Common%20Object%20Request%20Broker%20Architecture%20(CORBA)

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Common Object Request Broker Architecture (CORBA)

• CS-328

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What is CORBA

• Specification for a standard O-O architecture for
  applications
• 1990 - Object Management Group publishes Object
  Management Architecture Guide
• 1991 - Common Object Request Broker Architectural
  Specification

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CORBAs Goal

• Integrate existing mainframe applications with
  desktop based tools
• Access distributed information and resources
  from within desktop tools
• Make existing business data available as network
  resources
• Augment desktop applications with custom
  functions and capabilities for a particular
  business
• Change and evolve network based systems to
  reflect new topologies or resources

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Distributed Computing

• Two or more pieces of software sharing
  information
• could be on same machine
• could be on different machines on same network
• could be on different machines on different
  networks
• based on client/server concepts
• Benefits
• sharing of scarce resources
• load balancing
• run applications on most appriate hardware
• Existing Mechanisms - RMI, RPC, Corba, network
  APIs

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Distributed Object Computing

• Marriage of Distributed Computing with an Object
  Model
• Uses a broker to handle messages requests between
  clients and servers
• broker can choose server that best fits needs of
  client
• allows separation of interface and
  implementation
• allows building block approach to development and
  evolution

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Object Model

• Provides
• Abstraction
• group objects and focus on their similarities
• Encapsulation
• hide implementation details from the services
  provided
• Inheritance
• ability to pass along (object-to-object)
  capabilities and behaviors
• Polymorphism
• ability to substitute objects with matching
  interfaces at run time

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Object Management Architecture
Application Objects
Common Facilities
Object Request Broker
Object Services
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What does the ORB do?

• The ORB provides a communications hub for all
  objects
• analogous to a hardware bus
• Provides object services
• creation, access control and object tracking
• Common Facilities
• db access, printing, synchronization, document
  management

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CORBA Object Communications

• CORBA Objects communicate via their interfaces
• Interfaces are defined using Interface Definition
  Language
• Language independent
• Designed to be language agnostic
• Directly maps most common data types
• There are some datatype mapping problems,
  especially when using different languages for the
  client and server.

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CORBA Object Communications
IIOP
Object A
Object B
Interface
Interface
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Conceptually ...
Client
Server
Client
Server
ORB
Client
Server
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What happens...

• Server objects register their methods with the
  orb naming service
• Client binds to the orb naming server for a
  server object method
• Naming service return location of server object
  and disconnects
• Client connects to server at returned location

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Really though...
Registration
ORB
1
Client
Server
Services
2
Client
Server
3
Client
Server
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3-Tier Client Server
Sockets JavaIDL JavaRMI
JDBC Native
Client
Client
Server
DB
Client
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3-Tier Advantages

• Server can manage database connections
  efficiently
• Client can focus on presentation
• Server can preformat data for client and focus on
  business logic
• Running server on high performance hardware can
  improve perceived performance

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Interface Definition Language

• In Java we cannot separate a classs definition
  from its implementations as we can in C
• Header files
• Implementation files
• CORBA allows the separation of definition and
  implementation

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IDL (more)

• CORBA uses IDL for defining interfaces between
  clients and servers
• ORB Vendors provide specific IDL compilers for
  supported languages
• create target language stubs and skeletonsfor
  building CORBA clients and servers
• C, C, Smalltalk, Java, COBOL

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IDL/Java type mapping

• due to differences in heritage IDL and Java types
  dont have a 1 to 1 mapping

long Java int short Java short float Java
float double Java double char Java
char boolean Java boolean octet java
byte string java.lang.String any Special type
consisting of any of the above
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an IDL interface

• ex.

interface Cooler int getHotWaterLevel()
int getWarnWaterLevel() int
getColdWaterLevel() exception
NoMoreWaterException int getHotWater()
throws NoMoreWaterException int
getWarmWater() throws NoMoreWaterException
int getColdWater() throws NoMoreWaterException
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Compiling the IDL

• Compiling the IDL using the javatoidl compiler
  will produce six classes
• CoolerRef
• CoolerHolder
• CoolerOperations
• CoolerStub
• CoolerServant
• CoolerSkeleton

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The ORB

• What is it that the ORB does ?
• Object location
• Location transparency
• Object communications
• Marshalls and unmarshalls parameter and results
  data
• Services
• Object Life Cycle Management
• Naming
• Event decoupling
• Relationship management
• Externalization
• Transaction management
• Concurrency control
• Property management
• Trader locating objects based on properties
• Object query

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OEM Orb Info

• The Borland VisiBroker Orb is built into
  Netscape Communicator 4.x but can be purchased
  seperately
• CORBA 3.0 Compliant
• Language support C, Java, .NET (C and C)
• Orbix Orbacus (Progress Software (previously
  Iona Technologies)
• CORBA 2.6 Compliant
• Language support - C, C, Java, COBOL,
  Smalltalk, Ada, PL/1, Lisp, Python and IDLscript.
  In additionally mappings for Perl, Objective-C,
  Oberon, Eiffel, Modula3, Scheme, and Tcl.
• J2SE - org.omg.CORBA ( in the JDK) since JDK 1.3)
  
• Provides the mapping of the OMG CORBA APIs to the
  JavaTM programming language, including the class
  ORB
• class ORB is implemented such that a programmer
  can use it as a fully-functional Object Request
  Broker (ORB).