O'Reilly Book Excerpts: .NET Framework Essentials, 2nd Edition
Editor's Note: This is the first of four excerpts from the second edition of O'Reilly's .NET Framework Essentials, 2nd Ed. by Thuan L. Thai and Hoang Lam. The complete series of excerpts covers Web services in the context of the .NET framework. In this first part, they give a .NET Web services framework overview.
Web Services allow access to software components through standard web protocols such as HTTP and SMTP. Using the Internet and XML, we can now create software components that communicate with others, regardless of language, platform, or culture. Until now, software developers have progressed toward this goal by adopting proprietary componentized software methodologies, such as DCOM; however, because each vendor provides its own interface protocol, integration of different vendors' components is a nightmare. By substituting the Internet for proprietary transport formats and adopting standard protocols such as SOAP, Web Services help software developers create building blocks of software, which can be reused and integrated regardless of their location.
In this chapter, we describe the .NET Web Services architecture and provide examples of a Web Service provider and several Web Service consumers.
Web Services in Practice
You may have heard the phrase "software as services" and wondered about its meaning. The term service, in day-to-day usage, refers to what you get from a service provider. For example, you bring your dirty clothing to the cleaner to use its cleaning service. Software, on the other hand, is commonly known as an application, either off-the-shelf, or a custom application developed by a software firm. You typically buy the software (or in our case, build the software). It usually resides on some sort of media such as floppy diskette or CD and is sold in a shrink-wrapped package through retail outlets.
How can software be viewed as services? The example we are about to describe might seem far-fetched; however, it is possible with current technology. Imagine the following. As you grow more attached to the Internet, you might choose to replace your computer at home with something like an Internet Device, specially designed for use with the Internet. Let's call it an iDev. With this device, you can be on the Internet immediately. If you want to do word processing, you can point your iDev to a Microsoft Word service somewhere in Redmond and type away without the need to install word processing software. When you are done, the document can be saved at an iStore server where you can later retrieve it. Notice that for you to do this, the iStore server must host a software service to allow you to store documents. Microsoft would charge you a service fee based on the amount of time your word processor is running and which features you use (such as the grammar and spell checkers). The iStore service charges vary based on the size of your document and how long it is stored. Of course, all these charges won't come in the mail, but rather through an escrow service where the money can be piped from and to your bank account or credit card.
This type of service aims to avoid the need to upgrade your Microsoft Word application. If you get tired of Microsoft Word, you can choose to use a different software service from another company. Of course, the document that you store at the iStore server is already in a standard data format. Since iStore utilizes the iMaxSecure software service from a company called iNNSA (Internet Not National Security Agency), the security of your files is assured. And because you use the document storage service at iStore, you also benefit from having your document authenticated and decrypted upon viewing, as well as encrypted at storing time.
All of these things can be done today with Web Services.
In fact, Microsoft has launched a version of the "software as service" paradigm with its Passport authentication service. Basically, it is a centralized authentication service that you can incorporate into your web sites. At sites using the Passport authentication service, it's no longer necessary to memorize or track numerous username/password pairs.
Recently, Microsoft also announced .NET My Services (formerly codenamed "HailStorm"), a set of user-centric Web Services, including identification and authentication, email, instant messaging, automated alert, calendar, address book, and storage. As you can see, most of these are well-known services that are provided separately today. Identification and authentication is the goal of the Passport project. Email might map to Hotmail or any other web-based email services. Instant messaging and automated alert should be familiar to you if you use MSN Messenger Service or AOL Instant Messenger. A calendar and address book are usually bundled together with the web-based email service. Consolidating these user-centric services and exposing them as Web Services would allow the user to publish and manage his own information.
A .NET My Services customer can also control access permission to the data to allow or restrict access to content. These services also allow other users, organizations, and smart devices to communicate and retrieve information about us. For example, how many times have you been on the road with your mobile phone and wanted your contact list from Outlook? Your mobile phone should be able to communicate with your address book Web Service to get someone's phone number, right? Or better yet, if your car broke down in the middle of nowhere, you should be able to use your mobile phone to locate the nearest mechanic. The user is in control of what information is published and to whom the information will be displayed. You would probably have it set up so that only you can access your address book, while the yellow pages Web Service that publishes the nearest mechanic shop to your stranded location would be publicly accessible to all.
Currently, users store important data and personal information in many different places. With .NET My Services, information will be centrally managed. For example, your mechanic might notify you when it's time for your next major service. Or when you move and change your address, instead of looking up the list of contacts you wish to send the update to, .NET My Services will help you publish your update in one action.
The potential for consumer-oriented and business-to-business Web Services like .NET My Services is great, although there are serious and well-founded concerns about security and privacy. In one form or another, though, Web Services are here to stay, so let's dive in and see what's underneath.
Web Services Framework
Web Services combine the best of both distributed componentization and the World Wide Web, extending distributed computing to broader ranges of client applications. The best thing is that this is done by seamlessly marrying and enhancing existing technologies.
Web Services Architecture
Web Services are distributed software components accessible through standard web protocols. The first part of the definition is similar to COM/DCOM components. However, it is the second part that distinguishes Web Services from the crowd. Web Services enable software to interoperate with a much broader range of clients. While COM-aware clients can understand only COM components, Web Services can be consumed by any application that understands how to parse an XML-formatted stream transmitted through HTTP channels. XML is the key technology used in Web Services and is used in the following areas of the Microsoft .NET Web Services framework:
- Web Service wire formats
- The technology enabling universal understanding of how to perform data exchanges between the service provider and consumer; the format of data for the request and response.
- Web Service description in WSDL (Web Services Description Language)
- The language describing how the service can be used. Think of this as the instructions on the washing machine at the laundromat telling you where to put quarters, what buttons to push, etc.
- Web Service discovery
- The process of advertising or publishing a piece of software as a service and allowing for the discovery of this service.
Figure 6-1 depicts the architecture of web applications using Windows DNA, while Figure 6-2 shows .NET-enabled web applications architecture. As you can see, communication between components of a web application does not have to be within an intranet. Furthermore, intercomponent communication can also use HTTP/XML.