Distributed Component Object Model (DCOM) is a proprietary Microsoft technology for communication among software components distributed across networked computers. DCOM, which originally was called "Network OLE", extends Microsoft's COM, and provides the communication substrate under Microsoft's COM+ application server infrastructure. It has been deprecated in favor of the Microsoft .NET Remoting, a part of their .NET Framework.
The addition of the "D" to COM was due to extensive use of DCE/RPC (Distributed Computing Environment/Remote Procedure Calls) – more specifically Microsoft's enhanced version, known as MSRPC.
In terms of the extensions it added to COM, DCOM had to solve the problems of
- Marshalling – serializing and deserializing the arguments and return values of method calls "over the wire".
- Distributed garbage collection – ensuring that references held by clients of interfaces are released when, for example, the client process crashed, or the network connection was lost.
- Aggregating hundreds or potentially tens of thousands of references to objects held by clients of interfaces at a single host, into a single "ping" function, in order to minimise bandwidth utilisation.
One of the key factors in solving these problems is the use of DCE/RPC as the underlying RPC mechanism behind DCOM. DCE/RPC has strictly defined rules regarding marshalling and who is responsible for freeing memory.
DCOM was a major competitor to CORBA. Proponents of both of these technologies saw them as one day becoming the model for code and service-reuse over the Internet. However, the difficulties involved in getting either of these technologies to work over Internet firewalls, and on unknown and insecure machines, meant that normal HTTP requests in combination with web browsers won out over both of them. Microsoft, at one point, attempted and failed to head this off by adding an extra http transport to DCE/RPC called ncacn_http (Network Computing Architecture, Connection-based, over HTTP). This was later resurrected to support a Microsoft Exchange 2003 connection over HTTP.
Read more about Distributed Component Object Model: Alternative Versions and Implementations, Procedure
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