FAQ 34.38 What is DCOM?

Distributed COM the distributed version of COM.

Distributed COM extends COM to support communication between callers and components that are located on different computers. DCOM does not really change much about how callers create and interact with COM objects, and therefore, DCOM lets callers remain unaware of where a COM object is running. The one noticeable difference is, of course, that network latency means that accessing a remote object is slower than accessing a local object. DCOM does introduce some differences, especially in the areas of security and the registry.

DCOM uses reference counting and pinging (pinging is a technique whereby one computer or one program sends ping messages to another computer or another program to determine whether or not it is running and to verify that there is a valid connection between the two) to handle garbage collection of objects and the servers in which the objects are running. As always, callers call AddRef when they obtain pointers to interfaces and call Release when they no longer need the pointer to the interface. When the reference count on an object hits zero, it can be garbage collected and its resources can be freed. Since reference-counting schemes are vulnerable to machine crashes and network problems, DCOM also uses a mechanism that sends pings on a computer-by-computer basis to detect nonexistent callers and dangling references so that unneeded objects can be garbage collected.

Although DCOM extends COM across machine boundaries it does not try to solve every possible distributed programming problem. Some of these other problems are handled by Microsoft Transaction Server and Microsoft Message Queueing.

Software AG has ported DCOM to Sun Solaris, 64-bit Digital Unix, and IBM's OS 390.