- Web service
A Web service (also Web Service) is defined by the
W3Cas "a software system designed to support interoperable machine-to-machine interaction over a network" [cite web |url=http://www.w3.org/TR/ws-gloss/ |title=Web Services Glossary] . Web services are frequently just WebAPIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.
W3CWeb service definition encompasses many different systems, but in common usage the term refers to clients and servers that communicate over the HTTPprotocol used on the Web. Such services tend to fall into one of two camps: Big Web Services and RESTful Web Services.
"Big Web Services" use
XMLmessages that follow the SOAPstandard and have been popular with traditional enterprise. In such systems, there is often machine-readable description of the operations offered by the service written in the Web Services Description Language(WSDL). The latter is not a requirement of a SOAP "endpoint", but it is a prerequisite for automated client-sidecode generation in many Java and .NET SOAP frameworks (frameworks such as Spring, Apache Axis2 and Apache CXF being notable exceptions). Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service.
More recently, "RESTful Web services" have been regaining popularity, particularly with Internet companies. These also meet the W3C definition, and are often better integrated with
HTTPthan SOAP-based services. They do not require XML messages or WSDL service-API definitions.
To improve interoperability of Web Services, the
WS-Ipublishes profiles. A profile is a set of core specifications ( SOAP, WSDL, ...) in a specific version (SOAP 1.1, UDDI2, ...) with some additional requirements to restrict the use of the core specifications.The WS-I also publishes use cases and test tools to help deploying profile compliant Web Service.
Additional specifications, WS
Some specifications have been developed or are currently being developed to extend Web Services capabilities. These specifications are generally referred to as
WS-*. Here is a non-exhaustive list of these WS-* specifications.; WS-Security: Defines how to use XML Encryptionand XML Signaturein SOAPto secure message exchanges, as an alternative or extension to using HTTPSto secure the channel.; WS-Reliability: An OASIS standard protocol for reliable messaging between two Web services.; WS-Transaction: A way of handling transactions.
Some of these additional specifications have come from the W3C. There is much discussion around the organization's participation, as the general Web and the
Semantic Webstory appear to be at odds with much of the Web Services vision. This has surfaced most recently in February 2007, at the Web of Services for the Enterpriseworkshop. Some of the participants advocated a withdrawal of the W3C from further WS-* related work, and a focus on the core Web.
In contrast, OASIS has standardized many Web service extensions, including
Web Services Resource Frameworkand WSDM.
tyles of use
Web services are a set of tools that can be used in a number of ways. The three most common styles of use are RPC, SOA and REST.
Remote procedure calls
RPC Web services present a distributed function (or method) call interface that is familiar to many developers. Typically, the basic unit of RPC Web services is the WSDL operation.
The first Web services tools were focused on RPC, and as a result this style is widely deployed and supported. However, it is sometimes criticised for not being loosely coupled, because it was often implemented by mapping services directly to language-specific functions or method calls. Many vendors felt this approach to be a dead end, and pushed for RPC to be disallowed in the
WS-I Basic Profile.
Web services can also be used to implement an architecture according to
Service-oriented architecture(SOA) concepts, where the basic unit of communication is a message, rather than an operation. This is often referred to as "message-oriented" services.
SOA Web services are supported by most major software vendors and industry analysts. Unlike RPC Web services,
loose couplingis more likely, because the focus is on the "contract" that WSDL provides, rather than the underlying implementation details.
Representational state transfer
Finally, RESTful Web services attempt to emulate HTTP and similar protocols by constraining the interface to a set of well-known, standard operations (e.g., GET, PUT, DELETE). Here, the focus is on interacting with stateful resources, rather than messages or operations.RESTful Web services can use WSDL to describe SOAP messaging over HTTP, which defines the operations, or can be implemented as an abstraction purely on top of SOAP (e.g., WS-Transfer).
WSDL version 2.0 offers support for binding to all the HTTP request methods (not only GET and POST as in version 1.1) so it enables a better implementation of RESTful Web services [cite web |url=http://www.w3.org/TR/2007/REC-wsdl20-adjuncts-20070626/#_http_binding_default_rule_method |title=Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts] .However, support for this specification is still poor in software development kits, which often offer tools only for WSDL 1.1.
Critics of non-RESTful Web services often complain that they are too complex [ [http://www.tbray.org/ongoing/When/200x/2004/09/21/WS-Research ongoing · WS-Pagecount ] ] and based upon large software vendors or integrators, rather than
One big concern of the REST Web Service developers is that the SOAP WS toolkits make it easy to define new interfaces for remote interaction, often relying on introspection to extract the WSDL and service API from Java, C# or VB code. This is viewed as a feature by the SOAP stack authors (and many users) but it is feared that it can increase the brittleness of the systems, since a minor change on the server (even an upgrade of the SOAP stack) can result in different WSDL and a different service interface. The client-side classes that can be generated from WSDL and XSD descriptions of the service are often similarly tied to a particular version of the SOAP endpoint and can break if the endpoint changes or the client-side SOAP stack is upgraded. Well designed SOAP endpoints (with handwritten XSD and WSDL) do not suffer from this but there is still the problem that a custom interface for every service requires a custom client for every service.
There are also concerns about performance due to Web services' use of XML as a message format and SOAP and HTTP in enveloping and transport. At the same time, there are also emerging XML parsing/indexing technologies, such as
VTD-XML, that promise to address those XML-related performance issues.
There are several other approaches to solving the set of problems that Web services address, both preceding and contemporary to it. RMI was one of many
middlewaresystems that have seen wide deployment. More ambitious efforts like CORBAand DCOM attempted to effect distributed objects, which Web services implementations sometimes try to mimic.
More basic efforts include
XML-RPC, a precursor to SOAP that was only capable of RPC, and various forms of HTTPusage without SOAP.
List of Web service Frameworks
List of web service protocols
List of Web service specifications
Service Oriented Architecture
Enterprise Information Integration(EII)
Business Intelligence 2.0(BI 2.0)
Devices Profile for Web Services
Web Processing Service
Microsoft Connected Services Framework
Web Services Discovery
* [http://www.w3.org/2002/ws/ W3C Web Services Activity home page]
* [http://www.w3.org/TR/ws-arch/ Web Services Architecture] (W3C Working Group Note)
* [http://www.ibm.com/developerworks/webservices/library/ws-securtrans/ Secure, Reliable, Transacted Web Services] (IBM/Microsoft white paper)
* [http://www.ibm.com/developerworks/edu/ws-dw-ws-soa-autotest3.html?S_TACT=105AGY59&S_CMP=WIKITU&ca=dtl-1808tu1 Automate Web service testing, Part 3: Test a secured Web service with IBM Rational Software Architect and XMLUnit] (IBM developerWorks tutorial - advanced level)
* [http://webservices.sys-con.com/read/250512.htm The Performance Woe of Binary XML]
* [http://www2008.org/papers/fp389.html Where to find Web Services on the Web: Investigating Web Services on the World Wide Web (2008)]
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