Web Services

Web service is a way of communication that allows interoperability between different applications on different platforms, for example, a java based application on Windows can communicate with a .Net based one on Linux. The communication can be done through a set of XML messages over HTTP protocol.
Web services are browsers and operating system independent service, which means it can run on any browser without the need of making any changes. Web Services take Web-applications to the Next Level.
The World Wide Web Consortium (W3C) has defined the web services. According to W3C, “Web Services are the message-based design frequently found on the Web and in enterprise software. The Web of Services is based on technologies such as HTTP, XML, SOAP, WSDL, SPARQL, and others.” Let’s say you are a java developer and you can publish your functions on internet or LAN through java web service so any other developer(let’s say .Net developer) can access your function.

Why you need to learn web services

Reuse already developed(old) functionality into new software
Let’s understand with a very simple example.Let’s say you are developing a finance software for a company on java and you have old .net software which manages salary of employees.So rather than developing new software for employee part, you can use old software and for other parts of the infrastructure, you can develop your own functionalities.


Usability 

Web Services allow the business logic of many different systems to be exposed over the Web. This gives your applications the freedom to chose the Web Services that they need. Instead of re-inventing the wheel for each client, you need only include additional application-specific business logic on the client-side. This allows you to develop services and/or client-side code using the languages and tools that you want.


Interoperability 

This is the most important benefit of Web Services. Web Services typically work outside of private networks, offering developers a non-proprietary route to their solutions.Web Services also let developers use their preferred programming languages. In addition, thanks to the use of standards-based communications methods, Web Services are virtually platform-independent.


Loosely Coupled
Each service exists independently of the other services that make up the application. Individual pieces of the application to be modified without impacting unrelated areas.

Ease of Integration
Data is isolated between applications creating ’silos’. Web Services act as glue between these and enable easier communications within and across organizations.


Deployability 

Web Services are deployed over standard Internet technologies. This makes it possible to deploy Web Services even over the firewall to servers running on the Internet on the other side of the globe. Also thanks to the use of proven community standards, underlying security (such as SSL) is already built-in.


Some jargons used in Web services

Simple Object Access Protocol(SOAP)
SOAP is a protocol specification for exchanging structured information in the implementation of Web services in computer networks. It relies on XML as its message format.


Web Service Description Language(WSDL)

WSDL stands for Web Service Description Language. It is an XML file that describes
the technical details of how to implement a web service, more specifically the URI,
port, method names, arguments, and data types. Since WSDL is XML, it is both
human-readable and machine-consumable, which aids in the ability to call and bind to
services dynamically.

Elements of WSDL

Description
It is the root element of a WSDL 2.0 file. It usually contains a set of namespace declarations which are used throughout the WSDL file.  

Types
The WSDL types element describes the data types used by your web service.Data types are usually specified by XML schema.It can be described in any language as long as your web services API supports it.


Binding
The WSDL binding element describes how your web service is bound to a protocol. In other words, how your web service is accessible. To be accessible, the web service must be reachable using some network protocol. This is called “binding” the web service to the protocol.


Interface
The WSDL interface element describes the operations supported by your web service.It is similar to methods in a programming language.The client can only call one operation per request.


Service
It describes the endpoint of your web service. In other words, the address where the web service can be reached.


Endpoint
The endpoint element describes the address of the web service. The endpoint binding attribute describes what binding element this endpoint uses.i.e. protocol with which you will access web service. The address attribute describes the URI at which you can access the service.


Message
The message element describes the data being exchanged between the Web service providers and consumers.


Sample WSDL file

https://gist.github.com/IsurangaPerera/c043130cd2008db505db24b38b3c4cbd.js

Universal Description, Discovery, and Integration(UDDI)


UDDI stands for Universal Description, Discovery, and Integration.It is a directory service. Web services can register with a UDDI and make themselves available through it for discovery

Web service design approaches

Contract last or Bottom-up approach
When using contract last approach, you first write your java code then you create web service contract(WSDL).There are various kinds of tools which can generate WSDL on the basis of java code.


Contract first or Top Down Approach
It is the reverse of contract first.Here you first define web service contract.You define all the elements of WSDL first then after that you create your java logic.



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