A Web service is a method of communication between two electronic devices over a network.
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, known by the acronymWSDL). 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."
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."
Virtual private server (VPS) is a term used by internet hosting services to refer to a virtual machine. The term is used for emphasizing that the virtual machine, although running in software on the same physical computer as other customers' virtual machines, is functionally equivalent to a separate physical computer, is dedicated to the individual customer's needs, has the privacy of a separate physical computer, and can be configured to run as a server computer (i.e. to run server software). The term Virtual Dedicated Server or VDS is used less often for the same concept, however it may indicate that the server does not use burst/shared ram through multiple machines, as well as individual CPU cores.
In addition to reducing hardware and power expenditures, virtualisation allows businesses to run their legacy applications on older versions of an operating system on the same server as newer applications.
Each virtual server can run its own full-fledged operating system and can be independently rebooted.
Web API
Web API is a development in Web services (in a movement called Web 2.0) where emphasis has been moving away from SOAP based services towards representational state transfer(REST) based communications.REST services do not require XML, SOAP, or Web Service Description language service-API definitions.
Web APIs allow the combination of multiple Web services into new applications known as mashups.
When used in the context of web devlopment, 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, although this can still be handled by responding with an error object to the clients.
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 RESET.
Remote procedure calls
Main article: Remote procedure call
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 criticized 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-1 basic profile.
Other approaches with nearly the same functionality as RPC are: Object Management group's (OMG) Common Object Request Broker Architecture (COBRA), Open Software Foundation's (OSF) DCE/, Microsoft's RPC (a part of DCOM) or .NET Remoting and Sun Microsystem's Java/Remote Method Invocation (RMI).
Service-oriented architecture
Main article: 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 orinted" 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 analyst use enterprise service buses (ESBs) that combine message oriented processing and Web services to create an event driven SOA. One example of an open-source ESB are WSO2 ESB mule and Open ESB
Representational state transfer (REST)
Main article: representational space provider
REST attempts to describe architectures that 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. clean URLs are tightly associated with the REST concept.
An architecture based on REST (one that is 'RESTful') can use WSDL to describe SOAP messaging over HTTP, 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 method (not only GET and POST as in version 1.1) so it enables a better implementation of RESTful web services However, support for this specification is still poor in software devlopment skills, which often offer tools only for WSDL 1.1.
Automated design methodologies
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 strategy) or to generate a class skeleton given existing WSDL (a top-down strategy).
- A developer using a bottom up method 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 often the simpler approach.
- A developer using a top down method writes the WSDL document first and then uses a code generating tool to produce the class skeleton, to be completed as necessary. This way is generally considered more difficult but can produce cleaner designs
