Download XML Web Services - University of Windsor

What are Xml Web Services?
1.
A Web Service simply an application that exposes a Webaccessible 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;
communicate using platform-independent and languageneutral 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.
2.
loosely specified and coupled
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.
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:
•
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 commandline utility wsdl.exe.
•
Contains not only the default constructor, but also methods to
invoke the web service both synchronously and asynchronously.
–
–
–
–
–
Generate SOAP request
Send to the target service
Receive the SOAP response
Parse the SOAP Generate an object
Return the object as a result
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
UDDI continued
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
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: You 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: If 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