Web Services

Information about Web Services

Web services architecture

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 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 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.

Specifications

Core specifications

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:

SOAP: An XML-based, extensible message envelope format with "bindings" to underlying protocols. The primary protocols are HTTP and HTTPS, although bindings for others, including SMTP and XMPP, have been written.

Web Services Description Language (WSDL): An XML format that allows service interfaces to be described along with the details of their bindings to specific protocols. Typically used to generate server and client code, and for configuration.

Universal Description, Discovery, and Integration (UDDI): A protocol for publishing and discovering metadata about Web services that enables applications to find them, either at design time or runtime.

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.

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

Architectural elements involved in the XML-RPC.

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.

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^{[Confusing — Please clarify]} 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, 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. At the same time there are emerging XML parsing/indexing technologies, such as VTD-XML, that promise to address those XML-related performance issues.

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.

Notes

1. ^ Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts.
2. ^ [2]

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)
XMethods - Collection of public Web Services
The Performance Woe of Binary XML
What is a WebService?

World Wide Web Consortium

Consortium
Founded October 1994
Founder Tim Berners-Lee
Headquarters MIT/CSAIL in USA
ERCIM in France
Keio University in Japan
and many other offices around the world

Website www.w3.
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Interoperability is a property referring to the ability of diverse systems and organizations to work together (inter-operate). The term is often used in a technical systems engineering sense, or alternatively in a broad sense, taking into account social, political, and
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M2M refers to data communications between machines. M2M is most commonly translated as Machine-to-Machine but has sometimes been translated as Man-to-Machine, Machine-to-Man, Machine-to-Mobile and Mobile-to-Machine.
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as a college campus, industrial complex, or a military base. A CAN, may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN.
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WEB is a computer programming system created by Donald Knuth as the first implementation of what he called "literate programming": the idea that one could create software as works of literature, by embedding source code inside descriptive text, rather than the reverse (as is common
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An application programming interface (API) is a source code interface that an operating system or library provides to support requests for services to be made of it by computer programs.
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Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government
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A client is an application or system that accesses a (remote) service on another computer system known as a server by way of a network. The term was first applied to devices that were not capable of running their own stand-alone programs, but could interact with remote computers
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Server Computer

The inside/front of a server computer

Connects to:

Internet via one of

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Extensible Markup Language

File extension: .xml
MIME type: application/xml, text/xml (deprecated)
Uniform Type Identifier: public.xml
Developed by: World Wide Web Consortium
Type of format: Markup language
Extended from: SGML
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Simple Object Access Protocol, and lately also Service Oriented Architecture Protocol, but is now simply SOAP. The original acronym was dropped with Version 1.2 of the standard, which became a W3C Recommendation on June 24 2003, as it was considered to be misleading.
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Server Computer

The inside/front of a server computer

Connects to:

Internet via one of

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Web Services Description Language

File extension: .wsdl
MIME type: application/wsdl+xml
Developed by: World Wide Web Consortium
Contained by: XML
Standard(s): 2.
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In computer networking, the term client-side refers to operations that are performed by the client in a client-server relationship.

Typically, a client is a computer application, such as a web browser, that runs on a user's local computer or workstation and connects to a
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Java

Paradigm: Object-oriented, structured, imperative
Appeared in: 1995
Designed by: Sun Microsystems
Typing discipline: Static, strong, safe, nominative
Major implementations: Numerous
Influenced by: Objective-C, C++, Smalltalk, Eiffel,^[1]
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.NET Framework is a software component that can be added to or is included with Microsoft Windows operating system. It provides a large body of pre-coded solutions to common program requirements, and manages the execution of programs written specifically for the framework. The .
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Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML hypertext pages.
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https is a URI scheme used to indicate a secure HTTP connection. It is syntactically identical to the http:// scheme normally used for accessing resources using HTTP.
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Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.
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Extensible Messaging and Presence Protocol (XMPP) is an open, XML-inspired protocol for near-real-time, extensible instant messaging (IM) and presence information (a.k.a. buddy lists).
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Organization for the Advancement of Structured Information Standards (OASIS) is a global consortium that drives the development, convergence and adoption of e-business and web service standards.
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There are a variety of specifications associated with web services. These specifications are in varying degrees of maturity and are maintained or supported by various standards bodies and entities. Specifications may complement, overlap, and compete with each other.
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