| DCOM Explained by Rosemary Rock-Evans Digital Press ISBN: 1555582168 Pub Date: 09/01/98

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Microsoft Internet Information Server

Internet Information Server (IIS) is Microsoft’s Web server for Windows NT version 4 (and upwards). IIS was code-named Gibraltar and was launched in February 1996. IIS version 3.0 was released in December 1996. IIS version 4 and Windows NT 5.0 have more code in common than the previous version of IIS—again a precursor to the integration of Internet and Windows technologies now taking place in version 5 of Windows NT.

IIS supports ISAPI—a set of API extensions to the Windows API that developers can use instead of CGI or Perl to access other applications, or ISAPI can be used with Perl/CGI/Rex scripts. IIS can support ordinary “static” pages of information, or it can support Active Server Pages.

Static pages can be the normal HTML text files or images and so on. Pages can be created using any HTML generation tools, but Microsoft has a group of tools called Internet Assistants that can be used to convert documents in Word, Excel, or PowerPoint into HTML versions ready for loading onto a Web site. There is also an Internet Assistant for Access95, which converts data into HTML tables and others for Schedule Plus, Outlook, and SQL Server.

PowerPoint users can also use the ActiveX Animation Player, which shows Powerpoint’s animation, sound, and special effects. The presentation then appears in the Browser window.

Microsoft Active Server Pages (code-named Denali)—Support server-side generation of Web pages “on-the-fly.” The Web page is thus no longer static, but a dynamically created page of text, images, and data. Perhaps key is the fact that the state can be preserved in the server across pages.

Active Server Pages are the key to Internet support for Microsoft and form the foundation of their distributed application support from the Web. We will see this in a moment.

The developer can use server-side scripting and basic templates to create Active Server pages, generally called “active page generation.” Scripting languages that Microsoft supports include JavaScript and VBScript. In fact, Microsoft’s version of Java Script is called JScript and is slightly different from JavaScript. Both scripting languages can be used to control Java applets or ActiveX controls written in Visual Basic 5, C++, Borland Delphi, and so on.

Options for distributed application developer

We have seen how DCOM worked on Windows NT and how it worked when it ran on other platforms. Given what we have just learned about Microsoft’s strategy and its products, what are the options open to the developer if he or she wants to build distributed applications using DCOM that use the Internet? There are only two approaches that the developer can use, and they depend upon whether the user is a known user or an unknown user:

Unknown user—In this case, the developer has to use direct communication from a Web browser to the Web server. Where this approach is used, the user uses the Web browser to access a page of information that is downloaded from the Web server. Where communication is directly between Internet Explorer and the Web server (IIS), normal HTTP is used.

Figure 12.5 Example configuration for an unknown user

This page will be an Active Server Page, which, via a CGI Script or ISAPI, accesses a controlling server program on the Web server host. This program can then use COM to access components on Windows NT machines, other platforms, or on the MVS machine using Cedar. A connection exists between IIS and DCOM—particularly Microsoft’s Transaction Server, which means that server applications can be invoked via IIS and put under transaction control.

As we have seen in our description of other products, HTTP is a limited protocol supporting only a request/reply format and is also unable to sustain the sort of context information which may be needed if transactions are invoked over the Web. It is a useful method of communication, however, if only request/reply communication is needed. Microsoft has extended the capabilities of the Web server by adding Active Server page capability, so even a simple request/reply could produce quite a comprehensive result. But we must still realize the limitations of this approach in comparison with the approach used by other middleware vendors.

Known user—Where the user is known, the Internet may still be in use, but the desktop environment of the end user is under the control of the company and internal to the company. There are two basic assumptions made about the environment in which the user is working:

  • First, it is assumed the user will be using Windows NT or Windows 95, will have Internet Explorer, and will have a copy of the JVM. COM services will be already available as they are supplied with Windows NT and 95.
  • Second, a copy of the client application will not be available on the user’s machine. Thus the machine is configured for DCOM use but has not been configured with the application.

The user uses the Web browser to access a page of information that is downloaded from the Web server. The page contains Java applets or ActiveX controls (usually the latter) downloaded with the page. Once the controls have been downloaded, they then intercommunicate with each other and via components on server machines using DCOM. In effect, once the ActiveX controls have been downloaded, DCOM takes over, and the Web browser is bypassed. The Web browser has simply been used as a means of downloading Active X components to the user’s machine; from the point where communication via DCOM begins, the configuration behaves like an ordinary client server DCOM configuration.

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