Distributed Component Object Model

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Distributed Component Object Model (DCOM) is a Microsoft proprietary technology for software components distributed across several networked computers to communicate with each other. 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 Microsoft .NET.

The addition of the "D" to COM was due to extensive use of DCE/RPC – 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.

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 (initially, it was later resurrected to support an Exchange 2003 connection over HTTP) to head this off by adding an extra http transport to DCE/RPC called "ncacn_http" (Network Computing Architecture, Connection-based, over HTTP).

[edit] Alternative versions and implementations

The Open Group has a DCOM implementation called COMsource. The source code is available for COMsource, along with full and complete documentation, sufficient to use and also sufficient to implement an interoperable version of DCOM. According to that documentation, COMsource comes directly from the Windows NT 4.0 source code, and even includes the source code for a Windows NT Registry Service.

The Wine Team is also implementing DCOM. They are doing so for binary interoperability purposes, and are not currently interested in the networking side of DCOM, which is provided by MSRPC. They are restricted to implementing NDR (Network Data Representation) through Microsoft's API, but are committed to making it as compatible as possible with MSRPC.

j-Interop is an Open Source (LGPL) implementation of MSRPC purely in Java supporting DCOM client applications in Java on any platform communicating with DCOM servers.