large chunks of functionality to form ad hoc applications that are built almost entirely from existing software services. The larger the chunks, the fewer the interface points required to implement any given set of functionality; however, very large chunks of functionality may not prove sufficiently granular for easy reuse. Each interface brings with it some amount of processing overhead, so there is a performance consideration in choosing the granularity of services. The great promise of SOA suggests that the marginal cost of creating the n-th application is low, as all of the software required already exists to satisfy the requirements of other applications. Ideally, one requires only orchestration to produce a new application.
For this to operate, no interactions must exist between the chunks specified or within the chunks themselves. Instead, humans specify the interaction of services (all of them unassociated peers) in a relatively ad hoc way with the intent driven by newly emergent requirements. Thus the need for services as much larger units of functionality than traditional functions or classes, lest the sheer complexity of thousands of such granular objects overwhelm the application designer. Programmers develop the services themselves using traditional languages like Java, C, C++, C#, Visual Basic, COBOL, or PHP.
SOA services feature loose coupling, in contrast to the functions that a linker binds together to form an executable, to a dynamically linked library or to an assembly. SOA services also run in "safe" wrappers (such as Java or .NET) and in other programming languages that manage memory allocation and reclamation, allow ad hoc and late binding, and provide some degree of indeterminate data typing.
As of 2008[update], increasing numbers of third-party software companies offer software services for a fee. In the future, SOA systems may consist of such third-party services combined with others created in-house. This has the potential to spread costs over many customers and customer uses, and promotes standardization both in and across industries. In particular, the travel industry now has a well-defined and documented set of both services and data, sufficient to allow any reasonably competent software engineer to create travel-agency software using entirely off-the-shelf software services.[5] Other industries, such as the finance industry, have also started making significant progress in this direction.
SOA as an architecture relies on service-orientation as its fundamental design principle.[6] If a service presents a simple interface that abstracts away its underlying complexity, users can access independent services without knowledge of the service's platform implementation.[7]
In order to efficiently use a SOA, the architecture must meet the following requirements:
The following guiding principles define the ground rules for development, maintenance, and usage of the SOA[8]:
The first published research of service orientation from an industry perspective was provided by Thomas Erl of SOA Systems Inc. who defined eight specific service-orientation principles common to all primary SOA platforms. These principles were published in “Service-Oriented Architecture: Concepts, Technology, and Design”, on the www.soaprinciples.com research site, and in the September 2005 edition of the Web Services Journal (see Service-orientation).
Some authors also include the following principles:
The following references provide additional considerations for defining a SOA implementation:
In addition, one might take the following factors into account when defining a SOA implementation:
Web services can implement a service-oriented architecture. Web services make functional building-blocks accessible over standard Internet protocols independent of platforms and programming languages. These services can represent either new applications or just wrappers around existing legacy systems to make them network-enabled.
Each SOA building block can play one or both of two roles:
Implementors commonly build SOAs using web services standards (for example, SOAP) that have gained broad industry acceptance after recommendation of Version 1.2 from the W3C[11] (World Wide Web Consortium) in 2003. These standards (also referred to as Web service specifications) also provide greater interoperability and some protection from lock-in to proprietary vendor software. One can, however, implement SOA using any service-based technology, such as Jini, CORBA or REST.
Architectures can operate independently of specific technologies.[3] Designers can implement SOA using a wide range of technologies, including:
Implementations can use one or more of these protocols and, for example, might 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 that can be called to perform their tasks in a standard way, without a service having foreknowledge of the calling application, and without the application having or needing knowledge of how the service actually performs its tasks.
Many implementers of SOA have begun to adopt an evolution of SOA concepts into a more advanced architecture called SOA 2.0.
SOA enables the development of applications that are built by combining loosely coupled and interoperable services.[13]
These services inter-operate based on a formal definition (or contract, e.g., WSDL) that is independent of the underlying platform and programming language. The interface definition hides the implementation of the language-specific service. SOA-based systems can therefore function independently of development technologies and platforms (such as Java, .NET, etc.). Services written in C# running on .NET platforms and services written in Java running on Java EE platforms, for example, can both be consumed by a common composite application (or client). Applications running on either platform can also consume services running on the other as web services that facilitate reuse. Managed environments can also wrap COBOL legacy systems and present them as software services. This has extended the useful life of many core legacy systems indefinitely, no matter what language they originally used.
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-CDL and WS-Coordination extend the service concept by providing a method of defining and supporting orchestration of fine-grained services into more coarse-grained business services, which architects can in turn incorporate into workflows and business processes implemented in composite applications or portals.
As of 2008[update] researchers have started investigating the use of service component architecture (SCA) to implement SOA.
Service-oriented modeling [1] is a SOA framework that identifies the various disciplines that guide SOA practitioners to conceptualize, analyze, design, and architect their service-oriented assets. The Service-oriented modeling framework (SOMF) offers a modeling language and a work structure or "map" depicting the various components that contribute to a successful service-oriented modeling approach. It illustrates the major elements that identify the “what to do” aspects of a service development scheme. The model enables practitioners to craft a project plan and to identify the milestones of a service-oriented initiative. SOMF also provides a common modeling notation to address alignment between business and IT organizations.
SOMF addresses the following principles:
Commentators have provided multiple definitions of SOA.
The OASIS group[14] and the Open Group[15] have both created formal definitions. OASIS 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.
According to Thomas Erl:
SOA represents an open, agile, extensible, federated, composable architecture comprised of autonomous, QoS-capable, vendor diverse, interoperable, discoverable, and potentially reusable services, implemented as Web services. SOA can establish an abstraction of business logic and technology, resulting in a loose coupling between these domains. SOA is an evolution of past platforms, preserving successful characteristics of traditional architectures, and bringing with it distinct principles that foster service-orientation in support of a service-oriented enterprise. SOA is ideally standardized throughout an enterprise, but achieving this state requires a planned transition and the support of a still evolving technology set.[3]
A service contract needs[16] to have the following components:
As of 2008[update] the principles of SOA are being applied by network managers in their field. Examples of service-oriented network management architectures include TS 188 001 NGN Management OSS Architecture from ETSI and M.3060 Principles for the Management Of Next Generation Networks recommendation from the ITU-T.
Tools for managing SOA infrastructure include:
Some enterprise architects believe that SOA can help businesses respond more quickly and more cost-effectively to changing market conditions.[17] This style of architecture promotes reuse at the macro (service) level rather than micro (classes) level. It can also simplify interconnection to – and usage of – existing IT (legacy) assets.
In some respects, one can regard SOA as an architectural evolution rather than as a revolution. It captures many of the best practices of previous software architectures. In communications systems, for example, little development of solutions that use truly static bindings to talk to other equipment in the network has taken place. By formally embracing a SOA approach, such systems can position themselves to stress the importance of well-defined, highly inter-operable interfaces.[18]
Some have questioned whether SOA simply revives concepts like 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 maximize reuse of services.
SOA realizes its business and IT benefits by utilizing an analysis and design methodology when creating services. This methodology ensures that services remain consistent with the architectural vision and roadmap and that they adhere to principles of service-orientation. Arguments supporting the business and management aspects from SOA are outlined in various publications.[19]
A service comprises a stand-alone unit of functionality available only via a formally defined interface. Services can be some kind of "nano-enterprises" that are easy to produce and improve. Also services can be "mega-corporations" constructed as the coordinated work of subordinate services.
Services generally adhere to the following principles of service-orientation:[20]
A mature rollout of SOA effectively defines the API of an organization.
Reasons for treating the implementation of services as separate projects from larger projects include:
An indirect benefit of SOA involves dramatically simplified testing. Services are autonomous, stateless, with fully documented interfaces, and separate from the cross-cutting concerns of the implementation.
If an organization possesses appropriately defined test data, then a corresponding stub is built that reacts to the test data when a service is being built. A full set of regression tests, scripts, data, and responses is also captured for the service. The service can be tested as a 'black box' using existing stubs corresponding to the services it calls. Test environments can be constructed where the primitive and out-of-scope services are stubs, while the remainder of the mesh is test deployments of full services. As each interface is fully documented with its own full set of regression test documentation, it becomes simple to identify problems in test services. Testing evolves to merely validate that the test service operates according to its documentation, and finds gaps in documentation and test cases of all services within the environment. Managing the data state of idempotent services is the only complexity.
Examples may prove useful to aid in documenting a service to the level where it becomes useful. The documentation of some APIs within the Java Community Process provide good examples. As these are exhaustive, staff would typically use only important subsets. The 'ossjsa.pdf' file within JSR-89 exemplifies such a file.[22]
One obvious and common challenge faced involves managing services metadata. SOA-based environments can include many services that 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 can become complex. This becomes even more complicated when these services are delivered by different organizations within the company or even different companies (partners, suppliers, etc.). This creates huge trust issues across teams; hence SOA Governance comes into the picture.
Another challenge involves the lack of testing in SOA space. There are no sophisticated tools that provide testability of all headless services (including message and database services along with web services) in a typical architecture. Lack of horizontal trust requires that both producers and consumers test services on a continuous basis. SOA's main goal is to deliver agility to businesses. Therefore it is important to invest in a testing framework (build it or buy it) that would provide the visibility required to find the culprit in the architecture. Business agility requires SOA services to be controlled by the business goals and directives as defined in the business Motivation Model (BMM).[23]
Another challenge relates to providing appropriate levels of security. Security models built into an application may no longer suffice when an application exposes its capabilities as services that can be used by other applications. That is, application-managed security is not the right model for securing services. A number of new technologies and standards have started to emerge and provide more appropriate models for security in SOA.
Finally, the impact of changing a service that touches multiple business domains will require a higher level of change management governance [24]
As SOA and the WS-* specifications practitioners expand, update and refine their output, they encounter a shortage of skilled people to work on SOA-based systems, including the integration of services and construction of services infrastructure.
Interoperability becomes an important aspect of SOA implementations. The WS-I organization has developed basic profile (BP) and basic security profile (BSP) to enforce compatibility.[25] WS-I has designed testing tools to help assess whether web services conform to WS-I profile guidelines. Additionally, another charter has been established to work on the Reliable Secure Profile.
Significant vendor hype surrounds SOA, which can create exaggerated expectations. Product stacks continue to evolve as early adopters test the development and runtime products with real-world problems. SOA does not guarantee reduced IT costs, improved systems agility or shorter time to market. Successful SOA implementations may realize some or all of these benefits depending on the quality and relevance of the system architecture and design.[26][27]
Internal IT delivery organizations routinely initiate SOA efforts, and some do a poor job of introducing SOA concepts to a business with the result that SOA remains misunderstood within that business. The adoption of SOA starts to meet IT delivery needs instead of those of the business, resulting in an organization with, for example, superlative laptop provisioning services, instead of one that can quickly respond to market opportunities. Business leadership also frequently becomes convinced that the organization is executing well on SOA.
One of the most important benefits of SOA is its ease of reuse. Therefore accountability and funding models must ultimately evolve within the organization. A business unit needs to be encouraged to create services that other units will use. Conversely, units must be encouraged to reuse services. This requires a few new governance components:
Some criticisms[29] of SOA depend on conflating SOA with Web services. For example, some critics claim SOA results in the addition of XML layers, introducing XML parsing and composition. In the absence of native or binary forms of remote procedure call (RPC), applications could run slower and require more processing power, increasing costs. Most implementations do incur these overheads, but SOA can be implemented using technologies (for example, Java Business Integration (JBI) and data distribution service (DDS)) that do not depend on remote procedure calls or translation through XML. At the same time, emerging open-source XML parsing technologies (such as VTD-XML) and various XML-compatible binary formats promise to significantly improve SOA performance.[30][31][32]
Stateful services require both the consumer and the provider to share the same consumer-specific context, which is either included in or referenced by messages exchanged between the provider and the consumer. This constraint has the drawback that it could reduce the overall scalability of the service provider if the service-provider needs to retain the shared context for each consumer. It also increases the coupling between a service provider and a consumer and makes switching service providers more difficult.[33] Ultimately, some critics feel that SOA services are still too constrained by applications they represent.[34]
Another concern relates to the ongoing evolution of WS-* standards and products (e. g., transaction, security), and SOA can thus introduce new risks unless properly managed and estimated with additional budget and contingency for additional proof-of-concept work.
Some critics regard SOA as merely an obvious evolution of currently[update] well-deployed architectures (open interfaces, etc.).
IT system designs sometimes overlook the desirability of modifying systems readily. Many systems, including SOA-based systems, hard-code the operations, goods and services of the organization, thus restricting their online service and business agility in the global marketplace.
The next step in the design process covers the definition of a service delivery platform (SDP) and its implementation. In the SDP design phase one defines the business information models, identity management, products, content, devices, and the end-user service characteristics, as well as how agile the system is so that it can deal with the evolution of the business and its customers.
In October 2009, at the 2nd International SOA Symposium, a mixed group of 17 independent SOA practitioners and vendors, the "SOA Manifesto Working Group", announced the publication of the SOA Manifesto.[35] The SOA Manifesto is a set of objectives and guiding principles that aim to provide a clear understanding and vision of SOA and service-orientation. Its purpose is rescuing the SOA concept from an excessive use of the term by the vendor community and "a seemingly endless proliferation of misinformation and confusion". [2]
The manifesto provides a broad definition of SOA, the values it represents for the signatories and some guiding principles. The manifesto prioritizes:
As of September 2010, the SOA Manifesto had been signed by more than 700 signatories and had been translated to nine languages.
The roadmap to SOA involves:
The various entities would be in one of the SOA readiness states.
appropriately.
Web 2.0, a perceived "second generation" of web activity, primarily features the ability of visitors to contribute information for collaboration and sharing. Web 2.0 applications often use REST-ful web services and commonly feature AJAX based user interfaces, utilizing web syndication, blogs, and wikis. While there are no set standards for Web 2.0, it is characterized by building on the existing Web server architecture and using services. Web 2.0 can therefore be regarded as displaying some SOA characteristics.[36][37][38]
Some commentators also regard mashups as Web 2.0 applications. The term "Business Mashups" has been coined to describe web applications that combine content from more than one source into an integrated user experience that shares many of the characteristics of service-oriented business applications (SOBAs). SOBAs 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 realization of SOA composite and business applications.[39]
Tim O'Reilly coined the term "Web 2.0" to describe a perceived, quickly-growing set of web-based applications.[40] A topic that has experienced extensive coverage involves the relationship between Web 2.0 and Service-Oriented Architectures (SOAs). SOA is considered as the philosophy of encapsulating application logic in services with a uniformly defined interface and making these publicly available via discovery mechanisms. The notion of complexity-hiding and reuse, but also the concept of loosely coupling services has inspired researchers to elaborate on similarities between the two philosophies, SOA and Web 2.0, and their respective applications. Some argue Web 2.0 and SOA have significantly different elements and thus can not be regarded “parallel philosophies”, whereas others consider the two concepts as complementary and regard Web 2.0 as the global SOA.[37]
The philosophies of Web 2.0 and SOA serve different user needs and thus expose differences with respect to the design and also the technologies used in real-world applications. However, as of 2008[update], use-cases demonstrated the potential of combining technologies and principles of both Web 2.0 and SOA.[37]
In an "Internet of Services", all people, machines, and goods will have access via the network infrastructure of tomorrow. The Internet will thus offer services for all areas of life and business, such as virtual insurance, online banking and music, and so on. Those services will require a complex services infrastructure including service-delivery platforms bringing together demand and supply. Building blocks for the Internet of Services include SOA, Web 2.0 and semantics on the technology side; as well as novel business models, and approaches to systematic and community-based innovation.[41]
Even though Oracle indicates that Gartner is coining a new term, Gartner analysts indicate that they call this advanced SOA and refer to it as "SOA 2.0".[42] Most of the major middleware vendors (e. g., Red Hat, webMethods, TIBCO Software, IBM, Sun Microsystems, and Oracle) have had some form of SOA 2.0 attributes for years.
SOA implementations have been described as representing a piece of the larger vision known as the digital nervous system [43][44] or the Zero Latency Enterprise.[45]
A methodology to developing SOA employing Lean Software Development principles.[46]
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