CORBA

Background

• In assignment 1 you should have ordinary objects plus half-objects and servers to do the networking
• Remote Procedure Call uses a specification to generate the client-side half-object and the server
• RPC generates procedure calls, not method calls
• SOAP/WSDL are the web services version of RPC.
• WSDL is too hard to use as a direct specification, so a hack of "write the implementation and reverse engineer the spec" is used
• Servers such as Axis generate the server-side objects
• Tools such as xprcc.sh generate the client-side half-objects
• The main extension of RPC to use object methods instead of procedure calls is CORBA

CORBA

The Common Object Request Broker Architecture is a middleware system for building distributed applications. It is looked after by the Object Management Group (OMG) which has been in existence for over 15 years.

CORBA supports distributed objects. The "objects" may be written in C, C++, Ada, Java, SmallTalk, Python, etc. Some of these are not O/O languages. A single distributed application may use a mixture of languages.

Objects communicate by making method calls on one another. The CORBA "backplane" or Object Request Broker (ORB) looks after marshalling parameters, transferring the data, making the remote call and returning the results.

The ORB structure is

Objects may be located in a variety of ways. These include

• Portable "string-ified" references
• Naming service
• Trader service

CORBA IDL

To achieve language independence, CORBA applications are specified by interfaces written in an IDL (Interface Definition Language). CORBA IDL is different to Sun's RPC/ONC IDL and to DCE IDL. It is based on C++, with changes to accomodate distributed objects, such as in/out parameters instead of reference/value parameters.

A simple IDL is

    module HelloApp {
        interface Hello {
            string sayHello();
        };
    };

module RoomBooking {
 
    interface Meeting {
         readonly attribute string purpose;
         readonly attribute string participants;
    };
 
    interface MeetingFactory {
         Meeting CreateMeeting(in string purpose, in string participants);
    };
 
    enum Slot {am9, am10, am11, pm12, pm1, pm2, pm3, pm4};
 
    const short MaxSlots = 8;
 
    exception NoMeetingInThisSlot {};
    exception SlotAlreadyTaken {};
 
    interface Room {
        typedef Meeting Meetings[MaxSlots];
 
        readonly attribute string name;
 
        Meetings View();
 
        void Book(in Slot a_slot, in Meeting  a_meeting)
            raises(SlotAlreadyTaken);
 
        void Cancel(in Slot  a_slot)
            raises(NoMeetingInThisSlot);
    };
};               

The IDL is used to generate stubs and skeletons for the client and server.

Primitive types

There are mappings defined to types in all of the CORBA supported languages

Modules and Interfaces

An appplication may be made up from specifications from different sources. There may be a possibility of name clashes. CORBA uses "modules" to create separate "name spaces" so that the possibility of clashes is reduced.

e.g.

module a {
  module b {
    ...
  };
};

Classes are defined by "interfaces". For example,

module gui {
  interface pushButton {
    ...
  };

  interface textArea {
    ...
  };
};

C++ mapping

C++ was still evolving while CORBA was being developed. There are a number of possible mappings, depending on which version of the language/which compiler you target.

• C++ with namespaces: the modules form namespaces, the interfaces form classes. e.g.

        namespace gui {
            class pushButton {...};
        };
        

  and the class is referenced by

        gui::pushButton
        

• C++ without namespaces but with nested (inner) classes gives modules as classes and interfaces as inner classes
• C++ with neither concatenates the module name(s) with the interface name

        class gui_pushButton {...};
        

  This is the most portable in terms of working with the largest number of C++ compilers, but fails to use the newer features of C++

Java mapping

Modules map to Java packages. Interfaces map to Java interfaces

package gui;

public interface pushButton {
  ...
};

Inheritance

IDL supports multiple inheritance of interfaces

interface Rectangle: Shape {...};

C++:

    class Rectangle: Shape {...};

Java

    interface Rectangle extends Shape {...};

Attributes

Attributes are fields of objects. They may be readonly or read/write fields. e.g.

attribute long x;
readonly attribute long y; 

Mapping to C++

Attributes are hidden. They are accessible by "set" and "get" methods

CORBA_Long x();      // get
void x(CORBA_Long);  // set

CORBA_Long y();      // get only

Mapping to Java

Attributes map to protected fields of objects. There are "set" and "get" methods for attributes that are both readable and writable, but only "get" methods for readonly attributes.

Java Beans (and most style guides) use getX() and setX() methods to access an attribute. CORBA doesn't follow this. It uses X() for get methods and X(...) for set methods

int x();       // get
void x(int i); // set

int y();       // get only

Operations

CORBA uses the word operation for methods. These translate into operations in C++ and methods in Java. Parameters can be in, inout or out. in parameters translate according to the earlier tables and are passed by value. inout and out parameters translate differently because they have to carry return values

mapping to C++

Each out or inout basic type maps to a CORBA type followed by _out

typedef CORBA_Short& CORBA_Short_out;

e.g.

    long f(in short x, out short y, inout short z);

    virtual CORBA_Long f(CORBA_Short x,
                         CORBA_Short& y,
                         CORBA_Short& z);                                     

Mapping to Java

Java has a set of "helper" classes such as org.omg.CORBA.ShortHolder. These have a field value. An instance of these classes can be passed as an out parameter, and its value field can be changed

    public int
    f(short x,
      org.omg.CORBA.ShortHolder y,
      org.omg.CORBA.ShortHolder z); 

Similar helper classes are defined for non-primitive types (objects).

CORBA protocols

Two CORBA objects must communicate using a standardised format. CORBA 1.0 failed to define this, so objects from different vendors could not communicate (or even find each other). In CORBA 2.0 this was fixed by a number of different specification (see http://www.blackmagic.com/people/gabe/iiop.html)

• GIOP: General Inter-ORB Protocol defines the messages
• IIOP: the Internet Inter-ORB Protocol. IIOP maps GIOP messages to TCP/IP
• IOP: Interoperable Object Reference specification defines references that are understood by each ORB.

CORBA and C++

Idl compiler

Given an IDL file X.idl, an "idl to C++" compiler will generate 4 files

• a header file X.h of client-side type definitions
• a header file X_skel.h of server-side definitions
• an implementation file X.cpp used by both client and server
• a server skeleton X_skel.cpp for the server to inherit

e.g. Hello.idl results in

• Hello.h
• hello_skel.h
• Hello.cpp
• Hello_skel.cpp

Simple C++ Client

The client is an application starting from main(). The pseudocode is

    initialize CORBA ORB
    find a reference to the service
    cast the service to the right type
    call methods on the service

For the IDL

interface Hello {
    void sayHello();
};

  void sayHello(); 

The client is

// for Orbacus

#include <OB/CORBA.h>
#include "Hello.h"

#include <fstream.h>

int main(int argc, char **argv)
{
    // initialize CORBA ORB
    CORBA_ORB_var orb = CORBA_ORB_init(argc, argv);

    // find service - here pick a string form out of a file
    const char* refFile = "Hello.ref";
    ifstream in(refFile);
    char s[1000]; // must be big enough!
    in >> s;
    CORBA_Object_var obj = orb -> string_to_object(s);

    // cast to right type
    Hello_var hello = Hello::_narrow(obj);

    // invoke methods
    // NB: Prints on the server!
    hello -> sayHello();

    exit(0);
}

Simple C++ Server

Much of the server code is supplied by the IDL compiler and the CORBA library calls. Pseudocode for a server is

    initialize CORBA ORB
    initialize the BOA (Basic Object Adapter)
    create a reference to the service
    export the reference (so clients can find it)
    enter client request loop

For the hello example, the server is

// Orbacus server

#include <OB/CORBA.h>
#include <Hello_impl.h>

#include <fstream.h>

int main(int argc, char** argv) 
{
    // initialize CORBA ORB
    CORBA_ORB_var orb = CORBA_ORB_init(argc, argv);

    // initialize the BOA
    CORBA_BOA_var boa = orb -> BOA_init(argc, argv);

    // create a reference to the service
    Hello_var p = new Hello_impl;

    // export the reference
    CORBA_String_var s = orb -> object_to_string(p);
    const char* refFile = "Hello.ref";
    ofstream out(refFile);
    out << s << endl;
    out.close();

    // enter client request loop
    boa -> impl_is_ready(CORBA_ImplementationDef::_nil());
}

The Hello_impl class is defined in two files: a header file and an implementation file. The header file Hello_impl.h is

#include <Hello_skel.h>

class Hello_impl: public Hello_skel
{
 public:
    virtual void sayHello();
};

// Orbacus

#include <OB/CORBA.h>
#include <Hello_impl.h>

void Hello_impl::sayHello()
{
    cout << "Hello world" << endl;
}

CORBA and Java

Idl compiler

Given an IDL file Hello.idl, an "idl to Java" compiler will generate 5 files

• Hello.java - interface definition in Java
• HelloHelper.java - used on the client for "type casting"
• HelloHolder.java - used for out and inout parameters
• _HelloStub.java - client-side stub (the client HOPP) Sun
• _HelloImplBase.java - server-side implementation inherits from this (this is used for the server HOPP)

   idlj -f all -oldImplBase *.java

From the Hello.idl

interface Hello {
    void sayHello();
};

Simple Java client

The client starts from main(), and obtains a reference to a CORBA Object. It does this by reading the string version of the reference from a file, like the C++ case. It then casts it to an implementation of the Hello interface.

Simple Java server

The implementation of the service is

The server is

Different ORBs

Different vendors will need their own classes loaded in. This is generally done by setting properties to label the class files. For example, the Orbacus ORB requires this in the client

public class JClient {

    public static void main(String[] argv) {
        // Setup info about the CORBA implementation used
	java.util.Properties props = System.getProperties();
	props.put("org.omg.CORBA.ORBClass", "com.ooc.CORBA.ORB");
	props.put("org.omg.CORBA.ORBSingletonClass",
		  "com.ooc.CORBA.ORBSingleton");
	System.setProperties(props);

	org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(argv, props);
        // etc

The server changes are

public class JServer {

    static public void main(String[] argv) {
        // setup info about CORBA implementation
	java.util.Properties props = System.getProperties();
	props.put("org.omg.CORBA.ORBClass", "com.ooc.CORBA.ORB");
	props.put("org.omg.CORBA.ORBSingletonClass",
		  "com.ooc.CORBA.ORBSingleton");
	System.setProperties(props);

        // initialise ORB and BOA
	org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(argv, props);
	org.omg.CORBA.BOA boa = ((com.ooc.CORBA.ORB) orb).BOA_init(argv,props);
        // etc

Getting object references

Strings

The methods object_to_string() and string_to_object() can be used to store references to objects outside of a server so that clients can reconstruct them. These "stringified" references are portable to other machines - they contain location information about the service. For example

• The stringified reference may be stored in a file accessible to both server and clients
• The reference may form the contents of a Web page, so that it can be fetched as a URL
• The reference may be given as an applet parameter on a Web page, so that an applet can access the object

Naming service

A naming service is a process running independently of any other clients and servers. A service will register itself with the naming service, passing both a simple string as its name and an object reference to itself. A client will then ask the naming service for the service by using its name, and will get back an object reference.

Sun supply a naming service as part of JDK

tnameserv -ORBInitialPort nameserverport

All servers and clients will need to know the location of the naming service in order to find it and then register/query it. This is typically done by giving a port number or URL. e.g.

JClient -ORBInitialHost nameserverhost
        -ORBInitialPort nameserverport

Java resolution

The Naming Service is found this way in Java

org.omg.CORBA.Object obj;
org.omg.CosNaming.NamingContext ctx;

obj = orb.resolve_initial_references("NameService");
ctx = org.omg.CosNaming.NamingContextHelper.narrow(obj);

Java service

The service registers itself with the naming service by

// bind the Hello service to the name server
NameComponent nc = new NameComponent("Hello", "");
NameComponent path[] = {nc};
ctx.rebind(path, helloRef);

The Java Hello service using this is

import java.io.*;
import org.omg.CosNaming.*;

public class JServer {

    static public void main(String[] argv) {

	org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(argv, null);

	Hello_impl helloRef = new Hello_impl();

	// find the naming service
	org.omg.CORBA.Object obj = null;
	org.omg.CosNaming.NamingContext ctx;

	try {
	    obj = orb.resolve_initial_references("NameService");
	} catch(org.omg.CORBA.ORBPackage.InvalidName e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	ctx = org.omg.CosNaming.NamingContextHelper.narrow(obj);

	// bind the Hello service to the naming service
	NameComponent nc = new NameComponent("Hello", "");
	NameComponent path[] = {nc};
	try {
	    ctx.rebind(path, helloRef);
	} catch(Exception e) {
	    e.printStackTrace();
	    System.exit(1);
	}

        orb.connect(p);

        java.lang.Object sync = new java.lang.Object();
        synchronized(sync) {
            try {
                sync.wait();
            } catch(InterruptedException e) {
                // do nothing
            }
        }
    }
}

Java client

The client uses the Naming Service to get a reference to the Hello object by

NameComponent nc = new NameComponent("Hello", "");
NameComponent path[] = {nc};
org.omg.CORBA.Object obj = ctx.resolve(path);
Hello helloRef = HelloHelper.narrow(obj);

The Hello client using this is

import java.io.*;
import org.omg.CosNaming.*;

public class JClient {

    public static void main(String[] argv) {

	org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(argv, null);

	// find the naming service
	org.omg.CORBA.Object obj = null;
	org.omg.CosNaming.NamingContext ctx;
	
	try {
	    obj = orb.resolve_initial_references("NameService");
	} catch(org.omg.CORBA.ORBPackage.InvalidName e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	ctx = org.omg.CosNaming.NamingContextHelper.narrow(obj);

	// find the object reference
	NameComponent nc = new NameComponent("Hello", "");
	NameComponent path[] = {nc};
	org.omg.CORBA.Object helloObj = null;
	try {
	    helloObj = ctx.resolve(path);
	} catch(Exception e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	Hello helloRef = HelloHelper.narrow(helloObj);

	helloRef.sayHello();
    }
}

The naming service also supports a directory structure. For example, a list of shops coud be stored under the directory "shops", and the naming context can be asked to list the directory.

Trader

A Trader is a more advanced CORBA service than just Naming. A Naming Service associates a single string with a service. A Trader allows a set of properties to be associated with a service. A client can then search (using boolean operators) for services matching required properties. For example, a printer can register itself as a printer capable of printing Postscript and PDF files, with a speed of 24 ppm. A client can then search for a printer capable of printing PDF files with a speed of more than 20 ppm.

References

• A. Vogel and K. Duddy Java Programming with CORBA
• M. Henning Advanced CORBA Programming with C++
• Object Management Group The Common Object Request Broker: Architecture and Specification http://www.omg.org
• Object Management Group C++ Language mapping http://www.omg.org
• Object Management Group OMG IDL to Java Language Mapping http://www.omg.org