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, web technology such as the HTTP protocol, originally designed for human-to-machine communication, is utilized for machine-to-machine communication, more specifically for transfering machine readable file formats such as XML and JSON. In practice, the 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 application, 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 software system designed to support interoperable machine-to-machine interaction over a network.
In a 2002 document, the W3C Web Services Architecture Working Group defined a Web Services Architecture, requiring a standardized implementation of a "Web service." In this:
[a 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 (Simple Object Access Protocol) messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards.
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 representations of Web resources using a uniform set of stateless operations.
- Arbitrary Web services, in which the service may expose an arbitrary set of operations.
Explanation
The term "Web services" 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 used to tag the data, 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. The W3C defines a Web service generally as:
a software system designed to support interoperable machine-to-machine interaction over a network.
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:
- How one system can request data from another system.
- Which specific parameters are needed in the data request
- What would be the structure of the data produced. (Normally, data is exchanged in XML files, and the structure of the XML file is validated against an .xsd file.)
- What error messages to display when a certain rule for communication is not observed, to make troubleshooting easier
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.
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Web API
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 design methods
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).
- A developer using a bottom-up model writes implementing classes first (in some programming language), and then uses a WSDL generating tool to expose methods from these classes as a Web service. This is simpler to develop but may be harder to maintain if the original classes are subject to frequent change.
- A developer using a top-down model writes the WSDL document first and then uses a code generating tool to produce the class skeleton, to be completed as necessary. This model is generally considered more difficult but can produce cleaner designs and is generally more resistant to change. As long as the message formats between sender and receiver do not change, changes in the sender and receiver themselves do not affect the Web service. The technique is also referred to as contract first since the WSDL (or contract between sender and receiver) is the starting point.
- A developer using a Subset WSDL (SWSDL) (i.e. a WSDL with the subset operation in the original WSDL) can perform web service testing and top down development.
Web services that use markup languages
There are a number of Web services that use markup languages:
- JSON-RPC
- JSON-WSP
- Web template
- Web Services Description Language (WSDL) from the W3C
- XML Interface for Network Services (XINS) provides a POX-style Web service specification format
- Web Services Conversation Language (WSCL)
- Web Services Flow Language (WSFL) (superseded by BPEL)
- WS-MetadataExchange
- Representational state transfer (REST) versus remote procedure call (RPC)
- XML-RPC - XML - Remote Procedure Call
Criticisms
Critics of non-RESTful Web services often complain that they are too complex and based upon large software vendors or integrators, rather than typical open source implementations.
There are also concerns about performance due to Web services' use of XML as a message format and SOAP/HTTP in enveloping and transporting.
Regression Testing of Web service
Functional and non-functional Web service testing is done with the help of WSDL parsing. Regression testing is performed by identifying the changes made thereafter. Web service regression testing needs can be categorized in three different ways, namely, changes in WSDL, changes in code, and selective re-testing of Web service 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, by performing the selection of the relevant test cases to construct a reduced test suite from the old test suite.
Web Service Change Management
Work related to the dealing with the visualization and capturing changes in 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. Automated Web Service Change Management (AWSCM) is a tool that identify subset operations in a WSDL file to construct a Subset WSDL.
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