CORBA

1
CORBA

• Revision of
• Remote Method Invocation
• Interfaces
• If you understand this, you will be able to
• Explain the purpose of CORBA
• Explain how remote objects are found and used
• Compare CORBA and RMI
• What is an ORB?
• What does resolve mean?

2
Key Terms

• Heterogeneous peer-to-peer computing
• Interface Definition Language (IDL)
• Naming Service
• Common Facilities

3
The Problem

• Allowing interaction between components
• Different manufacturers
• Different languages (language transparency)
• Distributed computing
• (client-server/peer to peer)
• Location (location transparency)
• Communicating information

4
The Solution

• Put an intermediary between the objects
• Hide location of objects from client

Caller
Caller
Interconnection Network
Stub
Skeleton
Callee
Callee
5
CORBA

• Common Object Request Broker Architecture
• Standardised
• Object Management Group
• Collection of companies HP, DEC, SUN, ... (400)
• Manages the CORBA standards

6
Essence of CORBA

• An object can be
• Created in an application on one machine
• Used by another application
• Written in a different language
• On a different machine
• With a completely different architecture
• CORBA architecture defines interfaces.
• Implementation left to manufacturers

7
CORBA

• A framework and specification for object
  collaboration and sharing across a heterogeneous
  (what?) distributed network of machines and
  applications.
• What does CORBA do?
• Hides communication complexities
• Allow interaction between different languages
• Provides commonly useful services

8
CORBA Concepts

• Objects
• any entity in the system
• e.g. document, folder, application, window
  manager
• OID
• a unique identifier, constant during the objects
  life
• Operations
• methods or services that an object provides
• Operation Semantics
• operation behaviour under errors
• At most once
• Best Effort

9
CORBA Concepts

• Type (class)
• an object is an instance of a type
• Interface
• a description of what an object provides,
• not how it should be implemented.
• Object Request Broker
• manages links between objects.
• Common Facilities
• Support for data presentation, retrieval and
  storage
• Object Services
• extend the capabilities of an ORB

10
Object Request Brokers

• Purpose
• Allows objects to call methods of any other
  CORBA-compliant object without knowing how it is
  implemented.
• A request is similar to
• A SOAP request
• RMI marshalling of a method call

11
A Request

• An request contains
• Target object
• Method
• Parameters
• Request context

12
Types of ORB

• Client- or server-resident
• ORB implemented as libraries (routines) resident
  in the clients and the implementations (the
  servers)
• Server-based
• ORB is implemented as a server (separate process)
  that manages requests between client and
  implementation processes
• System-based
• ORB is part of the operating system.

13
CORBA Components
14
CORBA Components

• Object Adapter
• Provides services for managing objects
• creating, activating de-activating objects,
• generating and managing object references,
• routing requests to appropriate server methods,
• broadcasting the presence of object servers.
• Dynamic Invocation
• Construct and use a request at run-time.
• ORB Interface
• The set of ORB operations common to all objects
  (e.g. returning an object's interface type).

15
Interface Definition Language

• Allows an object to publish its interface
• telling ORB clients how to obtain its services
• Language-independent specification language
• Looks like C/Java declarations.
• modules,
• constants, exceptions,
• interfaces, methods, properties
• Mappings provided for many compilers
• i.e. translate IDL into C or Java
• Used to generate stubs, skeletons
• Supports static and dynamic requests.

16
CORBA Example

• Using the application

The server publishes a CORBA object with an
addText Method
The client gets the remote object on
Connect Calls its addText method on Send
17
CORBA Example

• The Interface
• module corbachatOps
• interface ICorbaChatOne
• void addText( in string s )
• Written in IDL
• What is an in parameter?
• What is a module?
• What is the equivalent in Java?

18
Remote Object (receiver)

• class CChatImpl extends _ICChatImplBase
• // _ICChatImplBase generated from the IDL
• public CChatImpl()
• //Display the frame, etc.
•  
• public void addText (String s)
• frame.displayText(s)

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The Server 1

• public class ChatOneServer
• public static void main(String args)
• try
• ORB orb ORB.init(args, null) 
• // register the CORBA component
• CChatOpsImpl corbaComp new
  CChatOpsImpl()
• orb.connect( corbaComp )
• // set up the naming service
• org.omg.CORBA.Object objRef
• orb.resolve_initial_references("NameSe
  rvice")
• // checked cast to make sure objRef is of
  right type
• NamingContext ncRef NamingContextHelper.n
  arrow(objRef)

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The Server 2

•   // Bind the object reference in the
  naming service
• NameComponent cc new NameComponent(
  "CChat", "" )
• NameComponent path cc
• ncRef.rebind( path, corbaComp )
•  
• // wait for clients
• java.lang.Object lock new
  java.lang.Object()
• synchronized( lock )
• lock.wait() // wait for a
  notify which will mark end
• catch( Exception e ) System.err.println(
  e )

21
Client 1 getting an ORB

• ORB orb
•  
• public SenderFrame(String args)
• ltsnipgt
• try
• // initialize the ORB using a static
  factory method
• orb ORB.init( args, null )
• jbInit() // Set up user interface
• ltsnipgt

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 Client 2 Connecting

• void connectBtn_actionPerformed(ActionEvent e)
  
• try
• ltsnipgt
• // get the root naming context
• org.omg.CORBA.Object objRef
• orb.resolve_initial_references(
  "NameService" )
• // What is this doing?
• NamingContext ncRef
• NamingContextHelper.narrow( objRef )
•  

23
Client 3 Getting the object

• // resolve name in the naming context
• NameComponent nc new NameComponent(
  "CChat", "" )
• NameComponent path nc
• org.omg.CORBA.Object obj
  ncRef.resolve(path)
•  
• // Cast the object to the correct type, with
  checking
• comp ICChatHelper.narrow(obj)
•  
• if (comp ! null)
• status.setText("Connected")
• else
• status.setText("")
• catch(Exception ex) ltsnip handle errorgt
  

24
 Client 4 Using the Object

• void sendBtn_actionPerformed(ActionEvent e)
• if (comp ! null)
• comp.addText(jTextArea1.getText())
• else
• status.setText("")

25
Services 1

• Object Services
• Services to extend the capabilities of an ORB
• Event Notification Services
• Permit objects to register and deliver events
• Persistence Services
• Support for storing objects in different ways
• e.g. relational databases
• Life-Cycle Management Services
• e.g. Support for activating and freeing objects
  after use
• Transaction Services
• Provide a two-phase commit protocol
• Nested transactions are supported

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Services 2

• Concurrency Control Services
• Provide lock management for transactions or
  threads
• Relationships Services
• Provide general services for creating and
  managing links between objects
• e.g. to manage referential integrity constraints
• Externalisation Services
• Provide an object stream interface to transfer
  data into and out of an object
• Other services
• e.g. security, time and licensing

27
Common Facilities

• General - purpose components
• Dealing with
• presentation of data
• retrieval and storage of information.

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Common Facilities

• Horizontal
• Services across a layer of the 'virtual machine'
• Common to range of application areas
• e.g. mobile agents, printing, internationalisation
  
• Vertical
• Services concerned with a specific application,
  potentially at different levels
• e.g. health, financial, legal, manufacturing,e-com
  merce

29
Summary

• CORBA is like RMI in purpose operation
• Interfaces
• Automatically generate Stubs
• CORBA can be used with any language
• Define interfaces in IDL
• Language mapping generates interfaces stubs
• CORBA provides a wider range of services