Introduction to CORBA

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Introduction to CORBA

• Organizational Communications and Technologies
• Prithvi N. Rao
• H. John Heinz III School of Public Policy and
  Management
• Carnegie Mellon University

2
Readings

• Posting on the Class Web Site

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Objectives

• Understand goals of distributed object technology
• Examine evolution of Software Bus
• Cite CORBA challenges
• Review OO support in CORBA

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Seven Layer Model Revisited
OSI Model
Revised Model
Application
Religion
Presentation
Politics
Session
Finance
Environment
Transport
Network
Stacks
Data Link
Interfaces
Physical
Substrates
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Evolution of the Software Bus
Remote Msgs
Remote Procedure
Distributed Objects
DLL
LPC
DDE
OLE1.0
OLE2.0
DCOM
DCE
COM/CORBA
NCS RPC
ONC RPC
ONC
CORBA/IIOP
UDP/TCP
SHL
RRBC
JAVA/RMI
IPC
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Terms
Name
Abbreviation
Purpose
LPC
Local Procedure Call
Parameterized code segments
UDP
User Datagram Protocol
Intermachine messaging
TCP
Transmission Control Protocol
Reliable delivery of UDP messages
Messaging between processes
IPC
Inter-process Communication
SHL
Shared Library/Object
Dynamic Linking of shared code
RRBC
Release to release binary compat.
Support for versioning of shared Libs.
DDE
Dynamic data exchange
uSofts messaging between procs.
DLL
Dynamic Link Library
uSofts shared library/object
OLE 1.0
Object Linking and Embedding
Extension of DDE for windows events
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Terms
Name
Abbreviation
Purpose
COM
Component Object Model
Extension DLL/C supporting versioning
DCOM
Distributed COM
Extension of OLE2.0 supporting IPC
RPC
Remote Procedure Call
Proxies making RPC look like LPC
NCS RPC
Apollos inter-machine marshalling
Network Computing Service
ONC RPC
Open Network Computing
Suns inter-machine marshalling
Common Object Request Broker Architecture
Extends NCS/DCE for dist. Object comm.
CORBA
Internet Inter-ORB Protocol
CORBA 2.0 standard marshalling protocol
IIOP
RMI
Remote Method Invocation
Dist. Java-java ipc marshalling protocol
SNMP
Simple Network Management Protocol
Remote attribute query mechanism
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Software Interconnect Bus
Application Objects
Common Facilities
Software Interconnect Bus
Services
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Distributed Objects
CORBA brings together three technologies
Objects
Remote Procedure Calls
Threaded Execution
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Why do we care?

• Interested in heterogeneous distributed
  application development
• Advanced complexity of applications
• Security
• Transactions
• Naming
• Open standard development

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Distributed Objects Combining Three Technologies
Objects Remote Procedure
Calls Threaded Execution
Concurrency Priorities Pre-emption
Inheritance Encapsulation Polymorphism
Distribution Heterogeneity Procedural Model
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The Basic Idea

• Take an Object
• Make it distributable
• Make it distributed

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Terminology

• DistributedObjects and components are dispersed
  across processes that may exist on the same
  machine or located across machines on a network.
• HeterogeneousApplication whose parts are
  located on machine that vary by hardware or by
  operating system or by programming language.
• Object-OrientedApplications written using
  objects.

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CORBA Challenges

• Presents two challenges of developing distributed
  systems
• Making distributed application development no
  more difficult than developing centralized
  programs
• Providing an infrastructure to integrate
  application components into a distributed
  application
• Challenges are easier said than done due to
• Partial failures
• Impact of latency
• Load balancing
• Event ordering

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General ORB Structure
Op(args)
CLIENT
Object Implementation
IDL Skeleton
Object Adapter
Dynamic II
IDL Stubs
ORB Interface
Object Request Broker
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CORBA Interface Definition Language (IDL)

• OMG IDL is an object-oriented interface
  definition language
• Used to specify interface containing methods and
  attributes
• OMG IDL support interface inheritance both single
  and multiple inheritance
• OMG IDL is designed to map onto multiple
  programming languages
• C, C, Smalltalk, COBOL, Modula 3, DCE

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OMG IDL Compiler

• OMG IDL compiler generates client stubs and
  server skeletons
• Stubs and skeletons automate the following
  activities in conjunction with the ORB
• Client proxy factories
• Parameter marshalling and unmarshalling
• Implementation class interface generation
• Object registration and activation
• Object location and binding

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Requesting Service Using CORBA (and DCE)
Application Code
Server Code
Client stub generated by IDL compiler
Server stub generated by IDL compiler
Runtime System
Runtime System
Client
Server
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CORBA Services (OMG specified)

• Services essential for implementing objects
• Concurrency control service protects integrity of
  an objects data when multiple requests to object
  are processed concurrently
• Event service that supports notification of
  interested parties when program-defined events
  occur
• Externalization service that supports conversion
  of object state to a form that can be transmitted
  between systems by means other than a request
  broker
• Life Cycle service that support creation,
  copying, moving and destruction of objects

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CORBA Services (OMG specified)

• Naming service that permits object reference to
  be retrieved through associations between names
  and objects and for those associations to be
  created and destroyed
• Persistent Object service that supports
  persistence of an objects state when the object
  is not active in memory and between application
  executions.
• Query service that supports operations on sets
  and collections of objects.
• Relationship services that provides for creating,
  deleting, navigating and managing relationships
  between objects
• Containment relationship between a folder
  object and document objects contained in that
  folder.

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CORBA Services (OMG Specified)

• Transaction service provides support for ensuring
  that a computation consisting of one or more
  operations on one or more objects satisfies the
  requirements of
• Atomicity in case of failure
• Isolation transactions run concurrently but
  results are the same as if they ran serially
• Durability result of successful transaction is
  never lost

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CORBA Services (OMG specified)

• Security service that supports authentication,
  authorization, integrity and privacy to degrees
  and using mechanisms yet to be determined.
• Time service that provides synchronized clocks to
  all objects regardless of location.
• Collection service that support creation and
  manipulation of collections of objects.
• Replication service that would provide for the
  explicit replication of objects in a distributed
  environment for the purpose of fault tolerance
  and for management of consistency of replicated
  copies.
• Trader service that provides matchmaking service
  between clients seeking services and objects
  offering services

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CORBA and Object Oriented Support

• EncapsulationData and functions manipulating
  data are encapsulated in object. This enforces
  data hiding since the only way to access an
  objects data is through the operations in the
  objects public interface.

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CORBA and Object Oriented Support

• Abstraction
• Abstraction of common features shared by objects
  in classes. A class definition describes the data
  associated with each instance of the class,
  defines the set of operations that can be invoked
  on an instance of the class and prescribes the
  functions that are executed in response to
  requests for those operations.

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CORBA and Object Oriented Support

• InheritanceInheritance of interfaces and
  implementations. This is the mechanism that
  supports the specialization or refinement of
  classes into subclasses. It is also one example
  of reuse inn object-oriented programming.

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CORBA and Object Oriented Support

• Polymorphism
• Polymorphism is the ability for a request to
  specific operation to be handled differently
  depending on the type of object on which it is
  invoked. Example subclasses of a common
  superclass may override functions defined by the
  superclass to differentiate how instances of the
  subclasses and superclass behave.

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CORBA and Object Oriented Support

• Creation of new objects and classes
• Late binding of operation invocations to function
  calls allow programs to be written without regard
  for types objects they will manipulate

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Summary

• Examined evolution of distributed object
  technology
• Introduced CORBA services
• Examined OO support in CORBA