Distributed Component Object Model
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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.
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.
DCOM is supported natively in Windows NT 4.0, Windows 2000, Windows XP, and Windows Server 2003. Besides this fact, the feature is still available on Windows 7, Windows 8, Windows Server 2008, Windows Server 2008 R2 and Windows Server 2012.
As part of the initiative that began at Microsoft as part of Secure Development Lifecycle to re-architect insecure code, DCOM saw some significant security-focused changes in Windows XP Service Pack 2. 
Alternative versions and implementations
COMsource: Its source code is available, along with full and complete documentation, sufficient to use and also 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 for binary interoperability purposes; they 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.
TangramCOM is a separate project from Wine, focusing on implementing DCOM on Linux-based smartphones.
The Samba Team is also implementing DCOM for over-the-wire interoperability purposes: unlike the Wine Team, they are not currently interested in binary-interoperability, as the Samba MSRPC implementation is far from binary-interoperable with Microsoft's MSRPC.
J-Integra for COM is a mature commercial pure Java implementation of the DCOM wire protocol allowing access to COM components from Java clients, and Java objects from COM clients.