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A web service is a method of communication between two electronic devices over the World Wide Web. A web service is a software function provided at a network address over the web or the cloud, it is a service that is "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:
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 light-weight interfaces.
XML web services 
XML web services use Extensible Markup Language (XML) messages that follow the SOAP standard and have been popular with the traditional enterprises. In such systems, there is often a 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-side code generation in many Java and .NET SOAP frameworks (frameworks such as Apache Axis2, Apache CXF, Spring, gSOAP being notable exceptions). Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a web service.
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 strategy) or to generate a class skeleton given existing WSDL (a top-down strategy).
- A developer using a bottom up method 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 method writes the WSDL document first and then uses a code generating tool to produce the class skeleton, to be completed as necessary. This way 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.
See also 
- Service-oriented architecture (SOA)
- List of web service specifications
- List of web service protocols
- List of web service frameworks
- "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.
- "Mashup Dashboard". ProgrammableWeb.com. 2009.
- "Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts". W3C.
- "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."
- 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