d Com Comparison

8/13/2019 d Com Comparison

1/22

DCOM-CORBA Comparison

Operating Systems II

Reference: P. Chung et al., DCOM and CORBA: side by side, step by

step,and layer by layer, C++ Report.

http://www.research.att.com/~ymwang/papers/HTML/DCOMnCORBA/S.html


2/22

DCOM vs CORBA 2

A COM object can have multiple interfaceseach representing a different view of the

objectA COM client interacts with a COM object by

obtaining a pointer to one of the object'sinterfaces

and invoking methods through that pointer

as if the object is in the client's address space


3/22

DCOM vs CORBA 3

The core of CORBA is the Object RequestBroker (ORB) that acts as the softwarebus over which objects interact with each

other transparently

The client of a CORBA object acquires anobject-reference and

uses it as a handle to make calls

as if it is in the client's local address space.

The ORB is responsible for facilitating the call


4/22

DCOM vs CORBA 4

Both DCOM and CORBA provides client-server relations between objects

The interface to a server is describedusing an IDL in both cases

CORBA supports multiple inheritance at

IDL level, DCOM does notDCOM however allows multiple interfaces to

an object


5/22

DCOM vs CORBA 5

Communication

CORBA and DCOM uses RPC style ofcommunication

The client makes a call to the client stub(called proxy in DCOM)

The stub marshals the parameters and sends it tothe server

At the server end, a server skeleton (calledstub in DCOM) demarshals the message andmakes the call


6/22

DCOM vs CORBA 6

Sample application

A Grid server maintains a two-dimensionalgrid of integers

Two views to the grid:View One:

Two methods: get and set to get and set gridpoint

View Two:

One method: reset to reset all grid points to agiven value


7/22

DCOM vs CORBA 7

The DCOM implementation uses twointerfaces, one for each view

Interfaces IGrid1 and IGrid2The CORBA implementation uses multiple

inheritance

Interface grid1 supports get and setInterface grid2 supports reset

Interface grid inherits from grid1 and grid2


8/22

DCOM vs CORBA 8

// uuid and definition of IGrid1

[ object,

uuid(3CFDB283-CCC5-11D0-BA0B-00A0C90DF8BC),

helpstring("IGrid1 Interface"),

pointer_default(unique)

]

interface IGrid1 : IUnknown {

import "unknwn.idl";

HRESULT get([in] SHORT n, [in] SHORT m, [out] LONG *value);

HRESULT set([in] SHORT n, [in] SHORT m, [in] LONG value);};

// uuid and definition of IGrid2

[ object,

uuid(3CFDB284-CCC5-11D0-BA0B-00A0C90DF8BC),

helpstring("IGrid2 Interface"),pointer_default(unique)

]

interface IGrid2 : IUnknown {

import "unknwn.idl";

HRESULT reset([in] LONG value);

};

DCOM IDL

Each interface and

class has GUIDs


9/22

DCOM vs CORBA 9

// uuid and definition of type library

[ uuid(3CFDB281-CCC5-11D0-BA0B-00A0C90DF8BC),

version(1.0),

helpstring("grid 1.0 Type Library)

]

library GRIDLib

{ importlib("stdole32.tlb");

// uuid and definition of class

[ uuid(3CFDB287-CCC5-11D0-BA0B-00A0C90DF8BC),

helpstring("Grid Class")

]

// multiple interfacescoclass CGrid

{ [default] interface IGrid1;

interface IGrid2;

};

};

DCOM IDL (continued)

multiple views for Grid


10/22

DCOM vs CORBA 10

interface grid1

{

long get(in short n, in short m);

void set(in short n, in short m, in long value);

};

interface grid2

{

void reset(in long value);

};

// multiple inheritance of interfaces

interface grid: grid1, grid2

{

};

CORBA IDL


11/22

DCOM vs CORBA 11

DCOM server class definition (cgrid.h)

#include "grid.h" // IDL-generated interface header file

class CGrid : public IGrid1, public IGrid2 {

public:// IUnknown

STDMETHODIMP QueryInterface(REFIID riid, void** ppv);

STDMETHODIMP_(ULONG) AddRef(void)

{ return InterlockedIncrement(&m_cRef); }

STDMETHODIMP_(ULONG) Release(void)

{ if (InterlockedDecrement(&m_cRef) == 0){ delete this; return 0; }

return 1; }

// IGrid1

STDMETHODIMP get(IN SHORT n, IN SHORT m,

OUT LONG *value);

STDMETHODIMP set(IN SHORT n, IN SHORT m,IN LONG value);

// IGrid2

STDMETHODIMP reset(IN LONG value);

// create and destroy methods, and private declarations are not shown

}


12/22

DCOM vs CORBA 12

CORBA server class definition (grid_i.h)

#include "grid.hh" // IDL-generated interface header file

class grid_i : public gridBOAImpl {

public:

virtual CORBA::Long get(CORBA::Short n, CORBA::Short m,

CORBA::Environment &env);

virtual void set(CORBA::Short n, CORBA::Short m,CORBA::Long value, CORBA::Environment &env);

virtual void reset(CORBA::Long value,

CORBA::Environment &env);

grid_i(CORBA::Short h, CORBA::Short w);

virtual ~grid_i();

private:

CORBA::Long **m_a;

CORBA::Short m_height, m_width;

};


13/22

DCOM vs CORBA 13

#include "cgrid.h"

STDMETHODIMP CGrid::QueryInterface(REFIID riid, void** ppv) {

if (riid == IID_IUnknown || riid == IID_IGrid1)

*ppv = (IGrid1*) this;else if (riid == IID_IGrid2) *ppv = (IGrid2*) this;

else { *ppv = NULL; return E_NOINTERFACE; }

AddRef(); return S_OK;

}

STDMETHODIMP CGrid::get(IN SHORT n, IN SHORT m, OUT LONG* value) {

*value = m_a[n][m];

return S_OK;

}

extern HANDLE hevtDone;

CGrid::~CGrid () {

for (int i=0; i < m_height; i++)

delete[] m_a[i];

delete[] m_a;

SetEvent(hevtDone);}

DCOM Server Implementation

create, set, reset method

implementations are not shown


14/22

DCOM vs CORBA 14

#include "grid_i.h"

CORBA::Long grid_i::get(CORBA::Short n, CORBA::Short m,

CORBA::Environment &) {return m_a[n][m];

}

void grid_i::set(CORBA::Short n, CORBA::Short m,

CORBA::Long value, CORBA::Environment &) {

m_a[n][m] = value;}

void grid_i::reset(CORBA::Long value, CORBA::Environment &) {

short n, m;

for (n = 0; n < m_height; n++)

for (m = 0; m < m_width; m++)

m_a[n][m]=value;

return;

}

// create and delete not shown

CORBA Server Implementation


15/22

DCOM vs CORBA 15

HANDLE hevtDone;

void main()

{

// Event used to signal this main thread

hevtDone = CreateEvent(NULL, FALSE, FALSE, NULL);

hr = CoInitializeEx(NULL, COINIT_MULTITHREADED);

CClassFactory* pcf = new CClassFactory;

hr = CoRegisterClassObject(CLSID_CGrid, pcf,

CLSCTX_SERVER, REGCLS_MULTIPLEUSE , &dwRegister);

// Wait until the event is set by CGrid::~CGrid()WaitForSingleObject(hevtDone, INFINITE);

CloseHandle(hevtDone);

CoUninitialize();

}

DCOM Server main program


16/22

DCOM vs CORBA 16

int main()

{

// create a grid object using the implementation class grid_i

grid_i ourGrid(100,100);

try {

// tell Orbix that we have completed the server's initialization:

CORBA::Orbix.impl_is_ready("grid");

} catch (...) {cout


17/22

DCOM vs CORBA 17

#include "grid.h"

void main(int argc, char**argv)

{

IGrid1 *pIGrid1;

IGrid2 *pIGrid2;

LONG value;

CoInitialize(NULL); // initialize COM

CoCreateInstance(CLSID_CGrid, NULL, CLSCTX_SERVER,

IID_IGrid1, (void**) &pIGrid1);

pIGrid1->get(0, 0, &value);

pIGrid1->QueryInterface(IID_IGrid2, (void**) &pIGrid2);pIGrid1->Release();

pIGrid2->reset(value+1);

pIGrid2->Release();

CoUninitialize();

}

DCOM client program


18/22

DCOM vs CORBA 18

CORBA client program

#include "grid.hh"

void main (int argc, char **argv)

{

grid_var gridVar;

CORBA::Long value;

// bind to "grid" object; Orbix-specific

gridVar = grid::_bind(":grid");

value = gridVar->get(0, 0);

gridVar->reset(value+1);

}


19/22

DCOM vs CORBA 19

Registration

In both CORBA and DCOM the serversneed to be registered

This establishes the mapping between theinterface name and the path name (inCORBA), and the CLSID and the path

name in DCOMDCOM uses registry (which is supported by

the Window o/s)

CORBA uses implementation repository


20/22

DCOM vs CORBA 20

Observations

DCOM: Client calls COM library'sCoCreateInstance() with CLSID_Grid andIID_IGrid1

CORBA: Client calls client stub's bind

DCOM: The client can get another interface

(IID_IGrid2) of the same object usingQueryInterface


21/22

DCOM vs CORBA 21

DCOM Middle layer

Upon receiving CoCreateInstance() call, COM librarydelegates the task to Service Control Manager (SCM)

SCM tries to map CLSID_Grid to its server path name by

consulting the registryIf an instance does not exist, SCM creates an instance

When the instance is created, COM creates an interface stub forIID_Grid1 interface

Then SCM ferries the marshalled pointer back to the client side;

COM on the client side creates a proxy for the interface andassociates it the with the RPC channel to the stub

COM library returns to the client an IID_IGrid1 pointer to theinterface proxy pIGrid1


22/22

DCOM vs CORBA 22

CORBA middle layer

Upon receiving bind() call, the client stub delegates thetask to ORB

ORB consults the implementation repository andactivates the server

Server instantiates all supported objects and creates aninstance of the skeleton

When the ORB ferries the object reference back to theclient side, it creastes an instance of the proxy class andregisters it in a proxy object table with its correspondingobject reference

Client stub returns to the client an object reference asgridVar

Documents

d Com Comparison