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

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## Chapter 12
DCOM and the Internet

• DCOM and ActiveX are intended to work with the Internet • But Microsoft’s approach to the Internet is different from other vendors • The main products are Internet Explorer and Information Internet Server • Script language used as VBScript and Jscript • The approach is unique to Microsoft

We have looked at the platform support for DCOM; the next big question most people are likely to ask is does it work over the Internet? This chapter thus looks at Microsoft’s approach for supporting the Internet, the products it has, the strategy it recommends its users take, and the links with DCOM.

We will be starting this chapter with a description of the approach taken by some of the other middleware vendors so that you can contrast the approach Microsoft has taken with that used by them. Microsoft’s approach is quite different, and we will be looking at the reasons why and the implications of this in this chapter.

### How Do Other Middleware Vendors Provide Internet Support?

Several middleware vendors currently support the Internet—NCR with TOP END using a module called Java Remote Client services, Transarc with Encina using DE-Light Web Client, and BEA with Tuxedo using a module called Jolt. All of them use an identical approach—an approach which uses Java, Web browsers, and Web servers—and all of the above products are aimed at organizations wanting to support commercial transactions on the Internet using the Web.

Figure 12.1 DCOM and the Internet

The Internet and its protocols are normally unable to recognize the state of a transaction, which makes the multistep, often complex interactions which take place within a transaction largely impossible to support. DE-Light Web Client, JRC, and Jolt solve this problem by combining distributed transaction processing with software that can sustain transaction interactions over the Web.

Let me start by explaining the way the Web normally works. If you are a Web expert you can ignore the next few subsections, otherwise read on for an explanation of Web and Internet technology.

### Internet Technology Explained

The Internet and the World Wide Web normally work using TCP/IP as the network protocol, Web browsers communicating with Web servers, URLs as the addressing mechanism, CGI to invoke programs, HTML to describe documents, and HTTP as the communication protocol. The Web is accessed using a Web browser on a client, which connects with a Web server. The Web browser (for example, Netscape’s Navigator or Microsoft’s Internet Explorer) running on a PC or similar machine is used as the user’s point of access to the Internet. The Web server is located elsewhere on the network and handles the storage of information and retrieval on behalf of the client.

URLs

When a user wants to connect to a Web server, he uses a “logical” address name called the URL—Uniform Resource Locator. The URL is used for all Internet communication and has a format like that shown below:

       aa://aa.aa.aa.aa/aa/aa/aa/aa.aa#aa

The first set of aas describe the data source/protocol. The Internet actually supports a number of protocols:

  • a file accessible through the File Transfer protocol (ftp)
  • a file accessible through the Gopher protocol (gopher)
  • a hypertext document—typically an HTML file accessible through the HyperText Transfer Protocol HTTP (of which more later)
  • mail
  • a news group accessible through the News Transfer Protocol (NNTP)
  • telnet

The Internet can thus act as an e-mail application, file transfer application, and data access application, among other things. The particular protocol used for Web access is the HTTP protocol.

The rest of the URL is then the address—in the case of Web access, the Web server name, the directory in which to find the Web page you’re looking for, and even down to a particular spot in the particular page. The address, in other words, is not just the address of the Web server, it is the address of the particular bit of information you’re wanting to look at on the Web server. This means that a company can set up a number of “Web sites” telling you about different things—support and help, general information, and so on.

HTTP

Communication from the PC to other nodes is via the protocol HTTP-HyperText Transfer Protocol. HTTP is just the particular protocol used for communication in the World Wide Web. It is not a network protocol, but an application protocol—it defines how Web browsers and Web servers talk to one another. The Internet is, of course, based on the network protocol TCP/IP.

Information exchanges on the Web happen in four parts all classed as message types for HTTP—connection, request, response, and close. These message types bear a strong similarity to the RPC mechanisms we have discussed in this book.

  • Connection-When connecting, the browser will display status messages on the connection—connecting, for example, or timed out if it can’t make the connection. The Web browser finds the server’s IP address from what is called a local domain name server. The Internet is based on the TCP/IP standard and the IP address is a unique number for each machine on the Internet. IP addresses are physical addresses, for example, 193.1.1.19.56. The Domain Name Server converts the logical name the user has used to the physical IP address for the machine. In effect, the DNS is a Directory Service.
  • Request-The client asks for the Web resource that it is looking for with the name of the protocol to use, the name of the object to find, and information about how the server should respond.
  • Response-After the Web browser has made a connection to the Web server, it downloads a Web page (or pages). If the server cannot deliver the information as requested by the client, it sends back an error message explaining why it couldn’t deliver.
  • Close-After the information requested has been delivered, the connection between client and server is closed.

Once HTTP has completely transferred the information requested, it has done its part. The browser then takes over to interpret and display the information.

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