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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. - PowerPoint PPT Presentation
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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
DCOM vs CORBA 2
A COM object can have multiple interfaces each representing a different view of the object A COM client interacts with a COM object
by obtaining a pointer to one of the object's interfacesand invoking methods through that pointeras if the object is in the client's address
space
DCOM vs CORBA 3
The core of CORBA is the Object Request Broker (ORB) that acts as the software bus over which objects interact with each other transparently
The client of a CORBA object acquires an object-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
DCOM vs CORBA 4
Both DCOM and CORBA provides client-server relations between objects
The interface to a server is described using an IDL in both cases
CORBA supports multiple inheritance at IDL level, DCOM does not DCOM however allows multiple interfaces
to an object
DCOM vs CORBA 5
CommunicationCORBA and DCOM uses RPC style of
communication The client makes a call to the client stub
(called proxy in DCOM)The stub marshals the parameters and
sends it to the server At the server end, a server skeleton
(called stub in DCOM) demarshals the message and makes the call
DCOM vs CORBA 6
Sample applicationA Grid server maintains a two-
dimensional grid of integersTwo views to the grid:
View One:Two methods: get and set to get and set grid
point View Two:
One method: reset to reset all grid points to a given value
DCOM vs CORBA 7
The DCOM implementation uses two interfaces, one for each view Interfaces IGrid1 and IGrid2
The CORBA implementation uses multiple inheritance Interface grid1 supports get and set Interface grid2 supports reset Interface grid inherits from grid1 and grid2
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 andclass has GUIDs
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 interfaces coclass CGrid { [default] interface IGrid1; interface IGrid2; }; };
DCOM IDL (continued)
multiple views for Grid
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 interfacesinterface grid: grid1, grid2 {};
CORBA IDL
DCOM vs CORBA 11
DCOM server class definition (cgrid.h) #include "grid.h" // IDL-generated interface header fileclass 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}
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; };
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
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
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
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 << "Unexpected exception" << endl; exit(1); }}
CORBA Server main program
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
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);}
DCOM vs CORBA 19
RegistrationIn both CORBA and DCOM the servers
need to be registeredThis establishes the mapping between
the interface name and the path name (in CORBA), and the CLSID and the path name in DCOM DCOM uses registry (which is supported by
the Window o/s) CORBA uses implementation repository
DCOM vs CORBA 20
ObservationsDCOM: Client calls COM library's
CoCreateInstance() with CLSID_Grid and IID_IGrid1
CORBA: Client calls client stub's bind
DCOM: The client can get another interface (IID_IGrid2) of the same object using QueryInterface
DCOM vs CORBA 21
DCOM Middle layer Upon receiving CoCreateInstance() call, COM library
delegates the task to Service Control Manager (SCM) SCM tries to map CLSID_Grid to its server path name by
consulting the registry If an instance does not exist, SCM creates an instance When the instance is created, COM creates an interface stub
for IID_Grid1 interface Then SCM ferries the marshalled pointer back to the client
side; COM on the client side creates a proxy for the interface and associates it the with the RPC channel to the stub
COM library returns to the client an IID_IGrid1 pointer to the interface proxy pIGrid1
DCOM vs CORBA 22
CORBA middle layer Upon receiving bind() call, the client stub delegates the
task to ORB ORB consults the implementation repository and activates
the server Server instantiates all supported objects and creates an
instance of the skeleton When the ORB ferries the object reference back to the
client side, it creastes an instance of the proxy class and registers it in a proxy object table with its corresponding object reference
Client stub returns to the client an object reference as gridVar