Chapter 5 Distributed Objects and Remote Invocation

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Chapter 5Distributed Objects and Remote
Invocation

• Choe, Hyun Jong
• Dept. of Computer Education
• Korea National Univ. Of Education

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Chapter Overview

• Middleware
• Interface definition
• Communication between distributed objects
• Java RMI case study

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Middleware

• A software layer on top of the OS that hides
  heterogeneity from the application programmer
• Location transparency(local vs. remote
  invocation location of server)
• Multiple languages(CORBA)

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Interfaces (1)

• An interface specifies the procedures and the
  variables that can be accessed from other modules
  of an application
• An implementation may change without effecting
  the module of the others.
• Interfaces in distributed systems
• No direct access to variables(RPC and RMI) but
  CORBAs IDL breaks this rule with attributess
• Call by value and call by reference are not
  suitable
• Describes the parameters as input or output or
  sometimes both

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Interfaces (2)

• RMIs Java Interfaces (Java)
• Public interface PersonList
• Void addPerson(string name)
• CORBAs IDL (Java)
• struct Person
• string name
• long year
• interface PersonList
• void addPerson(in Person p)

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Communication between distributed objects

• OO model
• Distributed objects
• Distributed object model
• Implementation
• Distributed garbage collection

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Object model

• An OO app is a collection of interacting objects
• Object references
• Objects are accessed via object references
• Interfaces
• Definition of the signatures of a set of methods
• Exceptions
• A clean way to deal with error conditions without
  complication the code
• Garbage collection
• Freeing the space occupied by objects when they
  are no longer needed(automatically)
• Java supports garbage collection but C doesnt

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Distributed objects

• Physical distribution of objects into different
  processes or computers is a natural extension of
  the OO model.
• Adopt the C/S architecture(C/S invoke methods)
• Encapsulation objects in different processes
  and therefore the state can be accessed my
  methods
• No unauthorized access to the state
• Concurrent access is synchronized

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Distributed object model (1)

• How to extend the OO model to make it applicable
  to distributed objects
• Two fundamental concepts
• Remote object reference
• Remote interfaces

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Distributed object model (2)

• First fundamental concepts Remote object
  reference
• A remote object reference is a unique identifier
  for a particular unique remote object
• Remote object references are analogous to local
  ones in that
• The remote object to receive a method invocation
  is specified as a remote object reference.
• Remote object references may be passed as
  arguments and results of remote method
  invocations

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Distributed object model (3)

• Second fundamental concepts Remote interfaces
• The class of a remote object implements the
  methods of its remote interface.
• Objects in other processes can only invoke the
  methods that belong to its remote interface
• Local objects can invoke methods in the remote
  interfaces as well as other methods implemented
  by a local object

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Distributed object model (4)

• Exceptions
• Unlike local invocations, remote invocations may
  fail more often because objects are being in
  different processes or computers
• Also, exceptions may be raised during the
  execution of a remote invocation(for examples,
  reading beyond the end of a file)

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Implementation (1)

• Several objects and modules involved in achieving
  a remote method invocation

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Implementation (2)

• Communication module
• Carries out the request/reply protocol

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Distributed garbage collection

• Garbage?
• When a client receives a remote reference to a
  particular remote object, it makes an addRef
  invocation to the server and then creates a proxy
• When a clients garbage collector notices that a
  proxy for remote object is no longer reachable,
  it makes a removeRef invocation to the
  corresponding server and then deletes the proxy.
  The server removes client skeleton from it.