Web service

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A Web service (also Web Service) is defined by the W3C as "a software system designed to support interoperable Machine to Machine interaction over a network." Web services are frequently just Web APIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.
The W3C Web service definition encompasses many different systems, but in common usage the term refers to clients and servers that communicate using XML messages that follow the SOAP standard. Common in both the field and the terminology is the assumption that there is also a machine readable description of the operations supported by the server 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 Spring and Apache CXF being notable exceptions). Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service.

Specifications

Profiles
To improve interoperability of Web Services, the WS-I publishes profiles. A profile is a set of core specifications (SOAP, WSDL, ...) in a specific version (SOAP 1.1, UDDI 2, ...) with some additional requirements to restrict the use of the core specifications. The WS-I also publishes use cases and test tools to help deploying profile compliant Web Service.

Additional specifications, WS-*
Some specifications have been developed or are currently being developed to extend Web Services capabilities. These specifications are generally referred to as WS-*. Here is a non-exhaustive list of these WS-* specifications.
WS-Security
Defines how to use XML Encryption and XML Signature in SOAP to secure message exchanges, as an alternative or extension to using HTTPS to secure the channel.
WS-Reliability
An OASIS standard protocol for reliable messaging between two Web services.
WS-ReliableMessaging
A protocol for reliable messaging between two Web services, issued by Microsoft, BEA and IBM it is currently being standardized by the OASIS organization [1].
WS-Addressing
A way of describing the address of the recipient (and sender) of a message, inside the SOAP message itself.
WS-Transaction
A way of handling transactions.
Some of these additional specifications have come from the W3C. There is much discussion around the organization's participation, as the general Web and the Semantic Web story appear to be at odds with much of the Web Services vision. This has surfaced most recently in February 2007, at the Web of Services for the Enterprise workshop. Some of the participants advocated a withdrawal of the W3C from further WS-* related work, and a focus on the core Web.
In contrast, OASIS has standardized many Web service extensions, including Web Services Resource Framework and WSDM.

Styles of use
Web services are a set of tools that can be used in a number of ways. The three most common styles of use are RPC, SOA and REST.

Remote procedure calls
RPC Web services present a distributed function (or method) call interface that is familiar to many developers. Typically, the basic unit of RPC Web services is the WSDL operation.
The first Web services tools were focused on RPC, and as a result this style is widely deployed and supported. However, it is sometimes criticised for not being loosely coupled, because it was often implemented by mapping services directly to language-specific functions or method calls. Many vendors felt this approach to be a dead end, and pushed for RPC to be disallowed in the WS-I Basic Profile.

Service-oriented architecture
Web services can also be used to implement an architecture according to Service-oriented architecture (SOA) concepts, where the basic unit of communication is a message, rather than an operation. This is often referred to as "message-oriented" services.
SOA Web services are supported by most major software vendors and industry analysts. Unlike RPC Web services, loose coupling is more likely, because the focus is on the "contract" that WSDL provides, rather than the underlying implementation details.

Representational state transfer
Finally, RESTful Web services attempt to emulate HTTP and similar protocols by constraining the interface to a set of well-known, standard operations (e.g., GET, PUT, DELETE). Here, the focus is on interacting with stateful resources, rather than messages or operations. RESTful Web services can use WSDL to describe SOAP messaging over HTTP, which defines the operations, or can be implemented as an abstraction purely on top of SOAP (e.g., WS-Transfer).
WSDL version 2.0 offers support for binding to all the HTTP request methods (not only GET and POST as in version 1.1) so it enables a better implementation of RESTful Web services[1] . However support for this specification is still poor in software development kits, which often offer tools only for WSDL 1.1.

Criticisms
Critics of non-RESTful Web services often complain that they are too complex[2] and biased towards large software vendors or integrators, rather than open source implementations.
One big concern of the REST Web Service developers is that the SOAP WS toolkits make it easy to define new interfaces for remote interaction, often relying on introspection to extract the WSDL and service API from Java, C# or VB code. This is viewed as a feature by the SOAP stack authors (and many users) but it is feared that it can increase the brittleness of the systems, since a minor change on the server (even an upgrade of the SOAP stack) can result in different WSDL and a different service interface. The client-side classes that can be generated from WSDL and XSD descriptions of the service are often similarly tied to a particular version of the SOAP endpoint and can break if the endpoint changes or the client-side SOAP stack is upgraded. Well designed SOAP endpoints (with handwritten XSD and WSDL) do not suffer from this but there is still the problem that a custom interface for every service requires a custom client for every service.
There are also concerns about performance due to Web services' use of XML as a message format and SOAP and HTTP in enveloping and transport.

Similar efforts
There are several other approaches to the set of problems that Web services attempts to address, both preceding and contemporary to it. RMI was one of many middleware systems that have seen wide deployment. More ambitious efforts like CORBA and DCOM attempted to effect distributed objects, which Web services implementations sometimes try to mimic.
More basic efforts include XML-RPC, a precursor to SOAP that was only capable of RPC, and various forms of HTTP usage without SOAP.

See also
List of Web service Frameworks
Service system
Service Oriented Architecture
Enterprise Information Integration (EII)
Business Intelligence 2.0 (BI 2.0)
Devices Profile for Web Services
Web Processing Service
Microsoft Connected Services Framework
Web Services Discovery
OAuth

Notes
^ Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts.
^ http://www.tbray.org/ongoing/When/200x/2004/09/21/WS-Research

External links
W3C Web Services Activity home page
Web Services Architecture (W3C Working Group Note)
Secure, Reliable, Transacted Web Services (IBM/Microsoft white paper)
Automate Web service testing, Part 3: Test a secured Web service with IBM Rational Software Architect and XMLUnit (IBM developerWorks tutorial - advanced level)
The Performance Woe of Binary XML
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Categories: Web services