This article needs to be updated. In particular: Needs to be less centred on SOAP.(January 2017)
A web service is a service offered by an electronic device to another electronic device, communicating with each other via the World Wide Web. In a web service, the Web technology such as HTTP--originally designed for human-to-machine communication--is utilized for machine-to-machine communication, more specifically for transferring machine-readable file formats such as XML and JSON. In practice, a web service typically provides an object-oriented web-based interface to a database server, utilized for example by another web server, or by a mobile app, that provides a user interface to the end user. Another common application offered to the end user may be a mashup, where a web server consumes several web services at different machines, and compiles the content into one user interface.
The W3C defines a web service generally as:
A web service is a software system designed to support interoperable machine-to-machine interaction over a network.-- W3C, Web Services Glossary
Web services may use SOAP over HTTP protocol, allowing less costly (more efficient) interactions over the Internet than via proprietary solutions like EDI/B2B. Besides SOAP over HTTP, web services can also be implemented on other reliable transport mechanisms like FTP. In a 2002 document, the Web Services Architecture Working Group defined a web services architecture, requiring a standardized implementation of a "web service." In this:
It [web service] has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the web service in a manner prescribed by its description using SOAP-messages, typically conveyed using HTTP with an XML serialization in conjunction with other web-related standards.-- W3C, Web Services Glossary
In a 2004 document, the W3C extended the definition:
We can identify two major classes of web services:
- REST-compliant web services, in which the primary purpose of the service is to manipulate XML representations of web resources using a uniform set of "stateless" operations; and
- arbitrary web services, in which the service may expose an arbitrary set of operations.-- W3C, Web Services Architecture
The term "web service" describes a standardized way of integrating web-based applications using the XML, SOAP, WSDL and UDDI open standards over an Internet Protocol backbone. XML is the data format used to contain the data and provide metadata around it, SOAP is used to transfer the data, WSDL is used for describing the services available and UDDI lists what services are available.
A web service is a method of communication between two electronic devices over a network. It is a software function provided at a network address over the web with the service always on as in the concept of utility computing.
Many organizations use multiple software systems for management. Different software systems often need to exchange data with each other, and a web service is a method of communication that allows two software systems to exchange this data over the internet. The software system that requests data is called a service requester, whereas the software system that would process the request and provide the data is called a service provider.
Different software may use different programming languages, and hence there is a need for a method of data exchange that doesn't depend upon a particular programming language. Most types of software can, however, interpret XML tags. Thus, web services can use XML files for data exchange.
Rules for communication between different systems need to be defined, such as:
All of these rules for communication are defined in a file called WSDL (Web Services Description Language), which has a
.wsdl extension. (Proposals for Autonomous Web Services (AWS) seek to develop more flexible web services which do not rely on strict rules.)
A directory called UDDI (Universal Description, Discovery and Integration) defines which software system should be contacted for which type of data. So when one software system needs one particular report/data, it would go to the UDDI and find out which other system it can contact for receiving that data. Once the software system finds out which other system it should contact, it would then contact that system using a special protocol called SOAP (Simple Object Access Protocol). The service provider system would first validate the data request by referring to the WSDL file, and then process the request and send the data under the SOAP protocol.
A Web API is a development in web services where emphasis has been moving to simpler representational state transfer (REST) based communications. Restful APIs do not require XML-based web service protocols (SOAP and WSDL) to support their interfaces.
Automated tools can aid in the creation of a web service. For services using WSDL, it is possible to either automatically generate WSDL for existing classes (a bottom-up model) or to generate a class skeleton given existing WSDL (a top-down model).
There are a number of web services that use markup languages:
There are also concerns about performance due to web services' use of XML as a message format and SOAP/HTTP in enveloping and transporting.
Functional and non-functional testing of web services is done with the help of WSDL parsing. Regression testing is performed by identifying the changes made to upgrade a software. Web service regression testing needs can be categorized in three different ways, namely, changes in WSDL, changes in code, and selective re-testing of operations. We can capture the above three needs in three intermediate forms of Subset WSDL, namely, Difference WSDL (DWSDL), Unit WSDL (UWSDL), and Reduced WSDL (RWSDL), respectively. These three Subset WSDLs are then combined to form Combined WSDL (CWSDL) that is further used for regression testing of the web service. This will help in Automated Web Service Change Management (AWSCM), by performing the selection of the relevant test cases to construct a reduced test suite from the old test suite. 
Work related to the capture and visualization of changes made to a Web service. Visualization and computation of changes can be done in the form of intermediate artifacts (Subset WSDL). The insight on computation of change impact is helpful in testing, top down development and reduce regression testing. AWSCM is a tool that can identify subset operations in a WSDL file to construct a subset WSDL.
Under the current Web Services, [...] stakeholder systems must follow the predefined rules for a particular business service including those about business protocols to send/receive messages and about system operation. [...] More flexible mechanism is desired where freely built and autonomously running systems can exchange business messages without pre-agreed strict rules. We call it Autonomous Web Services (AWS) and proposed the framework called Dynamic Model Harmonization (DMH) with its algorithm, which dynamically adjusts different business process models between systems [...].
The results presented in this paper show that the nature of response data has a greater impact on relative performance than has been allowed for in most previous studies.
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