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CS 5523 Lecture 21:
A CORBA Case Study
❚ CORBA overview
❚ CORBA objects and IDL
❚ The ShapeList example in CORBA
❚ CORBA naming service
❚ Other CORBA services
❚ Recommended reading
CORBA overview:
❚ Middleware that allows communication between programs
independent of language, OS, hardware, and network
❚ Applications are built from CORBA objects
❚ CORBA objects implement interfaces defined in IDL
❚ Clients access methods in the IDL interfaces by RMI
❚ RMI is implemented by an ORB (Object Request Broker)
Remote interfaces – Java RMI versus CORBA:
❚ CORBA – uses IDL to specify remote interfaces
❚ JAVA – uses ordinary interfaces that are extended by the
keyword remote.
CORBA objects versus Java RMI objects:
CORBA objects:
❚ implement an IDL interface
❚ have a remote object reference
❚ can respond to invocations of methods in the IDL interface
How does this differ from Java RMI?
❚ CORBA objects can be implemented in non-OO languages
❚ clients don’t have to be objects
❚ classes cannot be implemented in IDL – so no objects can be
passed, only data structures
CORBA IDL interfaces:
❚ specify a name and a set of methods
❚ parameters are marked with keywords in, out, or inout
❚ parameters can be of a primitive type or constructed type
❚ allows exceptions to be defined in interfaces and thrown by
methods
❚ invocation is at-most-once by default (can also specify oneway)
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 5.
CORBA IDL example
// In file Person.idl struct Person { string name; string place; long year; } ; interface PersonList { readonly attribute string listname; void addPerson(in Person p) ; void getPerson(in string name, out Person p); long number(); };
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
IDL interfaces Shape and ShapeList
struct Rectangle{ 1 long width; long height; long x; long y; } ;
struct GraphicalObject { 2 string type; Rectangle enclosing; boolean isFilled; };
interface Shape { 3 long getVersion() ; GraphicalObject getAllState() ; // returns state of the GraphicalObject };
typedef sequence <Shape, 100> All; 4 interface ShapeList { 5 exception FullException{ }; 6 Shape newShape(in GraphicalObject g) raises (FullException); 7 All allShapes(); // returns sequence of remote object references 8 long getVersion() ; };
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
Java interface ShapeList generated by idltojava from CORBA interface ShapeList
public interface ShapeList extends org.omg.CORBA.Object {
Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException;
Shape[] allShapes();
int getVersion();
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
ShapeListServant class of the Java server program for CORBA interface ShapeList
import org.omg.CORBA.*; class ShapeListServant extends _ShapeListImplBase { ORB theOrb; private Shape theList[]; private int version; private static int n=0; public ShapeListServant(ORB orb){ theOrb = orb; // initialize the other instance variables } public Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException { 1 version++; Shape s = new ShapeServant( g, version); if(n >=100) throw new ShapeListPackage.FullException(); theList[n++] = s; 2 theOrb.connect(s); return s; } public Shape[] allShapes(){ ... } public int getVersion() { ... } }
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
Java class ShapeListServer
import org.omg.CosNaming.; import org.omg.CosNaming.NamingContextPackage.; import org.omg.CORBA.*; public class ShapeListServer { public static void main(String args[]) { try{ ORB orb = ORB.init(args, null); 1 ShapeListServant shapeRef = new ShapeListServant(orb); 2 orb.connect(shapeRef); 3 org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); 4 NamingContext ncRef = NamingContextHelper.narrow(objRef); NameComponent nc = new NameComponent("ShapeList", ""); 5 NameComponent path[] = {nc}; 6 ncRef.rebind(path, shapeRef); 7 java.lang.Object sync = new java.lang.Object(); synchronized (sync) { sync.wait();} } catch (Exception e) { ... } } }
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
Java client program for CORBA interfaces Shape and ShapeList
import org.omg.CosNaming.; import org.omg.CosNaming.NamingContextPackage.; import org.omg.CORBA.*; public class ShapeListClient{ public static void main(String args[]) { try{ ORB orb = ORB.init(args, null); 1 org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); NamingContext ncRef = NamingContextHelper.narrow(objRef); NameComponent nc = new NameComponent("ShapeList", ""); NameComponent path [] = { nc }; ShapeList shapeListRef = ShapeListHelper.narrow(ncRef.resolve(path)); 2 Shape[] sList = shapeListRef.allShapes(); 3 GraphicalObject g = sList[0].getAllState(); 4 } catch(org.omg.CORBA.SystemException e) {...} }
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
The main components of the CORBA architecture
client server
proxy
or dynamic invocation
implementation repository (^) object adapter
ORB ORB
skeleton
or dynamic skeleton
client program
interface repository
Request
Reply
for A core core
Servant A
CORBA pseudo objects:
❚ provide interfaces to the functionality of the ORB
❚ have IDL interfaces, but cannot be passed as remote references
❚ examples:
❙ init – method to initialize the ORB
❙ connect – method used to register objects with the ORB
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Page 684
CORBA interoperable object references
IOR format
IDL interface type name Protocol and address details Object key interface repository identifier
IIOP host domain name
port number adapter name object name
CORBA naming service:
❚ binder providing facilities for servers to register remote objects
❚ provides facilities for clients to resolve names by name
❚ names are structured hierarchically
❚ each name in a path is inside a structure NameComponent
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
Naming graph in CORBA Naming Service
initial naming context
ShapeList C D E
B
initial naming context
P
R S^
T
V
Q U
initial naming context
XX
CORBA naming service (contined):
❚ initial naming context – provides a root for a set of bindings
❚ clients and servers request initial naming context
❚ an object of type NamingContext is returned and names are
relative to it
❚ an object is either a remote object or a NamingContext
❚ names are of type NameComponents and have a name and a
kind.
❚ a Name type is a sequence of NameComponents
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000
Figure 17.
Part of the CORBA Naming Service NamingContext interface in IDL
struct NameComponent { string id; string kind; };
typedef sequence Name;
interface NamingContext {
void bind (in Name n, in Object obj);
binds the given name and remote object reference in my context.
void unbind (in Name n);
removes an existing binding with the given name.
void bind_new_context(in Name n);
creates a new naming context and binds it to a given name in my context.
Object resolve (in Name n);
looks up the name in my context and returns its remote object reference.
void list (in unsigned long how_many, out BindingList bl, out BindingIterator bi);
returns the names in the bindings in my context.
CORBA notification services:
❚ extends the event server
❚ notifications may be data structures
❚ event consumers may use filters
❚ event suppliers can discover which events consumers are
interested in
❚ channel properties can be configured
❚ an event repository is provided
CORBA security services:
We’ll talk about at the end of the course….
CORBA recommended reading:
The October 1998 Issue of the Communications of the ACM was
devoted to new developments in CORBA. It contains many
excellent articles.
For next time:
❚ Read CDK 8.1-8.
