Component Object Model

COM arose out of the need for an efficient method of interprocess communication. COM provides a group of conventions and supporting libraries that allow interaction between different pieces of software in a consistent, object-oriented way. COM objects can be written in many languages, including C++, Java, and Visual Basic, and they can be implemented in dynamic link libraries (DLLs) or in their own executables, running as distinct processes.

A client using a COM object need not be aware of either what language the object is written in or whether it is running in a DLL or a separate process. This functionality is achieved because COM defines an object concept known as an interface, which is a collection of methods that a client application can call. Interfaces are assembled and exposed to the system in the same way, regardless of the language used to create the object. This design provides a system whereby any COM-based component can communicate with any other existing or future COM-compliant component. These components can be created and accessed both on local and remote computers.

Distributed COM (DCOM) allows objects to be created and accessed on remote computers. DCOM provides a standard protocol that can sit on top of any standard networking protocol. If connectivity is established between computers at the network layer, DCOM-based communications can occur.

DCOM runs on top of these network protocols:

DCOM communications also work between dissimilar computer hardware platforms and operating systems. If DCOM has been implemented on both ends of a communication, it does not matter to either the client or the component which operating system is executing the other.

These platforms currently support DCOM, or will support it in the future:

Developing DCOM servers capable of processing a few clients is manageable. However, developing servers capable of processing thousands of transactions can be daunting. In an effort to facilitate the development of scalable DCOM servers, Microsoft has developed and released MTS, which is designed to insulate developers from complex system-oriented tasks, such as process and thread management, and from involvement in development-intensive activities, such as directory management. MTS handles all of the applications' infrastructure, allowing developers to focus on business logic. Support is provided for accessing SQL Server and Oracle databases within MTS.