Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

COM (Component Object Model) is a Microsoft specification that defines the interaction between components in the Windows environment. A component is a self-contained coded module that provides some service to other components in an object-oriented environment. The topics "Object Technologies" and "Distributed Object Computing" describe this concept further.

COM was introduced in 1993, and its first major implementation was in Windows 95, although it is the basis of OLE (Object Linking and Embedding), which is an object-based service that allows applications running in the same computer to interact and share information. In 1996, Microsoft created ActiveX, which is basically OLE designed to work in Web-centric environments. ActiveX components are downloadable controls that enhance the Windows interface on a user's computer.

DCOM (Distributed Component Object Model) is the network version of COM that allows objects running in different computers attached to a network to interact. Microsoft describes DCOM as "COM with a long wire." DCOM works across a variety of network transports, but most important, it works across TCP/IP protocols and the Internet.

The latest version of COM, specifically designed to work in the Windows 2000 environment, is COM+. This new version has new services that enhance its ability to act as an ORB (object request broker). It supports messaging, directory services, and transaction services. In fact, MTS (Microsoft Transaction Server) was a separate product that Microsoft integrated with COM+. Other features include the ability to load balance traffic between servers in a clustered environment, enhanced security, and the ability of applications to operate in either a disconnected (asynchronous) or connected (synchronous) state (as described under the "Middleware and Messaging" topic).

One of the most important features of COM+ was added later. Microsoft replaced the interobject communication protocol with a message-passing approach that uses standard XML, thus advancing its component model into the realm of universal objects. Basically, it replaced RPC (remote procedure call) with a new protocol called SOAP (Simple Object Access Protocol).