Distributed Component Object Model (DCOM) is aproprietaryMicrosoft technology for communication betweensoftware components on networkedcomputers. DCOM, which originally was called "NetworkOLE", extends Microsoft'sCOM, and provides the communication substrate under Microsoft'sCOM+ application server infrastructure.
The extension COM into Distributed COM was due to extensive use ofDCE/RPC (Distributed Computing Environment/Remote Procedure Calls) – more specifically Microsoft's enhanced version, known asMSRPC.[1]
In terms of the extensions it added to COM, DCOM had to solve the problems of:
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.[2]
DCOM was a major competitor toCORBA.[3] Proponents of both of these technologies saw them as one day becoming the model for code and service-reuse over theInternet. However, the difficulties involved in getting either of these technologies to work over Internetfirewalls, and on unknown and insecure machines, meant that normalHTTP requests in combination withweb browsers won out over both of them.[4] Microsoft, at one point, attempted to remediate these shortcomings by adding an extra HTTP transport to DCE/RPC calledncacn_http (Network Computing Architecture connection-oriented protocol).[5]
DCOM was publicly launched as a beta for Windows 95 September 18, 1996.[6]
DCOM is supported natively in all versions of Windows starting from Windows 95, and all versions of Windows Server since Windows NT 4.0[7]
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.[8]
In response to a security vulnerability reported by Tencent Security Xuanwu Lab in June 2021,[9] Microsoft released security updates for several versions of Windows and Windows Server, hardening access to DCOM.[10]
COMsource is a Unix based implementation of DCOM, allowing interoperability between different platforms. Its source code is available, along with full and complete documentation, sufficient to use and also implement an interoperable version of DCOM. COMsource comes directly from theWindows NT 4.0 source code, and includes the source code for aWindows NT Registry Service.[11]
In 1995, Digital andMicrosoft announced Affinity for OpenVMS (also known as NT Affinity) which was intended to allow OpenVMS to serve as thepersistence layer for Windows NTclient-server applications.[12] As part of this initiative, an implementation of the Distributed Component Object Model (DCOM) was added to OpenVMS Alpha.[13] In order to support DCOM, VMS was provided with implementations of the Windows Registry,NTLM authentication, and a subset ofWin32 APIs needed to support COM.[14] DCOM was first added to OpenVMS V7.2-1 for the Alpha.[15] A similar implementation of DCOM was added toDigital Unix as part of the AllConnect program.[13]
TangramCOM was a separate project from Wine, focusing on implementing DCOM on Linux-based smartphones.[16]
