d com comparison
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
}
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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;
};
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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
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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
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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
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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
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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
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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);
}
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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
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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
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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
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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
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