A Web service is a method of communications between two electronic devices over the World Wide Web. 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 as:
a software system designed to support interoperable machine-to-machine interaction over a network. It 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.
The W3C also states:
We can identify two major classes of Web services:
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- Web Services Flow Language (WSFL) (superseded by BPEL)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 above three changes three intermediate forms of WSDL, namely, Difference WSDL (DWSDL), Unit WSDL (UWSDL), and Reduced WSDL (RWSDL), respectively can be used. These intermediate forms of WSDLs are then combined to form Combined WSDL (CWSDL) which 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. 
- Service-oriented architecture (SOA)
- List of web service specifications
- List of web service protocols
- List of web service frameworks
- Web Service Testing
- "Web Services Glossary". W3C. February 11, 2004. 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 (October 28, 2004). "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 January 11, 2011. "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.
|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
- Webzesty Pvt.Ltd. - Website Designing Company in India