DCOM-CORBA Comparison

1
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/DC
  OMnCORBA/S.html

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
  interfaces
• and invoking methods through that pointer
• as if the object is in the client's address space

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

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

5
Communication

• CORBA 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

6
Sample application

• A Grid server maintains a two-dimensional grid of
  integers
• Two 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

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

8
DCOM IDL
// 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)
Each interface and class has GUIDs
9
DCOM IDL (continued)
// 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
multiple views for Grid
10
CORBA IDL
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
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_cR
ef) 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
CORBA server class definition (grid_i.h) includ
e "grid.hh" // IDL-generated interface header
file class grid_i public gridBOAImpl
public virtual CORBALong get(CORBAShort
n, CORBAShort m,
CORBAEnvironment
env) virtual void set(CORBAShort n,
CORBAShort m, CORBALong value,
CORBAEnvironment env) virtual void
reset(CORBALong value,
CORBAEnvironment env) grid_i(CORBAShort
h, CORBAShort w) virtual grid_i() private
CORBALong m_a CORBAShort
m_height, m_width
13
include "cgrid.h" STDMETHODIMP
CGridQueryInterface(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
CGridget(IN SHORT n, IN SHORT m, OUT LONG
value) value m_anm
return S_OK extern HANDLE hevtDone
CGridCGrid () for (int i0 i lt
m_height i) delete m_ai
delete m_a SetEvent(hevtDone)
DCOM Server Implementation
create, set, reset method implementations are
not shown
14
CORBA Server Implementation
include "grid_i.h" CORBALong
grid_iget(CORBAShort n, CORBAShort m,
CORBAEnvironment )
return m_anm void grid_iset(CORBASh
ort n, CORBAShort m, CORBALong value,
CORBAEnvironment ) m_anm
value void grid_ireset(CORBALong value,
CORBAEnvironment ) short n, m for (n
0 n lt m_height n) for (m 0 m lt
m_width m) m_anmvalue
return // create and delete not shown
15
DCOM Server main program
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 CGridCGrid()
WaitForSingleObject(hevtDone, INFINITE)
CloseHandle(hevtDone)
CoUninitialize()
16
CORBA Server main program
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
CORBAOrbix.impl_is_ready("grid") catch
(...) cout ltlt "Unexpected exception" ltlt
endl exit(1)
17
DCOM client program
include "grid.h" void main(int
argc, charargv)
IGrid1 pIGrid1
IGrid2 pIGrid2 LONG
value
CoInitialize(NULL) // initialize COM
CoCreateInstance(CLSID_CGrid,
NULL, CLSCTX_SERVER,
IID_IGrid1, (void) pIGrid1)
pIGrid1-gtget(0, 0, value)
pIGrid1-gtQueryInterface(IID_IGrid2,
(void) pIGrid2)
pIGrid1-gtRelease()
pIGrid2-gtreset(value1)
pIGrid2-gtRelease()
CoUninitialize()
18
CORBA client program
include "grid.hh" void main (int argc, char
argv) grid_var gridVar
CORBALong value
// bind to "grid"
object Orbix-specific gridVar
grid_bind("grid") value gridVar-gtget(0,
0) gridVar-gtreset(value1)
19
Registration

• In both CORBA and DCOM the servers need to be
  registered
• This 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

20
Observations

• DCOM 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

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

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