ASP provided a much easier method of development, used a commonly known language, and also provided a rich set of built-in objects to help developers quickly put together applications. Finally, developers had a standard way in which to describe data and services. From there it was not long before someone, actually several someones, began to write applications that communicated with each other by passing XML documents. These were the first Web services. As you were reading about XML Web services' role in creating platform-independent applications and their use of HTTP protocols to communicate with other applications, you probably noticed the need for a standard format in which clients can actually send requested data, such as method calls with parameters, and receive Web service data, such as strings, arrays, and record sets.
Well, that formatting issue was solved with XML.
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SOAP documents are simply a way of marking up a call to an XML Web service method or function, including all of its required arguments. SOAP is also used to mark up all of the returned results so that the client can understand what is being sent back to it. Data is marked up with XML tags denoting the type of data being sent back, such as string or long, and the variable name to which the data was sent, in the case of parameters. This helps XML Web services and their client applications maintain type safety even across differing platforms.
When you develop an XML Web service, you will generate more than just the code that provides the functionality you wish to expose. You will also be creating a host of files, and in the case of UDDI, entries in existing files that will help support your service and describe it to the intended user community.
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Some of the files that you will create or edit when working with Web services are. NET application files. These are files that are created either by you using a simple text editor or, if you are using Visual Studios. This is the global file of your XML Web services. This file handles application-level events such as requests and sessions, both of which you will learn more about as you progress through this book. XML is also used to help prospective users find a given Web service.
Disco files, short for discovery files, are created and exposed to the Internet as the primary way of advertising a service. Disco files contain links that point to the service and the service's WSDL files.
Hour 5 covers the syntax and usage of Disco files. This repository allows developers looking to consume Web services to search for functionality that matches their needs and to contact the Web service owner about its use. WSDL documents list the exposed methods of a service and any parameters and return types that those methods expose. The contract is a promise that the listed methods will exist in the format described by the WSDL document.
The WSDL can be used either by developers or, as is more likely the case, by tools such as Visual Studios to create consumers client applications for the service and guarantee that requested services both exist and function in the manner that the client developer expects them to. Another key feature of the XML Web services framework is that they are built as components. What that means is that XML Web services are not built as complete, stand-alone programs, but rather as small building blocks to be used in the creation of other programs.
Many such blocks can be used from all over the Internet, an intranet, or both in order to create a single application. In fact, an XML Web service itself may make use of several other Web services in providing its functionality. Suppose you wished to develop an application that allowed users to track sports scores.
A new Universal API with supporting technologies that allows applications to be developed using Internet based methods. Below is a graphical depiction of the Web Services architecture, which some refer to as a Service Oriented architecture.
From a high level view, the Provider publishes their Web Services in the Registry, where the Requestor can find it. With the details provided by the registry the Requestor is able to bind to that service offered from the Provider and access the Web Service. The major advantage of Web Services is based on independence, not being tied to any specific implementation of the following:. The follow is a list of companies that have a Web Services presence.
The list is not extensive, but does identify the key major players, and what they offer. The new MS. NET have Web Services integrated into its core. IBM is also considered to be one of the leading Web Services companies. For development and testing purposes the Web Services Toolkit is used as a platform for testing new specifications, such as the new WS-Security specification.
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Sun created the Internet programming language Java in the early 90s. Sun originally positioned their new language for web based client applications based on the applet technology, but is more prevalent today in servers using servlet technology. Apache Open Software Foundation has many successful projects ranging from the http web server, to the Tomcat reference Servlet container. One of the advantages of Axis project is source code is published allowing developers to know exactly what is going on.
XML is one of the main enabler for Web Services. For the purpose of the paper we will focus on a couple components that are critical to using and understanding Web Services. The Structure of XML documents are made up of entities in a markup format that provides the storage layout and logical structure 2.
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XML can be thought of a common meta-language used to define other languages. Tim Bray, co-editor of the XML 1. The best way to understand this principle is to look at an example XML file. Below is an example ArcXML request. As with most XML languages it is easier to provide an example of what a namespace is. Below are two versions of the same fictitious XML document fragments that contain contact information for a Forest Tenure Licensee. With the second code fragment adding the namespace constructs. Also notice it is in the format of a web URL. Unfortunately people get the wrong impression that it must point to a valid address, when it only needs to be a unique identifier.
Namespaces enable programs to parse concisely and completely define all elements of an XML file given. XML Schema is a very complex specification, broken in to these 3 parts including hundreds of pages of technical specifications for each part. XSD for it schema file extension. This schema can then be used to validate a group of documents based on this one schema or combined with other schemas through the use of namespaces.
To help understand XSD functionality the following is a simple schema used to define the types, structure and validates the fictional Licensee from the earlier example, Code 3. Because this schema is a valid XML document. The schema has changed a lot over the course of its development, so you might see a different namespace that refers to earlier versions of the XML Schema specification. The contact structure is defined to be made up of a sequence of elements. XML schema allows you to place a restriction of how many occurrences of elements or structures.
The address string for the contact type has being given a maximum string length of 50 chars. The AAC element has being defined to use a range of 1 to A type could be defined to include a pattern, such as one for the telephone number string of xxx xxx-xxxx. Besides clarifying the type and structure for each element, XSD provides validation functionality that has traditional had been done in computer programs and applications.
This separates validation from the individual programs and associates it more closely with data for all programs that use the data. In the current W3C working draft the Working Group has jointly come to agree on the following working definition for a Web Service:. A Web service is a software application identified by a URI, whose interfaces and binding are capable of being defined, described and discovered by XML artifacts and supports direct interactions with other software applications using XML based messages via internet-based protocols The Server has a manager that listens for requests that translates the request into a form it can understand.
For certain types of distributed and peer-to-peer applications, web services play an important role. If you use distributed computing over an uncontrolled network such as the Internet rather than over a LAN or corporate network, you might use web services. Web services do not require state maintenance, thus offering potentially improved performance, particularly where a request-response behavior is not absolutely required. For applications that require strict request-response behavior and high security, you should consider using an older, more controlled model, such as COM or.
NET remoting. Certain files are automatically generated when you create applications with ASP. These files enable the ASP. NET Web Services to render their services through a web server. The following table lists the files and their descriptions. Building an ASP. Accessing an ASP. Microsoft Overview of Web Services. RAD Studio for Microsoft. Collapse All. Web Service Prerequisites. Web Service Scenarios. When you create an ASP.
NET Web Services application, a text file is automatically generated with the. The required Web Services directive is placed at the top of this file to correlate between the URL address of the web service and its implementation. Within the. This file is compiled into an assembly, along with other files, for deployment. NET Web Service application, a code-behind file is generated with a language-specific extension. You add your Web Services logic to the public method to process Web Services requests and responses.
This file is generated when you click the Add Web Reference feature to add the web service to your client application. It describes the Web Services interface available to the client. This file enables the discovery of a web service that is exposed on a given server. It also contains links to other resources that describe the web service. See Also.