Distributed Programming CORBA

1
Distributed Programming - CORBA

• Marc Conrad
• D104 (Park Square Building)
• Marc.Conrad_at_beds.ac.uk
• These slides
• CORBA and the OMG
• An example CORBA implementation

2
The Common Object Request Broker Architecture

CORBA
3
The Common Object Request Broker Architecture

• CORBA is not a programming language but an
  architecture.
• CORBA is an (elegant and sophisticated) way to
  describe and link the use of objects in a
  distributed environment.
• The "dirty work", the implementation of the
  behaviour of the objects is out of the scope of
  CORBA.

4
The Common Object Request Broker Architecture

• CORBA is in the context of Object Oriented
  Design, Analysis, Programming.

BUT The objects can be located on different
machines all over the world and implemented in a
variety of languages!
5
The Common Object Request Broker Architecture

• The Object Request Broker (ORB) is itself an
  object which provides the services for accessing
  remote objects and transmitting data.
• Note that the implementation of the ORB depends
  on the specific vendor. The CORBA standard only
  defines the interface.

6
The Common Object Request Broker Architecture

• Task Compare with XML-RPC and RMI!

7
The Common Object Request Broker Architecture

• CORBA is a standard which has been developed by
  the OMG, the Object Management Group.
• The OMG is the world's largest computer industry
  consortium with over 800 members.
• See http//www.omg.org/cgi-bin/apps/membersearch.p
  l

8
The History of the OMG

• Founded 1989 by eleven companies as non profit
  organisation.
• Aims on standards for object oriented software
  production.
• Other projects
• MDA http//www.omg.org/mda/
• (Model Driven Architecture)
• UML http//www.omg.org/uml/
• (Unified Modeling Language)
• MOF http//www.omg.org/mof/
• (MetaObject Facility)

9
OMG technology

• The framework within which all OMG adopted
  technology fits is the OMA the Object
  Management Architecture
• http//www.omg.org/oma/
• So, learning CORBA is also on learning about
  Object Oriented technologies from a language
  independent view.

10
OMG goals

• The goals of the OMG are promotion of the
  object-oriented approach to software engineering
  and development of a common architectural
  framework for writing distributed object-oriented
  applications based on interface specifications
  for the object in the application.

11
The Common Object Request Broker Architecture

• Common also means something else
• CORBA is an architecture which integrates code
  written in languages as Java, C, C, Smalltalk,
  COBOL, Lisp, Python.

We focus here mainly on Java
12
CORBA Some TLAs (Three Letter Acronyms)

• OMG - Object Management Group
• ORB - Object Request Broker
• IDL - Interface Definition Language
• IOR - Interoperable Object Reference
• POA - Portable Object Adapter
• IIOP - Internet Inter-ORB Protocol

13
CORBA Development Process
0. Specify Service
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Server
14
The Interface Definition Language (IDL)

• The IDL provides a class definition.
• It defines the behaviour but not the
  implementation!
• The IDL is then translated (mapped) to a specific
  programming language.

15
Or, to rephrase the previous slide more
professionally

• OMG IDL is a purely declarative language designed
  for specifying programming-language-independent
  operational interfaces for distributed
  applications. OMG specifies a mapping from IDL to
  several different programming languages,
  including C, C, Smalltalk, COBOL, Ada, and
  Java. When mapped, each statement in OMG IDL is
  translated to a corresponding statement in the
  programming language of choice.
• from http//java.sun.com/j2se/1.3/docs/guide/idl/
  tutorial/GSIDL.html

?
16
The CORBA Development Process

• Write some IDL that describes the interfaces to
  the object or objects that will be used or
  implemented.
• Compile the IDL file.
• Identify the IDL compiler generated classes and
  interfaces.
• Write additional code.
• Run the application.

17
IDL and Java

• We discuss in this lecture the IDL to Java
  mapping, that means using an IDL for producing
  Java classes which are used in a Java
  environment.
• The idlj from the jdk maps the IDL to Java.
• Other languages which are similarly supported are
  C, Lisp, Smalltalk, ...

18
IDL Overview.
(diagram Java Programming with CORBA, OMG press,
page 143)
19
IDL Example
A module is a namespace similar to a Java
packages

• module Example
• interface Hello
• string sayHello()

Declares the application interface. The idlj maps
interfaces to Java classes.
Operations are mapped to (Java, C, ...) methods.
This is an example of an IDL which provides one
method, namely sayHello(). This code is saved in
a file Hello.idl
20
CORBA Development Process
0. Specify Service
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Server
21
Mapping Hello.idl to Java(Here with idlj)

• Go to a command line prompt and type in the
  following commandidlj fall oldImplBase
  Hello.idl

Name of Compiler
Name of the idl file
Option to generate also files only relevant for
Server
Means Do not use POA (well discuss this later)
22
Mapping Hello.idl to Java(Here with idlj)

• These Files are generated in the folder Example

23
Mapping Hello.idl to Java(Here with idlj)

• This abstract class is the server skeleton,
  providing basic CORBA functionality for the
  server. It implements the Hello.java interface.
  The server class HelloServant extends
  _HelloImplBase.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

24
Mapping Hello.idl to Java(Here with idlj)

• This class is the client stub, providing CORBA
  functionality for the client. It implements the
  Hello.java interface

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

25
Mapping Hello.idl to Java(Here with idlj)

• This signature interface contains the Java
  version of our IDL interface. The Hello.java
  interface extends org.omg.CORBA.Object, providing
  standard CORBA object functionality. This class
  is the distributed CORBA object.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

26
Mapping Hello.idl to Java(Here with idlj)

• This operations interface contains the single
  method sayHello(). The IDL-to-Java mapping puts
  all of the operations defined on the IDL
  interface into this file.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

27
Mapping Hello.idl to Java(Here with idlj)

• This final class provides auxiliary
  functionality, notably the narrow() method
  required to cast CORBA object references to their
  proper types.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

28
Mapping Hello.idl to Java(Here with idlj)

• This final class holds a public instance member
  of type Hello. It provides operations for out and
  inout arguments, which CORBA allows, but which do
  not map easily to Java's semantics.
• Note that CORBA allows call-by-value method
  parameters which are not directly available in
  Java.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

29
Writing the Server.
Server
Servant

• For the implementation of the Server we have to
  implement two classes
• The Servant, i.e. the object which has been
  specified by the Hello.idl
• The Server itself, i.e. the program running in
  the background which reacts on client requests.

30
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

31
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• Makes the classes accessible which have been
  generated by the idlj compiler.

32
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• Note that _HelloImplBase is one of the classes
  which has been generated by idlj -fall.

33
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• This class extends _HelloImplBase, that means it
  has to implement the interface provided by
  _HelloImplBase, which has been produced by
  Hello.idl via idlj.

34
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• This was the file Hello.idl
• module Example
• interface Hello
• string sayHello()

35
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• This was the file Hello.idl
• Example
• interface Hello
• string sayHello()

36
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• This was the file Hello.idl
• Example
• interface Hello
• string sayHello()

37
Writing the Servant

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

• This was the file Hello.idl
• Example
• interface Hello
• string sayHello()

38
Writing the Servant

• This is the Implementation of the Hello.idl
  object.

• import Example.
• public class HelloServant extends _HelloImplBase
  
• public String sayHello()
• return "\nHello world !!\n"

39
Implementing the Server

• The Server has to perform the following tasks
• Initialise the ORB
• Instantiate a HelloServant object.
• Register this object in the ORB.
• Publish the object to the rest of the world.
• Wait for client requests.

40
Implementing the Server

• The file HelloServer.java

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

41
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• We import
• the java.io for file input/output
• CORBA for the ORB stuff
• the files generated by the idlj.

42
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• The Server runs as an application and has only a
  main method.

43
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• Initialise the ORB. Here we are using the ORB
  provided by the JDK.

44
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• Make an instance of the Servant class, and ...

45
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• tell the ORB about it.

46
Implementing the Server

• We generate a stringified reference of the
  object (and write it to a file).

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new FileOutputStream(fi
  lename)
• PrintStream ps new PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

47
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• Keep the server alive.

48
Implementing the Server

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(hello
  Ref)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

• Catch any errors.

49
Implementing the Server

• Initialise the ORB
• Instantiate a HelloServant object.
• Register this object in the ORB.
• Publish the object to the rest of the world
• Wait for client requests.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• HelloServant helloRef new
  HelloServant()
• orb.connect(helloRef)
• String str orb.object_to_string(helloRef)
• String filename "A//HelloIOR"
• FileOutputStream fos new
  FileOutputStream(filename)
• PrintStream ps new
  PrintStream(fos)
• ps.print(str) ps.close()
• java.lang.Object sync new
  java.lang.Object()
• synchronized (sync)
• sync.wait()
• catch (Exception e)

50
Implementing the Client

• A client has to perform the following tasks
• Initialise the ORB.
• Obtaining an object reference.
• Narrowing (casting) the reference.
• Using the object.

51
Implementing the Client.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

• The file HelloClient.java

52
Implementing the Client.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

• Initialising the ORB.

53
Implementing the Client.

• Reading the stringified object reference from the
  file.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new FileInputStream(fil
  ename)
• java.io.DataInputStream dis
• new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

54
Implementing the Client.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

• Generating an object reference from the string

55
Implementing the Client.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

• Casting the object to the expected type.

56
Implementing the Client.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

• Using the object.

57
Implementing the Client.

• Initialising the ORB.
• Retrieving an object reference.
• Narrowing the object.
• Using the object.

• import java.io. import org.omg.CORBA. import
  Example.
• public class HelloClient
• public static void main(String args)
• try
• ORB orb ORB.init(args, null)
• String filename "A//HelloIOR"
• FileInputStream fis new
  FileInputStream(filename)
• java.io.DataInputStream dis new
  java.io.DataInputStream(fis)
• String ior dis.readLine()
• org.omg.CORBA.Object obj
  orb.string_to_object(ior)
• Hello helloRef HelloHelper.narrow(ob
  j)
• String str helloRef.sayHello()
• System.out.println(str)
• catch (Exception e)
• System.out.println("ERROR " e)
• e.printStackTrace(System.out)

58
Running the Application

• Run the Server
• (Send the disk with the stringified object
  reference via Royal Mail to the remote machine)
• Better link the object to a naming service
• Run the Client
• (There is no additional naming service required
  as stringified object references are used)

59
CORBA Development Process
0. Specify Service
IDLkdfjshkdsahfkdashfkajshfkdsjhkjsahfkdshfkhasdf
kashfkdjsahfkshdfkhsafkjshfkdsfhakjdhfksjdhfakhsfk
jsdfkdasjfkdasjksdajfkdsafhksajhk assfksakshfdkjf
Server
60
A stringified reference

• Here is an example of an stringified object
  reference, generated by the example code
• IOR000000000000001649444c3a4578616d706c652f48656c
  6c6f3a312e3000000000000001000000000000005800010100
  0000000f3139342e38302e3231352e32323300000682000000
  000018afabcaff0000000225163ffd00000008000000000000
  00000000000100000001000000140000000000010020000000
  000001010000000000

Observe, that the stringified object reference is
in plain text. It can be passed by email or a
sheet of paper.
61
The CORBA services

• Instead of sending a stringified object reference
  we can use CORBA services for obtaining object
  references
• The CORBA naming service, which finds objects by
  name, similar to a "white page" phone book.
• The CORBA trading service, which finds references
  that match search criteria, similar to a "yellow
  page" service.
• Question How do we find the naming service
  without a naming service?

62
CORBA Services

• The initial references to instances of those
  services can be found via operations which are
  part of the ORB interface ("bootstrapping"),
  namely
• list_initial_services()
• resolve_initial_references()
• The association between an ORB and services is
  outside the CORBA definition.

63
CORBA Development Process
0. Specify Service
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kashfkdjsahfkshdfkhsafkjshfkdsfhakjdhfksjdhfakhsfk
jsdfkdasjfkdasjksdajfkdsafhksajhk assfksakshfdkjf
Server
64
IIOP

• See http//www.omg.org/library/iiop4.html if you
  really want to know.

65
Summary

• The IDL provides class definitions for objects.
• A Compiler compiles the IDL into several files.
• The server and the client are then implemented by
  using these files.
• The ORB plays a crucial role in running the
  application both for the server and for the
  client.