XML - Managing Data Exchange/Web Services

Web Services Overview
Web Services are a new breed of Web application. They are self-contained, self-describing, modular applications that can be published, located, and invoked across the Web. Web services perform functions, which can be anything from simple requests to complicated business processes. Once a Web service is deployed, other applications (and other Web services) can discover and invoke the deployed service. Web services make use of XML to describe the request and response, and HTTP as its network transport.

The primary difference between a Web Service and a web application relates to collaboration. Web applications are simply business applications which are located or invoked using web protocols. Similarly, Web Services also perform computing functions remotely over a network. However, Web Services use internet protocols with the specific intent of enabling inter operable machine to machine coordination.

Web Services have emerged as a solution to problems associated with distributed computing. Distributed computing is the use of multiple systems to perform a function rather than having a single system perform it. The previous technologies used in distributed computing, primarily Common Object Request Broker Architecture (CORBA) and Distributed Component Object Model (DCOM), had some limitations. For example, neither has achieved complete platform independence or easy transport over firewalls. Additionally, DCOM is not vendor independent, being a Microsoft product.

Some of the primary needs for a distributed computing standard were:

 Cross-platform support for Business to Business, as well as internal, communication. Concordance with existing Internet infrastructure as much as possible. Scalability, both in number and complexity of nodes. Internalization. Tolerance of failure. Vendor independence. Suitability for trivial and non-trivial requests. 

Over time, business information systems became highly configured and differentiated. This inevitably made system interaction extremely costly and time consuming. Developers began realizing the benefits of standardizing Web Service development. Using web standards seemed to be an intuitive and logical step toward attaining these goals. Web standards already provided a platform independent means for system communication and were readily accepted by information system users.

The end result was the development of Web Services. A Web Service forms a distributed environment, in which objects can be accessed remotely via standardized interfaces. It uses a three-tiered model, defining a service provider, a service consumer, and a service broker. This allows the Web Service to be a loose relationship, so that if a service provider goes down, the broker can always direct consumers to another one. Similarly, there are many brokers, so consumers can always find an available one. For communication, Web Services use open Web standards: TCP/IP, HTTP, and XML based SOAP.

At higher levels technologies such as XAML, XLANG, (transactional support for complex web transactions involving multiple web services) and XKMS (ongoing work by Microsoft and Verisign to support authentication and registration) might be added.

SOAP


Simple Object Access Protocol (SOAP) is a method for sending information to and from Web Services in an extensible format. SOAP can be used to send information or remote procedure calls encoded as XML. Essentially, SOAP serves as a universally accepted method of communication with web services. Businesses adhere to the SOAP conventions in order to simplify the process of interacting with Web Services.

A SOAP message contains either a request method for invoking a Web Service, or contains response information to a Web Service request.

Adhering to this layout when developing independent Web Services provides notable benefits to the businesses. Due to the fact that Web Applications are designed to be utilized by a myriad of actors, developers want them to be easily adoptable. Using established and familiar standards of communication ultimately reduces the amount of effort it takes users to effectively interact with a Web Service.

The SOAP Envelope is used for defining and organizing the content contained in Web Service messages. Primarily, the SOAP envelope serves to indicate that the specified document will be used for service interaction. It contains an optional SOAP Header and a SOAP Body. Messages are sent in the SOAP body, and the SOAP head is used for sending other information that wouldn't be expected in the body. For example, if the SOAP:actor attribute is present in the SOAP header, it indicates who the recipient of the message should be.

A web service transaction involves a SOAP request and a SOAP response. The example we will be using is a Web Service provided by Weather.gov. The input is latitude, longitude, a start date, how many days of forecast information desired, and the format of the data. The SOAP request will look like this:

The startDate was left empty because this will automatically get the most recent data. The format data type is not defined because it is defined in the WSDL document.

The response SOAP looks like this.

SOAP handles data by encoding it on the sender side and decoding it on the receiver side. The data types handled by SOAP are based on the W3C XML Schema specification. Simple types include strings, integers, floats, and doubles, while compound types are made up of primitive types.

Because they are text based, SOAP messages generally have no problem getting through firewalls or other barriers. They are the ideal way to pass information to and from web services.

Service Description - WSDL
Web Service Description Language (WSDL) was created to provide information about how to connect to and query a specific Web Service. This document also adheres to strict formatting and organizational guidelines. However, the methods, parameters, and service information are application specific. Web Services perform different functionality and contain independent information, however they are all organized the same way. By creating a standard organizational architecture for these services, developers can effectively invoke and utilize them with little to no familiarization. To use a web service, a developer can follow the design standards of the WSDL to easily determine all the information and procedures associated with its usage.

Essentially, a WSDL document serves as an instruction for interacting with a Web Service. It contains no application logic, giving the service a level of autonomy. This enables users to effectively interact with the service without having to understand its inner workings.

The following is an example of a WSDL file for a web service that provides a temperature, given a U.S. zip code.

The WSDL file defines a service, made up of different endpoints, called ports. The port is made up of a network address and a binding.

The binding identifies the binding style and protocol for each operation. In this case, it uses Remote Procedure Call style binding, using SOAP.

Port Types are abstract collections of operations. In this case, the operation is getTemp.

Finally, messages are used by the operations to communicate - in other words, to pass parameters and return values.

From the WSDL file, a consumer should be able to access data in a web service.

For a more detailed analysis of how this particular web service, please visit Weather.gov

Service Discovery - UDDI
You've seen how WSDL can be used to share interface definitions for Web Services, but how do you go about finding a Web Service in the first place? There are countless independent Web Services that are developed and maintained by just as many different organizations. Upon adopting Web Service practices and methodologies, developers sought to foster the involvement and creative reuse of their systems. It soon became apparent that there was a need for an enumerated record of these services and their respective locations. This information would empower developers to leverage the best practices and processes of Web Services quickly and easily. Additionally, having a central reference of current Web Service capabilities enables developers avoid developing redundant applications.

UDDI defines registries in which services can be published and found. The UDDI specification was creaed by Microsoft, Ariba, and IBM. UDDI defines a data structure and Application Programming Interface (API).

In the three-tier model mentioned before, UDDI is the service broker. Its function is to enable service consumers to find appropriate service providers.

Connecting to UDDI registries using Java can be accomplished through the Java API for XML Registries (JAXR). JAXR creates a layer of abstraction, so that it can be used with UDDI and other types of XML Registries, such as the ebXML Registry and Repository standard.

Using Java With Web Services
To execute a SOAP message, an application must be used to communicate with the service provider. Due to its flexibility, almost any programming language can be used to execute SOAP message. For our purposes, however, we will be focusing on using Java to interact with Web Services.

Using Java with web services requires some external libraries.

<ul> <li>Apache SOAP Toolkit</li> <li>Java Mail Framework</li> <li>JavaBeans Activation Framework</li> <li>Xerces XML parser</li> </ul>

Let's go through using Java to query the Temperature Web Service we talked about earlier.

This Java code effectively hides all the SOAP from the user. It invokes the target object by name and URL, and sets the parameter zipcode. But what does the underlying SOAP Request look like?

As you see, the SOAP request uses the parameters passed in by the Java Call to fill out the SOAP envelope and direct the message. Similarly, the response comes back into the Java program as '70.0'. The response SOAP is also hidden by the Java program.

Here's an additional example of using Java and SOAP to interact with Web Services. This particular Web Service is called the "US Zip Validator" and takes a ZipCode as a parameter, which then returns a corresponding latitude and longitude. When developing applications to interact with Web Services, the first step should be to review the WSDL document.

The WSDL document for this service is located here: http://www.webservicemart.com/uszip.asmx?WSDL

This document will contain all the necessary instructions for interacting with the "US Zip Validator" Web Service.

SOAPClient4XG

Modified by - Duncan McAllister From: http://www.ibm.com/developerworks/xml/library/x-soapcl/

This Java class refers to an XML document(SOAPRequest.xml), which is used as the SOAP message. This document should be included in the same project folder as the Java application invoking the service.

After reviewing the "US Zip Validator" WSDL document, it is clear that we would like to invoke the "getTemp" method. This information is contained within the SOAP body and includes the appropriate parameters.

SOAPRequest.xml

Following a successful interaction, the Web Service provider will provide a response that is similar in format to the user request. When developing in NetBeans, run this project and examine the subsequent SOAP message response in the Tomcat output window.

Web Services with Netbeans
The Netbeans version used for this explanation is 5.0.

After Netbeans is open, click on the "Runtime" tab on the left pane, then right-click "Web Services" and select "Add Web Service." In the "URL" field, enter the address of the web service WSDL file, in our example above it is "http://www.weather.gov/forecasts/xml/DWMLgen/wsdl/ndfdXML.wsdl" and click Get Web Service Description. This will bring up the information of the web service.

References and Links
<ul> <li>Weather.gov</li> <li>WebServices.org</li> <li>W3C's Web Services Reference</li> <li>UDDI.org</li> <li>XMethods.net</li> <li>Java API for XML Registries</li> <li>Apache SOAP Toolkit</li> <li>JavaMail Framework</li> <li>Java Activation Framework</li> <li>Xerces Java Parser</li> <li>Jasnowski, Mike. Java, XML, and Web Services Bible</li> <li>Microsoft Web Services</li> <li>http://www.xml.com/</li> <li>http://www.w3schools.com/soap/soap_intro.asp</li> <li>http://en.wikibooks.org/wiki/XML_-_Managing_Data_Exchange/Web_services</li> <li>http://www.w3.org/TR/2007/REC-soap12-part0-20070427/</li> <li>http://www.eweek.com/article2/0,1895,1589730,00.asp</li> <li>http://www.ibm.com/developerworks/xml/library/x-soapcl/</li> </ul>