Editing Distributed Component Object Model

{{Short description|Software for communication between software components}}

'''Distributed Component Object Model''' ('''DCOM''') is a [[proprietary software|proprietary]] [[Microsoft]] technology for communication between [[software componentry|software components]] on networked [[computer]]s. DCOM, which originally was called "Network [[Object Linking and Embedding|OLE]]", extends Microsoft's [[Component Object Model|COM]], and provides the communication substrate under Microsoft's [[Component Object Model#COM+|COM+]] application server infrastructure.

The extension COM into Distributed COM was due to extensive use of [[DCE/RPC]] (Distributed Computing Environment/Remote Procedure Calls) – more specifically Microsoft's enhanced version, known as [[MSRPC]]. <ref>{{Cite web |title=What is Distributed Component Object Model (DCOM)? |url=https://www.techtarget.com/whatis/definition/DCOM-Distributed-Component-Object-Model |access-date=2022-08-03 |website=WhatIs.com |language=en}}</ref>

In terms of the extensions it added to COM, DCOM had to solve the problems of:
*[[Marshalling (computer science)|Marshalling]] – serializing and deserializing the arguments and return values of method calls "over the wire".
*Distributed [[Garbage collection (computer science)|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.
*Combining significant numbers of objects in the client's browser into 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.<ref>{{Cite web |title=DCE 1.1: Remote Procedure Call - Introduction to the RPC API |url=https://pubs.opengroup.org/onlinepubs/9629399/chap2.htm |access-date=2022-08-03 |website=pubs.opengroup.org}}</ref>

DCOM was a major competitor to [[CORBA]].<ref>{{Cite web |last1=Nowak |first1=Filip |last2=Qasim |first2=Mohsin |title=A Comparison of Distributed Object Technologies CORBA vs DCOM |url=https://middlewares.files.wordpress.com/2008/04/dcom_corba.pdf |archive-url=https://web.archive.org/web/20170829054415/https://middlewares.files.wordpress.com/2008/04/dcom_corba.pdf |archive-date=2017-08-29 |url-status=live |journal=}}</ref> 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 [[Firewall (networking)|firewalls]], and on unknown and insecure machines, meant that normal [[HTTP]] requests in combination with [[web browser]]s won out over both of them.<ref>{{Cite web |title=Object Technology Jeff Sutherland: Web Services: Better than CORBA or DCOM? |url=http://jeffsutherland.com/2003/10/web-services-better-than-corba-or-dcom.html |access-date=2022-08-03 |language=en}}</ref> Microsoft, at one point, attempted to remediate these shortcomings by adding an extra HTTP transport to DCE/RPC called ''ncacn_http'' (Network Computing Architecture connection-oriented protocol).<ref>{{Cite web |last=stevewhims |title=Remote Procedure Calls Using RPC over HTTP - Win32 apps |url=https://docs.microsoft.com/en-us/windows/win32/rpc/remote-procedure-calls-using-rpc-over-http |access-date=2022-08-03 |website=docs.microsoft.com |date=23 August 2019 |language=en-us}}</ref>

DCOM was publicly launched as a beta for Windows 95 September 18, 1996.<ref>[https://news.microsoft.com/1996/09/18/microsoft-releases-beta-version-of-dcom-for-windows-95/#Microsoft Press Release Announcing DCOM Beta]</ref>

DCOM is supported natively in all versions of Windows starting from Windows 95, and all versions of Windows Server since Windows NT 4.0<ref>{{Cite web |date=June 25, 2021 |title=Distributed Component Object Model (DCOM) Remote Protocol |url=https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-DCOM/%5bMS-DCOM%5d.pdf |archive-url=https://web.archive.org/web/20210509140147/https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-DCOM/%5BMS-DCOM%5D.pdf |archive-date=2021-05-09 |url-status=live |access-date=August 3, 2022 |website=Microsoft - Distributed Component Object Model (DCOM) Remote Protocol}}</ref>

== Security improvements ==
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.<ref>[https://technet.microsoft.com/en-us/library/bb457156.aspx#EIAA DCOM Security Enhancements]</ref>

In response to a security vulnerability reported by Tencent Security Xuanwu Lab in June 2021,<ref>{{Cite web |title=Security Update Guide - Microsoft Security Response Center |url=https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26414 |access-date=2022-08-03 |website=msrc.microsoft.com}}</ref> Microsoft released security updates for several versions of Windows and Windows Server, hardening access to DCOM.<ref>{{Cite web |title=KB5004442—Manage changes for Windows DCOM Server Security Feature Bypass (CVE-2021-26414) |url=https://support.microsoft.com/en-us/topic/kb5004442-manage-changes-for-windows-dcom-server-security-feature-bypass-cve-2021-26414-f1400b52-c141-43d2-941e-37ed901c769c |access-date=2022-08-03 |website=support.microsoft.com}}</ref>

== Alternative versions and implementations ==
'''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 the [[Windows NT]] 4.0 source code, and includes the source code for a [[Windows Registry|Windows NT Registry Service]]. <ref>{{Cite web |title=DataSheet COMsource for Solaris, Tru64 UNIX and other Platforms |url=http://www.opengroup.org/comsource/datasheet.htm |website=DataSheet COMsource for Solaris, Tru64 UNIX and other Platforms}}</ref>

In 1995, Digital and [[Microsoft]] announced Affinity for OpenVMS (also known as NT Affinity) which was intended to allow OpenVMS to serve as the [[persistence layer]] for Windows NT [[Client–server model|client-server applications]].<ref name="vms-20-years">{{cite web|url=https://h50146.www5.hpe.com/products/software/oe/openvms/history/vaxvms20/pdf/vmsbook.pdf |archive-url=https://web.archive.org/web/20210419084702/https://h50146.www5.hpe.com/products/software/oe/openvms/history/vaxvms20/pdf/vmsbook.pdf |archive-date=2021-04-19 |url-status=live|date=October 1997|title=OpenVMS at 20 Nothing stops it|publisher=Digital|access-date=2021-02-12}}</ref> As part of this initiative, an implementation of the Distributed Component Object Model (DCOM) was added to OpenVMS Alpha.<ref name="com-compaq">{{cite book|title=COM Beyond Microsoft: Designing and Implementing COM Servers on Compaq Platforms|url=https://books.google.com/books?id=i47yKPSDd1gC|isbn=1555582265|author=Gene Cronin, Terence P. Sherlock|date=2000| publisher=Digital Press }}</ref> In order to support DCOM, VMS was provided with implementations of the Windows Registry, [[NTLM]] authentication, and a subset of [[Win32]] APIs needed to support COM.<ref>{{cite web|url=http://h30266.www3.hpe.com/odl/vax/opsys/vmsos73/vmsos73/6539/6539pro.html|title=OpenVMS Connectivity Developer Guide|date=July 2000|access-date=2021-01-01|website=hpe.com|archive-date=2020-12-04|archive-url=https://web.archive.org/web/20201204205526/http://h30266.www3.hpe.com/odl/vax/opsys/vmsos73/vmsos73/6539/6539pro.html|url-status=dead}}</ref> DCOM was first added to OpenVMS V7.2-1 for the Alpha.<ref>{{cite web|url=http://odl.sysworks.biz/disk$vaxdocmar002/opsys/vmsos721/6614/6614pro_contents.html|title=OpenVMS Alpha V7.2-1 Release Notes|website=odl.sysworks.biz|access-date=2021-01-01|date=January 1999}}</ref> A similar implementation of DCOM was added to [[Digital Unix]] as part of the AllConnect program.<ref name="com-compaq" />

'''TangramCOM''' was a separate project from Wine, focusing on implementing DCOM on Linux-based smartphones.<ref>{{Cite web |title=Home - TangramCOM |url=http://www.andjoin.com/Home |access-date=2022-08-03 |website=www.andjoin.com}}</ref>

== See also ==
* [[ActiveX]]
* [[Dynamic Data Exchange]] (DDE)
* [[.NET Remoting]]
* [[OLE for process control|OLE for Process Control]]

==References==
{{Reflist}}

== External links ==
* [https://tools.ietf.org/html/draft-brown-dcom-v1-spec-03 Distributed Component Object Model Protocol -- DCOM/1.0]
* [http://opengroup.org/comsource The Open Groups COMsource] {{Webarchive|url=https://web.archive.org/web/20200113034910/http://opengroup.org/comsource |date=2020-01-13 }}
* [https://web.archive.org/web/20091213022641/http://tcom.andjoin.com/ TangramCOM]

{{Microsoft APIs}}
{{Microsoft Windows components}}
{{Authority control}}

[[Category:Component-based software engineering]]
[[Category:Inter-process communication]]
[[Category:Windows communication and services]]
[[Category:Object models]]
[[Category:Object request broker]]