What Is DCOM? | C++Builder 5 Developers Guide

Distributed COM is Microsoft's solution enabling COM objects to work on different machines across the network. It extends COM by enabling objects to be accessed remotely.

If you take a COM object and put it out somewhere separated from the client across the network, you get a DCOM object. DCOM is just plain COM with distribution and security added to it. Technically speaking, DCOM was made possible by the extension of Remote Procedure Calls (RPCs) to Object RPC, and it first appeared when Microsoft released Windows NT 4.0.

DCOM has been ported to some Unix flavors and can, with some work and platform restriction, be used as a cross-platform solution for distributed systems.

Despite other great competing technologies, DCOM is a mainstream technology. This is becuase of the growing acceptance by the corporate market of the Windows NT and Windows 2000 operating system family.

DCOM uses the concept of location transparency, which means that you can move a COM server to a different machine on the network without requiring code modification. Neither the server nor the client needs to be concerned about where the other is running. They can be on the same machine, on a local area network, or in a wide area network such as the Internet. For the client, the location of the server is a Registry setting that can be easily modified. This is unlike more comprehensive distributed solutions such as CORBA or J2EE that use a name server to identify the location of a remote object and, therefore, offer a truer location transparency.

DCOM clients and servers must abide by the rules of authentication and authorization. This implies that clients must perform some sort of log in to access the machine where the server lives. Servers must authenticate the client, verifying whether he is really who he says he is, and authorize access by checking whether the user has the necessary privileges. DCOM applications can rely on security settings stored in the Registry or can change their security settings through code. These two kinds of security are known as declarative security and programmatic security , respectively.

Unfortunately, security is often one of the biggest challenges in DCOM ”DCOM itself is not that complex. This means a significant amount of this chapter isn't about DCOM per se, but actually is about making security work for DCOM objects.

Windows OS Family and DCOM

DCOM comes standard with Windows 98, Windows NT 4.0, Windows 2000, and Windows XP, but you have to add it to Windows 95. You can download DCOM95 from Microsoft's Web site at http://www.microsoft.com/com or you can do a full install of Internet Explorer 4/5 in Windows 95 to get DCOM95 installed.

NOTE

If your network has a Domain Controller, you should configure your Windows 95 and 98 machines to use user-level security; otherwise , you won't be able to perform authenticated DCOM calls.

Windows 95 and 98 are really only suited for use as DCOM clients because they can't dynamically launch and execute DCOM servers and don't have native security.

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