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:-
The W3C Web Services Architecture Working Group defined a Web Services Architecture, requiring a specific 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.
Most Web services do not adopt this complex architecture. This article describes it in more detail.
The W3C also states:
We can identify two major classes of Web services:
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 might be built using 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 the extension .wsdl.
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 of all 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 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.
Web services that use markup languages
There are a number of Web services that use markup languages:
- 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)
- Representational state transfer (REST) versus remote procedure call (RPC)
- XML-RPC - XML - Remote Procedure Call
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 and 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. To capture the above, three intermediate forms of WSDL, namely, Difference WSDL (DWSDL), Unit WSDL (UWSDL), and Reduced WSDL (RWSDL), respectively can be used. These intermediate forms are then combined to form Combined WSDL (CWSDL) that is further used for regression testing of the Web service. This will help in Automatic 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. 
- "Web Services Glossary". W3C. 2004-02-11. Retrieved 2011-04-22.
- "Relationship to the World Wide Web and REST Architectures". Web Services Architecture. W3C. Retrieved 2011-04-22.
- Benslimane, D.; Dustdar, S.; Sheth, A. (2008). "Services Mashups: The New Generation of Web Applications". IEEE Internet Computing 10 (5): 13–15. doi:10.1109/MIC.2008.110.
- "Help - Creating bottom-up Web services". Eclipse. Retrieved 2011-04-22.
- "Help - Creating top-down Web services". Eclipse. Retrieved 2011-04-22.
- Bray, Tim (2004-10-28). "WS-Pagecount". TBray.org. Retrieved 2011-04-22.
- Gray, N. A. B. (2005). "Performance of Java Middleware - Java RMI, JAXRPC, and CORBA". University of Wollongong. pp. 31–39. Retrieved 2011-01-11. "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."
- Chaturvedi, A.; Gupta, A. (2013). "A tool supported approach to perform efficient regression testing of web services". "2013 IEEE 7th International Symposium on the Maintenance and Evolution of Service-Oriented and Cloud-Based Systems". p. 50. doi:10.1109/MESOCA.2013.6632734. ISBN 978-1-4673-4889-8.
- List of Web service frameworks.
- List of Web service protocols
- List of Web service specifications
- Service-oriented architecture (SOA)
|Wikiversity has learning materials about Web service|
- Messaging Design Pattern and a distributed component/service model
- W3C Web Services Activity home page
- Web Services Architecture (W3C Working Group Note)
- Where to find Web Services on the Web: Investigating Web Services on the World Wide Web (2008)
- NIST SP800-95 Guide to Secure Web Services