What are Xml Web Services?

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3 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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What are Xml Web Services?

1.
A Web Service simply an application that exposes a Web
-
accessible API

.


2.
Web services are a new, standard


platform for building
interoperable distributed applications.




The Web services platform is a set of standards that
applications follow to achieve interoperability via the
Web.



You write your Web services in whatever language and
on any platform you like, as long as those Web services
can be viewed and accessed according to the Web
services standards.

About Web Services


Web Services, unlike DCOM or CORBRA, are founded on
universal, nonproprietary standards including XML and HTTP
and offer a model that is platform independent;
c
ommunicate using platform
-
independent and language
-
neutral Web protocols.



The Web services platform uses XSD as its type system.
XML provides a simple way of representing data, but it says
nothing about the standard set of data types available and
how to extend that set. The data types you use must be
translated to XSD types to conform to the Web services
standards.



Web services are not exclusive to .NET.


NET includes several tools and a degree of support which
simplify development of Web Services by automating many
tasks involved and shielding the developer form many of the
technical details.

More About Web Services


Your Web service will sit behind a Web server, typically
Internet Information Server (IIS
).


Support loosely coupled connections between systems.


Works through existing proxies and firewalls.


Can take advantage of HTTP authentication.


Encryption for free with SSL.


Web services only receive and return XML.


Provide a stateless model similar to the
singlecall

activation
mode. When a client invokes a remote method,the Server
automatically constructs the relevant object, executes the
method, returns any results, and discards the object. It is
possible however, to maintain between method calls using
the ASP.NET session object.

How are web services different
from earlier models of distributed
computing, such as Corbra?

Key differences:


1.
loosely specified and coupled

2.
build on top of existing, ubiquitous infrastructure
like HTTP and XML.

How are Web Services Described?


Web Services are described using WSDL (Web Services
Description language). These descriptions may be stored with
the service itself or published in a UDDI registry.



WSDL is
an XML
-
based grammar for describing Web services,
their functions, parameters, and return values.



.NET’s web service infrastructure will automatically generate
the necessary WSDL to fully describe a .NET web service.



.NET also provides tools that can consume WSDL descriptions
of services and use these descriptions to generate proxy
classes for use in client applications. So developers using .NET
can remain ignorant of WSDL (and even SOAP) and create
Web Services.

Example of a Service Description

<?xml version="1.0"?>

<serviceDescription


xmlns:s0="http://tempuri.org/"


name="TerraService"


targetNamespace="http://tempuri.org/"


xmlns="urn:schemas
-
xmlsoap
-
org:sdl.2000
-
01
-
25">


<httpget xmlns="urn:schemas
-
xmlsoap
-
org:get
-
sdl
-
2000
-
01
-
25">


<service>


<requestResponse


name="GetPlaceList"


href="http://207.46.235.37/terranet/terraservice.asmx/GetPlaceList">


<request>


<param name="placeName"/>


<param name="MaxItems"/>


<param name="imagePresence"/>


</request>


<response>


<mimeXml ref="s0:ArrayOfPlaceFacts"/>


</response>


</requestResponse>


</service>


</httpget>

</serviceDescription>

How to Create a Web Service

To build a .NET Web service you create a file with the .asmx
extension.

Viewing a Test Page for the Web Service

The framework provides built
-
in support for viewing and

accessing methods exposed by a Web service. By accessing the

Web service .asmx file, the framework automatically displays

the public methods, as shown in figure below.

Testing the Web Service Continued


The service method help page shows the XML representation of the
request and response for the Web service, its arguments, and return
types. You can also use the displayed
HTML form to test the method.

How are Web Services Invoked?


Soap (
Simple Object Access Protocol)

is the communications
protocol for Xml Web Services.


The Soap
specification provides standards for the format of
a SOAP message and how SOAP should be used over HTTP,
which it uses as the transport layer to move structured type
information across the Internet.


SOAP also builds on XML and XSD to provide standard rules
for encoding data as XML.




Invoking a Web service from either a Web or desktop
application is a simple matter of object instantiation and
invocation. But which Object? This is where the proxy class
comes into play.

Example of a SOAP Request

Proxy Classes


The Proxy class is responsible for the following:


Generate SOAP request


Send to the target service


Receive the SOAP response


Parse the SOAP Generate an object


Return the object as a result



The developer tools provided with the .NET SDK and Visual Studio
.NET remove the need for the developer to work at the SOAP level.
Instead, clients may calls against a proxy object derived from
System.Web.Services.Protocols.SoapHttpClientProtocol

and .NET takes care of
setting up, formatting and transmitting messages.



The proxy class can be generated automatically using the command
-
line utility
wsdl.exe
.



Contains not only the default constructor, but also methods to
invoke the web service both synchronously and asynchronously.


Example of a Proxy Class

Example for invoking a temperature Web
service from an ASP.NET Web page

What is UDDI?


Result of the combined efforts of several technology
companies (chiefly IBM and Microsoft) to produce a
global Internet
-
based registry of businesses and the
web services they provide.



UDDI represents a set of protocols and a public
directory for the registration and real
-
time lookup of
web services and other business processes.



“Yellow pages” of Web Services

Consists of 2 parts:


1.
First, UDDI is a technical specification for building a
distributed directory of businesses and Web services. Data
is stored within a specific XML format, and the UDDI
specification includes API details for searching existing data
and publishing new data.


2.
Second, the UDDI Business Registry is a fully operational
implementation of the UDDI specification. The UDDI
registry enables anyone to search existing UDDI data, for
example, for providers of service in a specified geographic
location or for businesses of a specified type. It also enables
any company to register themselves and their services.

UDDI continued

UDDI continued

The data captured within UDDI is divided into three main

categories:



White Pages
: This includes general information about a
specific company. For example, business name, business
description, and address.



Yellow Pages
: This includes general classification data for
either the company or the service offered. For example, this
data may include industry, product, or geographic codes
based on standard taxonomies.



Green Pages
: This includes technical information about a
Web service. Generally, this includes a pointer to an
external specification, and an address for invoking the Web
service.


When to Use Web Services?

Web Services offer the most benefit in cases where

applications need to communicate across platform boundaries

and over the Internet:


Business to Business Integration


Integrating business processes across multiple businesses is

commonly referred to as business
-
to
-
business integration. By

using Web services, a business can expose vital business

processes to authorized suppliers and customers.


Example
: Y
ou can expose electronic ordering and invoicing

thereby enabling your customers to electronically send you

purchase orders and your suppliers to electronically send you

invoices.

Application Integration


A good portion of development efforts is spent integrating

applications written in various languages and running on

different systems.



Example
: You need to get data into your application from a

legacy application running on an IBM mainframe or you need to

send data from your application to a mainframe or Unix
-
based

application somewhere. Even on the same platform, applications

from different vendors often need to be integrated. By

exposing some of its functionality and data via a Web service,

an application provides a standard mechanism for other

applications to integrate with it.



Communicating through a firewall


Software Reuse

Software reuse has always been limited by one key factor: you

can reuse the code but not the data behind the code. The

reason for this is you cannot easily distribute data unless it is

fairly static data not expected to change much.
Web services

let you reuse code along with the data it needs
.


Example
: I
f your application’s user enters a mailing address that you

want to validate, you could send it to an address verification Web

service. This service can lookup the street address, city, state and zip

code to make sure the address exists and in the specified zipcode. A

service like this is not possible with component reuse, you need to have

the current database of street addresses, cities, states and zip codes.


Another scenario for software reuse is when you are building

an application that aggregates the functionality of several

other applications: you can use Web services to aggregate

functionality from many applications into one, consistent, user

interface.

When Not to Use Web Services

Situations where using Web Services will cost you
performance:


Single Machine Applications

Applications that need to communicate with others running
on the same machine. In this case, it is almost always better
to use a native API rather than a Web service

because they
require relatively little overhead.


Homogeneous Applications on a LAN

Homogenous applications running on the same machine or on
different machines that need to communicate on the same
LAN.

Web Service Review

Service Side

•Write the service code


Expects SOAP as input

•Define the externally visible methods

•Create the WSDL description of those methods

•Publish the WSDL

•Run the service

Client Side


•Locate the WSDL

•Load the WSDL

•Convert to a client proxy

•Write the client code, calling proxy


Generates and sends SOAP