CORBA.ppt

12/15/2022 Distributed Computing, M. L. Liu 1
The Common Object Request
Broker Architecture (CORBA)
M. L. Liu

CORBA
 The Common Object Request Broker
Architecture (CORBA) is a standard
architecture for a distributed objects system.
 CORBA is designed to allow distributed
objects to interoperate in a heterogenous
environment, where objects can be
implemented in different programming
language and/or deployed on different
platforms

CORBA vs. Java RMI
 CORBA differs from the architecture of Java
RMI in one significant aspect:
 RMI is a proprietary facility developed by Sun
MicroSystems, Inc., and supports objects written
in the Java programming langugage only.
 CORBA is an architecture that was developed by
the Object Management Group (OMG), an
industrial consortium.

CORBA
 CORBA is not in inself a distributed objects facility;
instead, it is a set of protocols.
 A distributed object facility which adhere to these
protocols is said to be CORBA-compliant, and the
distributed objects that the facility support can
interoperate with objects supported by other CORBA-
compliant facilities.
 CORBA is a very rich set of protocols. We will
instead focus on the key concepts of CORBA related
to the distributed objects paradigm. We will also
study a facility based on CORBA: the Java IDL.

The Basic Architecture
object client
naming service
naming
lookup
stub
ORB
network
operating
system
object
implementation
skeleton
ORB
network
operating
system
logical data flow
physical data flow

CORBA Object Interface
 A distributed object is defined using a software file similar to
the remote interface file in Java RMI.
 Since CORBA is language independent, the interface is
defined using a universal language with a distinct syntax,
known as the CORBA Interface Definition Language (IDL).
 The syntax of CORBA IDL is similar to Java and C++.
However, object defined in a CORBA IDL file can be
implemented in a large number of diverse programming
languages, including C, C++, Java, COBOL, Smalltalk, Ada,
Lisp, Python, and IDLScript.
 For each of these languages, OMG has a standardized mapping
from CORBA IDL to the programming language, so that a
compiler can be used to process a CORBA interface to
generate the proxy files needed to interface with an object
implementation or an object client written in any of the
CORBA-compatible languages.

Cross-language CORBA application
object client written in Java
stub in Java generated by compiling
the CORBA object interface
ORB written in Java
object implementation written
in C++
skeleton in C++ generated by
compiling the CORBA object
interface
ORB written in C++

ORB Core Feature Matrix
ORB Core Feature Matrix
http://www.jetpen.com/~ben/corba/orbmatrix.html

Inter-ORB Protocols
 To allow ORBs to be interoperable, the
OMG specified a protocol known as the
General Inter-ORB Protocol (GIOP), a
specification which “provides a general
framework for protocols to be built on top
of specific transport layers.”
 A special case of the protocol is the Inter-
ORB Protocol (IIOP), which is the GIOP
applied to the TCP/IP transport layer.

Inter-ORB Protocols
The IIOP specification includes the following elements:
1. Transport management requirements: specifies the
connection and disconnection requirements, and the
roles for the object client and object server in making
and unmaking connections.
2. Definition of common data representation: a coding
scheme for marshalling and unmarshalling data of each
IDL data type.
3. Message formats: different types of message format
are defined. The messages allow clients to send
requests to object servers and receive replies. A client
uses a Request message to invoke a method declared in
a CORBA interface for an object and receives a reply
message from the server.

Object Bus
An ORB which adheres to the specifications of the IIOP may
interoperate with any other IIOP-compliant ORBs over the
Internet. This gives rise to the term “object bus”, where the
Internet is seen as a bus that interconnects CORBA objects
The Internet
CORBA
object
CORBA
object
CORBA
object
ORB ORB ORB
...

ORB products
There are a large number of proprietary as well
as experimental ORBs available:
(See CORBA Product Profiles,
http://www.puder.org/corba/matrix/)
 Orbix IONA
 Borland Visibroker
 PrismTech’s OpenFusion
 Web Logic Enterprise from BEA
 Ada Broker from ENST
 Free ORBs

Object Servers and Object Clients
 As in Java RMI, a CORBA distributed object
is exported by an object server, similar to the
object server in RMI.
 An object client retrieves a reference to a
distributed object from a naming or directory
service, to be described, and invokes the
methods of the distributed object.

CORBA Object References
 As in Java RMI, a CORBA distributed object
is located using an object reference. Since
CORBA is language-independent, a CORBA
object reference is an abstract entity mapped to
a language-specific object reference by an
ORB, in a representation chosen by the
developer of the ORB.
 For interoperability, OMG specifies a protocol
for the abstract CORBA object reference
object, known as the Interoperable Object
Reference (IOR) protocol.

Interoperable Object Reference (IOR)
 For interoperability, OMG specifies a protocol
for the abstract CORBA object reference
object, known as the Interoperable Object
Reference (IOR) protocol.
 An ORB compatible with the IOR protocol
will allow an object reference to be registered
with and retrieved from any IOR-compliant
directory service. CORBA object references
represented in this protocol are called
Interoperable Object References (IORs).

An IOR is a string that contains encoding
for the following information:
 The type of the object.
 The host where the object can be found.
 The port number of the server for that object.
 An object key, a string of bytes identifying the
object.
The object key is used by an object server to
locate the object.

The following is an example of the string
representation of an IOR [5]:
IOR:000000000000000d49444c3a677269643a312e3000000
00000000001000000000000004c0001000000000015756c74
72612e6475626c696e2e696f6e612e6965000009630000002
83a5c756c7472612e6475626c696e2e696f6e612e69653a67
7269643a303a3a49523a67726964003a
The representation consists of the
character prefix “IOR:” followed by a
series of hexadecimal numeric characters,
each character representing 4 bits of
binary data in the IOR.

CORBA Naming Service
 CORBA specifies a generic directory service.
The Naming Service serves as a directory for
CORBA objects, and, as such, is platform
independent and programming language
independent.
 The Naming Service permits ORB-based clients
to obtain references to objects they wish to use.
It allows names to be associated with object
references. Clients may query a naming service
using a predetermined name to obtain the
associated object reference.

 To export a distributed object, a CORBA object
server contacts a Naming Service to bind a symbolic
name to the object The Naming Service maintains a
database of names and the objects associated with
them.
 To obtain a reference to the object, an object client
requests the Naming Service to look up the object
associated with the name (This is known as resolving
the object name.)
 The API for the Naming Service is specified in
interfaces defined in IDL, and includes methods that
allow servers to bind names to objects and clients to
resolve those names.

To be as general as possible, the CORBA object naming
scheme is necessary complex. Since the name space is
universal, a standard naming hierarchy is defined in a manner
similar to the naming hierarchy in a file directory
naming context1
naming context1 naming context2
naming context1 naming context1
object
name1
object
namen
...
...
...
...

A Naming Context
 A naming context correspond to a folder or directory
in a file hierarchy, while object names corresponds to
a file.
 The full name of an object, including all the
associated naming contexts, is known as a compound
name. The first component of a compound name
gives the name of a naming context, in which the
second component is accessed. This process
continues until the last component of the compound
name has been reached.
 Naming contexts and name bindings are created using
methods provided in the Naming Service interface.

A CORBA object name
The syntax for an object name is as follows:
<naming context > …<naming context><object name>
where the sequence of naming contexts
leads to the object name.

Example of a naming hierarchy
As shown, an object representing the men’s
clothing department is named
store.clothing.men, where store and clothing
are naming contexts, and men is an object
name.
store
clothing Appliances
women men
...
television
...

Interoperable Naming Service
The Interoperable Naming Service (INS) is a URL-
based naming system based on the CORBA Naming
Service, it allows applications to share a common
initial naming context and provide a URL to access a
CORBA object.

CORBA Object Services
CORBA specify services commonly needed in distributed
applications, some of which are:
 Naming Service:
 Concurrency Service:
 Event Service: for event synchronization;
 Logging Service: for event logging;
 Scheduling Service: for event scheduling;
 Security Service: for security management;
 Trading Service: for locating a service by the type (instead of by
name);
 Time Service: a service for time-related events;
 Notification Service: for events notification;
 Object Transaction Service: for transactional processing.
Each service is defined in a standard IDL that can be
implemented by a developer of the service object, and whose
methods can be invoked by a CORBA client.

Object Adapters
In the basic architecture of CORBA, the implementation of a
distributed object interfaces with the skeleton to interact with
the stub on the object client side. As the architecture evolved,
a software component in addition to the skeleton was needed
on the server side: an object adapter.
distributed object
implementation
object adapter
ORB

Object Adapter
 An object adapter simplifies the
responsibilities of an ORB by assisting an
ORB in delivering a client request to an object
implementation.
 When an ORB receives a client’s request, it
locates the object adapter associated with the
object and forwards the request to the adapter.
 The adapter interacts with the object
implementation’s skeleton, which performs
data marshalling and invoke the appropriate
method in the object.

The Portable Object Adapter
 There are different types of CORBA object
adapters.
 The Portable Object Adapter, or POA, is a
particular type of object adapter that is defined
by the CORBA specification. An object
adapter that is a POA allows an object
implementation to function with different
ORBs, hence the word portable.

The Java IDL (Java 1.4 version)

Java IDL – Java’s CORBA Facility
 IDL is part of the Java 2 Platform, Standard
Edition (J2SE).
 The Java IDL facility includes a CORBA
Object Request Broker (ORB), an IDL-to-Java
compiler, and a subset of CORBA standard
services.
 In addition to the Java IDL, Java provides a
number of CORBA-compliant facilities,
including RMI over IIOP, which allows a
CORBA application to be written using the
RMI syntax and semantics.

Key Java IDL Packages
 package org.omg.CORBA – contains interfaces and
classes which provides the mapping of the OMG
CORBAAPIs to the Java programming language
 package org.omg.CosNaming - contains interfaces
and classes which provides the naming service for
Java IDL
 org.omg.CORBA.ORB - contains interfaces and
classes which provides APIs for the Object Request
Broker.

Java IDL Tools
Java IDL provides a set of tools needed for
developing a CORBA application:
 idlj - the IDL-to-Java compiler (called idl2java in
Java 1.2 and before)
 orbd - a server process which provides Naming
Service and other services
 servertool – provides a command-line interface for
application programmers to register/unregister an
object, and startup/shutdown a server.
 tnameserv – an olderTransient Java IDL Naming
Service whose use is now discouraged.

A Java IDL application example

The CORBA Interface file Hello.idl
01. module HelloApp
02. {
03. interface Hello
04. {
05. string sayHello();
06. oneway void shutdown();
07. };
08. };

Compiling the IDL file (using Java 1.4)
The IDL file should be placed in a directory dedicated to the
application. The file is compiled using the compiler idlj using
a command as follows:
idlj -fall Hello.idl
The –fall command option is necessary for the compiler to
generate all the files needed.
In general, the files can be found in a subdirectory named <some
name>App when an interface file named <some name>.idl is
compiled.
If the compilation is successful, the following files can be found
in a HelloApp subdirectory:
HelloOperations.java Hello.java
HelloHelper.java HelloHolder.java
_HelloStub.java HelloPOA.java
These files require no modifications.

The *Operations.java file
 There is a file HelloOperations.java
 found in HelloApp/ after you compiled using
idlj
 It is known as a Java operations interface in
general
 It is a Java interface file that is equivalent to
the CORBA IDL interface file (Hello.idl)
 You should look at this file to make sure that
the method signatures correspond to what you
expect.

HelloApp/HelloOperations.java
The file contains the methods specified in the original IDL file:
in this case the methods sayHello( ) and shutdown().
package HelloApp;
01. package HelloApp;
04. /**
05. * HelloApp/HelloOperations.java
06. * Generated by the IDL-to-Java compiler (portable),
07. * version "3.1" from Hello.idl
08. */
09.
10. public interface HelloOperations
11. {
12. String sayHello ();
13. void shutdown ();
14. } // interface HelloOperations

HelloApp/Hello.java
The signature interface file combines the characteristics of
the Java operations interface (HelloOperations.java) with
the characteristics of the CORBA classes that it extends.
01. package HelloApp;
03. /**
04. * HelloApp/Hello.java
05. * Generated by the IDL-to-Java compiler (portable),
06. * version "3.1" from Hello.idl
07. */
09. public interface Hello extends HelloOperations,
10. org.omg.CORBA.Object,
11. org.omg.CORBA.portable.IDLEntity
12. { …
13. } // interface Hello

HelloHelper.java, the Helper class
 The Java class HelloHelper (Figure
7d) provides auxiliary functionality needed to
support a CORBA object in the context of the
Java language.
 In particular, a method, narrow,allows a
CORBA object reference to be cast to its
corresponding type in Java, so that a CORBA
object may be operated on using syntax for
Java object.

HelloHolder.java, the Holder class
 The Java class called HelloHolder
(Figure 7e) holds (contains) a reference
to an object that implements the Hello
interface.
 The class is used to handle an out or an
inout parameter in IDL in Java syntax
( In IDL, a parameter may be declared to be
out if it is an output argument, and inout if the
parameter contains an input value as well as
carries an output value.)

_HelloStub.java
 The Java class HelloStub (Figure 7e) is
the stub file, the client-side proxy, which
interfaces with the client object.
 It extends
org.omg.CORBA.portable.ObjectImpl
and implements the Hello.java interface.

HelloPOA.java, the server skeleton
 The Java class HelloImplPOA (Figure
7f) is the skeleton, the server-side proxy,
combined with the portable object adapter.
 It extends org.omg.PortableServer.Servant,
and implements the InvokeHandler interface
and the HelloOperations interface.

The application
Server-side Classes
 On the server side, two classes need to be
provided: the servant and the server.
 The servant, HelloImpl, is the implementation
of the Hello IDL interface; each Hello object
is an instantiation of this class.

The Servant - HelloApp/HelloImpl.java
// The servant -- object implementation -- for the Hello
// example. Note that this is a subclass of HelloPOA,
// whose source file is generated from the
// compilation of Hello.idl using j2idl.
06. import HelloApp.*;
07. import org.omg.CosNaming.*;
08. import java.util.Properties; …
15. class HelloImpl extends HelloPOA {
16. private ORB orb;
18. public void setORB(ORB orb_val) {
19. orb = orb_val;
20. }
22. // implement sayHello() method
23. public String sayHello() {
24. return "nHello world !!n";
25. }
27. // implement shutdown() method
28. public void shutdown() {
29. orb.shutdown(false);
30. }
31. } //end class

The server - HelloApp/HelloServer.java
public class HelloServer {
public static void main(String args[]) {
try{
// create and initialize the ORB
ORB orb = ORB.init(args, null);
// get reference to rootpoa & activate the POAManager
POA rootpoa =
(POA)orb.resolve_initial_references("RootPOA");
rootpoa.the_POAManager().activate();
// create servant and register it with the ORB
HelloImpl helloImpl = new HelloImpl();
helloImpl.setORB(orb);
// get object reference from the servant
org.omg.CORBA.Object ref =
rootpoa.servant_to_reference(helloImpl);
// and cast the reference to a CORBA reference
Hello href = HelloHelper.narrow(ref);

HelloApp/HelloServer.java - continued
// get the root naming context
// NameService invokes the transient name service
org.omg.CORBA.Object objRef =
orb.resolve_initial_references("NameService");
// Use NamingContextExt, which is part of the
// Interoperable Naming Service (INS) specification.
NamingContextExt ncRef =
NamingContextExtHelper.narrow(objRef);
// bind the Object Reference in Naming
String name = "Hello";
NameComponent path[] = ncRef.to_name( name );
ncRef.rebind(path, href);
System.out.println
("HelloServer ready and waiting ...");
// wait for invocations from clients
orb.run();

The object client application
 A client program can be a Java application, an applet,
or a servlet.
 The client code is responsible for creating and
initializing the ORB, looking up the object using the
Interoperable Naming Service, invoking the narrow
method of the Helper object to cast the object
reference to a reference to a Hello object
implementation, and invoking remote methods using
the reference. The object’s sayHello method is
invoked to receive a string, and the object’s shutdown
method is invoked to deactivate the service.

// A sample object client application.
import HelloApp.*;
import org.omg.CosNaming.*; …
public class HelloClient{
static Hello helloImpl;
public static void main(String args[]){
try{
ORB orb = ORB.init(args, null);
org.omg.CORBA.Object objRef =
orb.resolve_initial_references("NameService");
NamingContextExt ncRef =
NamingContextExtHelper.narrow(objRef);
helloImpl =
HelloHelper.narrow(ncRef.resolve_str(“Hello”));
System.out.println(helloImpl.sayHello());
helloImpl.shutdown();

Compiling and Running a Java IDL application
1. Create and compile the Hello.idl file on the server
machine:
idlj -fall Hello.idl
2. Copy the directory containing Hello.idl (including
the subdirectory generated by idlj) to the client
machine.
3. In the HelloApp directory on the client machine:
create HelloClient.java. Compile the *.java files,
including the stubs and skeletons (which are in the
directory HelloApp):
javac *.java HelloApp/*.java

4. In the HelloApp directory on the server machine:
 Create HelloServer.java. Compile the .java files:
javac *.java HelloApp/*.java
 On the server machine: Start the Java Object Request
Broker Daemon, orbd, which includes a Naming Service.
To do this on Unix:
orbd -ORBInitialPort 1050 -ORBInitialHost
servermachinename&
To do this on Windows:
start orbd -ORBInitialPort 1050 -ORBInitialHost
servermachinename

5. On the server machine, start the Hello server, as
follows:
java HelloServer –ORBInitialHost <nameserver host
name> -ORBInitialPort 1050
6. On the client machine, run the Hello application
client. From a DOS prompt or shell, type:
java HelloClient -ORBInitialHost nameserverhost
-ORBInitialPort 1050
all on one line.
Note that nameserverhost is the host on which the
IDL name server is running. In this case, it is the
server machine.

7. Kill or stop orbd when finished. The name
server will continue to wait for invocations
until it is explicitly stopped.
8. Stop the object server.

Summary-1
 You have been introduced to
 the Common Object Request Broker
Architecture (CORBA), and
 a specific CORBA facility based on the
architecture: Java IDL

Summary-2
 The key topics introduced with CORBA are:
 The basic CORBA architecture and its emphasis on object
interoperability and platform independence
 Object Request Broker (ORB) and its functionalities
 The Inter-ORB Protocol (IIOP) and its significance
 CORBA object reference and the Interoperable Object
Reference (IOR) protocol
 CORBA Naming Service and the Interoperable Naming
Service (INS)
 Standard CORBA object services and how they are
provided.
 Object adapters, portable object Adapters (POA) and
their significance.

Summary-3
 The key topics introduced with Java IDL are:
 It is part of the Java TM 2 Platform, Standard Edition (J2SE)
 Java packages are provided which contain interfaces and
classes for CORBA support
 Tools provided for developing a CORBA application
include idlj (the IDL compiler) and orbd (the ORB and
name server)
 An example application Hello
 Steps for compiling and running an application.
 Client callback is achievable.
 CORBA tookits and Java RMI are comparable and alternative
technologies that provide distributed objects. An applicaton
may be implemented using either technology. However, there
are tradeoffs between the two.

CORBA.ppt

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