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The W3C defines a Web service (many sources also capitalize the second word, as in Web Services) 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, a description 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 the mainstream Java and .NET SOAP frameworks. Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service.
The specifications that define Web services are intentionally modular, and as a result there is no one document that contains them all. Additionally, there is neither a single, nor a stable set of specifications. There are a few "core" specifications that are supplemented by others as the circumstances and choice of technology dictate, including:
Most of these core specifications have come from W3C, including XML, SOAP, and WSDL; UDDI comes from OASIS.
See List of Web service specifications for a more complete listing.
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.
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.
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.
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.
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 function 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.
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.
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).
Critics of non-RESTful Web services often complain that they are too complex[1] 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 or C# 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, in which a minor change on the server (even an upgrade of the SOAP stack) can result in different WSDL and a different service interface. Similarly, the client-side classes that can be generated from WSDL and XSD descriptions of the service are often 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, because of Web services' use of XML as a message format and SOAP and HTTP in enveloping and transport. At the same time, there are also emerging XML parsing/indexing technologies, such as VTD-XML, that promise to address those XML-related performance issues.
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 both 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 using HTTP without SOAP.
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