This chapter is from the book
This chapter is from the book
This chapter is from the book
The Vision
The Web has evolved a long way from browsing static Hypertext Markup Language (HTML) pages. Today, users can download music, participate in auctions, buy items online, and even talk to their family face-to-face over the Internet. Even businesses are not behind. They have been implementing business-to-business (B2B) and business-to-consumer (B2C) applications that communicate over the Internet.
Microsoft believes that the Internet will evolve from a collection of isolated Web sites and applications into a general "communication bus" for distributed applications. Individual parts of the distributed application could be running on different hardware and software platforms. The computing devices include your desktop systems as well as mobile devices such as cellular phones, Pocket PCs, personal digital assistants (PDAs), and so on. Even household appliances such as microwaves and dishwashers will participate in this communication over the Internet.
Web Services
To be fair, this vision of anytime, anywhere, any-device computing is also shared by many other software companies, such as IBM and Hewlett-Packard, and many respected computer scientists around the world. A key technology enabler for this distributed computing model is Web services. A Web service can be defined as a service that can be accessed programmatically over the Web. Companies can make their business applications available as Web services. These Web services, for example, can be used to integrate applications within various divisions of the same company. The Web services can also be used to automate communication over the Internet between two companies.
To be able to develop distributed applications that transcend geographical, hardware, and OS boundaries, Web services need to be based on universally accepted standards. Table 1.1 lays out the foundation elements of Web services.
TABLE 1.1. Web Services Foundation
|
Standard |
Purpose |
|
Internet |
Ubiquitous communication |
|
Extensible Markup Language (XML) |
Universal data format |
|
Simple Object Access Protocol (SOAP) |
Communication protocol |
|
Web Services Description Language (WSDL) |
Describe the semantics of the methods available on a Web service |
|
Universal Description, Discovery, and Integration (UDDI) |
Publish and find Web services |
In the "anywhere computing" vision, clients that wish to access Web services can be geographically distant from the servers. As the Internet has a broad geographical reach, it makes sense to deliver the services over the Internet.
To develop distributed client–server applications that transcend hardware and OS boundaries, Extensible Markup Language (XML) has been accepted as the universal language for defining data formats. XML provides a common data format that does not require business partners or customers to use a particular programming language, application, OS, or hardware.
XML by itself is not enough to achieve the client–server communication. To access a Web service, a client has to make a procedural call to the server, pass in the needed parameters, and get back the return value. A protocol has to be defined for such an exchange of information. To this effect, the W3C1 has defined a protocol called Simple Object Access Protocol (SOAP). SOAP is a lightweight protocol for exchange of information in a decentralized, distributed environment. It specifies how a remote procedure call can be expressed in XML format. It is an XML-based protocol that consists of three parts:
An envelope that defines a framework for describing what is in a message and how to process it.
A set of encoding rules for expressing instances of application-defined data types.
A convention for representing remote procedure calls and responses.
Although the SOAP specification is independent of the underlying transport protocol, Hypertext Transport Protocol (HTTP) has been the sweet spot for the industry. Most companies let HTTP traffic pass through the firewall. Contrast this to other distributed object technologies such as Distributed Component Object Model (DCOM) and Common Object Request Broker Architecture (CORBA) that require opening ports on the firewall, thus compromising security.
Also note that although the client and the server can communicate with each other using raw SOAP packets, helper utilities are available on most platforms to hide the grunge work of creating SOAP packets:
The client makes a method call passing in the required parameters.
A helper utility on the client side packages the method call and its parameters into a SOAP-compliant XML format and sends the SOAP packet to the remote server over a network protocol, preferably HTTP.
A helper utility on the server side unpackages the SOAP packet and calls the actual method, passing in the method parameters. On returning from the method, the utility repackages the return value into a SOAP packet and sends it back to the client.
The client-side utility unpackages the SOAP packet and returns the value to the client.
From a programming perspective, using the SOAP helper utilities makes calling a method to a remote system as simple as making a local method call.
Why is SOAP important? Because it provides the foundational invocation mechanism for application-to-application computing, irrespective of the underlying hardware or operating system platforms.
The SOAP specification is a work in progress. The current draft of the specification can be found at W3C's Web site [W3C-01].
Now we know how to make method calls on a Web service programmatically. However, we still don't know what methods are available as part of the Web service. We need a mechanism that describes the programmatic "interface" of the Web service; that is, the methods available on the Web service, the parameters for each method, and the return value of each method. A popular choice is to define this interface in Web Services Description Language (WSDL), an XML-based language that lets you express the functions and formats supported at any endpoint of the service. This programmatic interface is referred to as the contract of the Web service.
At this point, we know how to obtain method information on a Web service and how to make the method call. The remaining problem is to identify the server running the Web service.
It is likely that in some cases the server is known to the client. However, it is possible that the client is not particularly happy with the quality of the service or the cost of accessing the service, and may wish to use a different server. The beauty of the Web services programming model is that it doesn't matter which server provides the service, as long as the server adheres to the Web service contract. Coding-wise, all that is needed is to point to the right server. There is no change required to the rest of the code.
An industry-wide effort is underway to promote e-commerce among businesses. This project, called Universal Description, Discovery, and Integration (UDDI2 ), is an initiative to create an open framework for describing Web services, discovering businesses, and integrating business services over the Internet. UDDI enables business applications to do the following:
Discover each other.
Define how they interact over the Internet.
Share information in a global registry that will more rapidly accelerate the global adoption of B2B e-commerce.
Essentially, UDDI provides the "yellow pages" on the Internet for the industry. UDDI has also embraced SOAP and WSDL, making it convenient to obtain information from its repository programmatically.
Note that standards such as XML, SOAP, WSDL, UDDI, and so on, are not proprietary to Microsoft, although Microsoft has been a major contributor in driving these standards.
Microsoft's .NET initiative is built around XML, SOAP, and WSDL. The .NET technology and tools make it easy for companies to develop Web services and to consume other Web services.
Heterogeneous Environment
It is possible that Web services and other future applications may run on a variety of computing devices, not just PCs or mainframes. These devices need not run the same operating system. Microsoft Windows is not the only choice for the OS. Therefore, jointly with Intel and Hewlett-Packard, Microsoft has submitted the core .NET Framework specifications to European Computer Manufacturer's Association (ECMA3 ).This ECMA specification is referred to as the Common Language Infrastructure (CLI). The CLI specifications are not wedded to any OS. The .NET runtime is Microsoft's implementation of the CLI for Windows OS. However, Microsoft has also made available the source code to a working implementation of ECMA CLI that builds and runs on FreeBSD, a variation of the UNIX OS. Currently, there are various other initiatives underway to implement CLI on other variations of UNIX such as Linux.
Among other things, the CLI also specifies that a CLI-compliant application must run on different platforms without being rewritten for each specific platform. A .NET application, for example, can run on many processors and platforms (currently, only x86 compatible CPUs are supported) as long as no OS-specific calls are made. So, if things go as expected by various implementers of CLI, you will be able to take a .NET executable that is built on one OS and run it on many other Windows and non-Windows OSs.
Smart Devices
In the not so distant future, Microsoft expects that PCs will be joined by many new kinds of smart devices such as data-enabled wireless phones, handheld computers, tablet PCs, home appliances, and so on. If an application has to run on all these devices, the application will have to automatically adapt its user interface to the capabilities of the device it runs on. This not only means adapting to each device's display and input capabilities, but also supporting new modes of communication such as spoken language and handwritten text.
To support software development for the smart devices, Microsoft has announced to release a subset of the .NET Framework called the .NET Compact Framework.
Compelling User Experience
Microsoft believes that, in this new distributed computing world, the experience should be very simple and compelling for the end users. To provide such experience, Microsoft intends to host a set of foundation services or building-block services. These building-block services will act as a central repository of data for users, allowing them to store e-mail, calendar information, contacts, and other important data, and present this data as needed to other Web sites.