Distributed Component Object Model 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. 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. It had to combine Hundreds/Tens of
Thousands objects held in the client's browser with a single transmission in
order to minimize bandwidth utilization. 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. 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, as well as Windows 7, Windows 8, Windows 10, Windows Server
2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2 and
the Windows Server 2016 Technical Preview.
Hardening 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 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.
See also ActiveX
Component Object Model Dynamic Data Exchange
.NET Remoting OLE for Process Control
References External links
DCOM Remote Protocol Specification The Open Groups COMsource
COMsource TangramCOM
EntireX DCOM