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Microsoft’s Distributed Component Object Model (DCOM). A semi-technical overview. Jim Ries JimR@acm.org Updated 10/5/1999. Genealogy. DCOM comes from COM and OSF (now Open Group ) DCE DCE Remote Procedure Calls (RPC) Interface Definition Language (IDL) Component Object Model (COM)

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Microsoft’s Distributed Component Object Model (DCOM)

A semi-technical overview

Jim Ries

JimR@acm.org

Updated 10/5/1999


Genealogy

  • DCOM comes from COM and OSF (now Open Group) DCE

    • DCE

      • Remote Procedure Calls (RPC)

      • Interface Definition Language (IDL)

    • Component Object Model (COM)

    • ORPC

  • OMG CORBA - a parallel standard

    • Different RPC

    • Different IDL

    • COM proxy == CORBA stub

    • COM stub == CORBA skeleton


Microsoft proprietary, but . . .

  • Open Group’s COMSource: http://www.opengroup.org/comsource/

  • Software AG’s EntireX: http://www.softwareag.com/entirex/default.htm

  • The Active Group: http://www.activex.org/


COM Goals

  • Encapsulation (separate implementation from interface)

  • Versioning

  • Execution context independence

  • Language independence

  • Object Creation / Lifetime Control

  • Standard error code (HRESULT)

  • Solve object discovery problem

  • Scripting

  • The Holy Grail of Reuse


Alphabet soup:COM/OLE/ActiveX

  • COM is a binary standard and a style for creating objects.

  • OLE is (was) a set of COM interfaces for embedding documents (originally “Object Linking and Embedding”).

  • ActiveX is a marketing buzz-word meaning COM and/or OLE, but usually applied to Internet-oriented components.


Later and later binding

  • “Editor inheritance” binds at compile time.

  • Link libraries (.LIB) bind to “components” at link time.

  • Dynamic link libraries (.DLL) bind at run time, but need a header at compile time, and path at runtime.

  • COM components bind at runtime and may need neither a header, nor a path! (though typelib contains header-like “meta-data”)


Interfaces

  • COM enforces the concept of interfaces being separate from implementation.

  • Interface == Abstract Base Class

  • Objects support multiple interfaces through multiple inheritance.

  • Interfaces NEVER change!

  • A “control” is just a COM component with the right interfaces.


GUID’s (or UUID’s)

  • Globally Unique Identifiers (Universally Unique Identifiers)

  • Needed to avoid name collisions

  • A class is associated with a GUID (CLSID).

  • An interface is associated with a GUID (IID).

  • The Windows Registry: a hierarchical database.


Execution Context

  • In proc - DLL’s (no marshalling)

  • Out of proc - EXE’s (LRPC)

  • Remote - EXE’s using DCOM

    • RPC

    • DCE “compatible” (see “Interconnecting Personal Computers with the Distributed Computing Environment” by Jim Ries, UMC Thesis, 1998.)


Coding Tools

  • C - Raw

  • C++ - Raw

  • C++ - ATL

  • C++ - MFC

  • Visual Basic

  • J++ (pseudo Java)

  • Binary standard ==> any language COULD produce COM components.


Platforms

  • Win32

    • Windows 95 (DCOM as separate download; included in OSR2)

    • Windows NT 4.0

    • Windows 98

    • Windows 2000

  • Unix platforms (with some help)


Nuts and Bolts

  • CoInitialize()

  • CoCreateInstance()

  • IUnknown

    • QueryInterface()

    • AddRef()

    • Release()

  • CoUninitialize()


Demonstration - IDL

[

object,

uuid(75D873CD-7B63-11D3-9D43-00C0F031CDDE),

helpstring("IServer Interface"),

pointer_default(unique)

]

interface IServer : IUnknown

{

HRESULT Hello([in, string] char * pszMessage);

};


Demonstration - Server Code

// Prototype

class CServer :

public IServer, public CComObjectRoot,public CComCoClass<CServer,&CLSID_Server>

{

// . . . Some code omitted for brevity

// IServer

public:

HRESULT STDMETHODCALLTYPE Hello(unsigned char * pszMessage);

};

// Code

HRESULT STDMETHODCALLTYPE CServer::Hello(unsigned char * pszMessage)

{

char szBuf[256];

wsprintf(szBuf,"%s",pszMessage);

::MessageBox(0,szBuf,"Server",MB_OK);

return(S_OK);

}


Demonstration - Client Code

if (SUCCEEDED(

hr=CoCreateInstance(CLSID_Server,NULL,

CLSCTX_LOCAL_SERVER,

IID_IServer,(void **)&pServer)))

{

if (SUCCEEDED(hr=pServer->Hello((unsigned char *)"Hello from the client")))

MessageBox("Client: Server printed the message");

else

{

wsprintf(szBuffer,"Hello() method failed: 0x%lX.\n",hr);

MessageBox(szBuffer);

}

pServer->Release();

}

else

{

wsprintf(szBuffer,"Unable to create a server: 0x%lX.\n",hr);

MessageBox(szBuffer);

}


Distributed Scenario

  • From “DCOM Architecture” a Microsoft white paper.


Demonstration

  • Run DCOM “Hello world” demo here.


Additional Technologies

  • COM+

    • MTS - Microsoft Transaction Server

    • MSMQ - Microsoft Message Queue

    • Compiler supported IUnknown, etc.

  • ADS - Active Directory Service

    • As “distributed registry”

    • As namespace abstraction

  • All Microsoft products are COM based:

    • IIS - Internet Information Server

    • Exchange

    • Internet Explorer

    • Word, Excel, etc.


References

  • Microsoft DCOM page

  • IETF DCOM Standard Proposal

  • Inside OLE by Kraig Brockschmidt, Microsoft Press, 1995.

  • Essential COM by Don Box, Addison Wesley, 1998.

  • Inside COM by Dale Rogerson, Microsoft Press, 1997.

  • Don Box homepage

  • ActiveX COM Control Programming by Sing Li and Panos Economopoulos, Wrox Press, 1997.

  • COM-CORBA Interoperability by Geraghty, et. al., Prentice Hall, 1999.

  • Microsoft Developer Network


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