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Web service - Wikipedia

Web service

From Wikipedia, the free encyclopedia

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 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, 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 web service is a software system designed to support interoperable machine-to-machine interaction

over a network.

— W3C, Web Services Glossary[1]

Web services may use SOAP over HTTP protocol, allowing less costly 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 W3C Web Services Architecture Working Group

(http://www.w3.org/2002/ws/arch/) 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[1]

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[2]

Contents

1 Explanation

2 Web API

3 Automated design methods

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4 Web services that use markup languages

5 Regression testing of Web services

6 Web service change management

7 See also

8 References

9 External links

Explanation

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:

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.[3])

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.

Web API

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A Web API is a development in web services where emphasis has been

moving to simpler representational state transfer (REST) based

communications.[4] 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).

Web services architecture: the service

provider sends a WSDL file to UDDI. The

service requester contacts UDDI to find out

who is the provider for the data it needs,

and then it contacts the service provider

using the SOAP protocol. The service

provider validates the service request and

sends structured data in an XML file, using

the SOAP protocol. This XML file would

be validated again by the service requester

using an XSD file.

web services in a service-oriented architecture.

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.[5]

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.[6]

A developer using a Subset WSDL (SWSDL)[7] (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)

Critics of non-RESTful web services often complain that they are too complex[8] 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.[9]

Regression testing of Web services

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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,[7] 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,[10] by performing the selection

of the relevant test cases to construct a reduced test suite from the old test suite. [11]

Web services testing can also be automated using several test automation tools like SOAP UI, Oracle Application

Testing Suite (OATS),[12][13] Unified Functional Testing, Selenium, etc.

Web service change management

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).[7] The insight on computation of change

impact is helpful in testing, top down development and reduce regression testing. Automated Web Service Change

Management (AWSCM)[10] is a tool that identify subset operations in a WSDL file to construct a subset WSDL.