Web service

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Web services architecture.

Web services today are frequently just Application Programming Interfaces (API) or web APIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.

In common usage the term refers to clients and servers that communicate over the Hypertext Transfer Protocol (HTTP) protocol used on the web. Such services tend to fall into one of two camps: Big Web Services[1] and RESTful Web Services.

"Big Web Services" use Extensible Markup Language (XML) messages that follow the Simple Object Access Protocol (SOAP) standard and have been popular with traditional enterprise. 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 Spring, Apache Axis2 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.

File:SOA Detailed Diagram.png
Web services in a service-oriented architecture.

More recently, REpresentational State Transfer (RESTful) web services have been regaining popularity, particularly with Internet companies. By using the PUT, GET and DELETE HTTP methods, alongside POST, these are often better integrated with HTTP and web browsers than SOAP-based services. They do not require XML messages or WSDL service-API definitions.

Web API is a development in web services (in a movement called Web 2.0) where emphasis has been moving away from Simple Object Access Protocol (SOAP) based services towards more direct Representational State Transfer (REST) style communications.[2] Web APIs allow the combination of multiple web services into new applications known as mashups.[3]

When used in the context of web development, web API is typically a defined set of Hypertext Transfer Protocol (HTTP) request messages along with a definition of the structure of response messages, usually expressed in an Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format.

When running composite web services, each sub service can be considered autonomous. The user has no control over these services. Also the web services themselves are not reliable; the service provider may remove, change or update their services without giving notice to users. The reliability and fault tolerance is not well supported; faults may happen during the execution. Exception handling in the context of web services is still an open research issue.

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 Web Services Description Language 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."[4]

The W3C also states, "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."[5]




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 the deployment of profile compliant web services. The WS is the editing 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.

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.
An OASIS standard protocol for reliable messaging between two web services.
A way of handling transactions.
Is a standard way to insert address in the SOAP header.

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 paradigms appear to be at odds with much of the Web Services vision. This has surfaced most recently in February 2007, at the W3C Workshop on Web of Services for Enterprise Computing[6]. Some of the participants advocated a withdrawal of the W3C from further WS-* related work, and a focus on the core Web.[7]

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

File:Webservice xrpc.png
Architectural elements involved in the XML-RPC.

RPC Web services present a distributed function (or method) call interface that is familiar with 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.

Other approaches with nearly the same functionality as RPC are Object Management Group's (OMG) Common Object Request Broker Architecture (CORBA), Microsoft's Distributed Component Object Model (DCOM) or Sun Microsystems's Java/Remote Method Invocation (RMI).

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.

Middleware Analysts use Enterprise Service Buses which combine message-oriented processing and Web Services to create an Event-driven SOA. One example of an open-source ESB is Mule, another one is Open ESB.

Representational state transfer

Finally, Representational State Transfer (REST) attempts to describe architectures which use HTTP or similar protocols by constraining the interface to a set of well-known, standard operations (like GET, POST, PUT, DELETE for HTTP). Here, the focus is on interacting with stateful resources, rather than messages or operations. An architecture based on REST (one that is 'RESTful') can use WSDL to describe SOAP messaging over HTTP, which defines the operations, can be implemented as an abstraction purely on top of SOAP (e.g., WS-Transfer), or can be created without using SOAP at all.

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.[8] However, support for this specification is still poor in software development kits, which often offer tools only for WSDL 1.1.

Design methodologies

Web services can be written in two ways:

  • A developer using the "bottom up method" first writes the implementing class in a programming language, and then uses a WSDL generating tool to expose its methods as a web service[1]. This is often the simpler approach.
  • A developer using the "top down method" first writes the WSDL document and then uses a code generating tool to produce the class skeleton, which she later completes. This way is more difficult but produces cleaner designs[2]


Critics of non-RESTful Web services often complain that they are too complex[9] and based upon large software vendors or integrators, rather than typical open source implementations. There are open source implementations like Apache Axis and Apache CXF.

One key 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. It is argued 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[10]. 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, however emerging XML parsing/indexing technologies, such as VTD-XML, promise to address those XML-related performance issues..

Similar efforts

Several other approaches exist to solve the set of problems that Web services 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


External links

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