Distributed Computing Class: BIT5 Instructor: Aatif Kamal

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Distributed ComputingClass BIT5Instructor
Aatif Kamal
Dated Nov 2006

• Chapter 05
• Distributed Objects and Remote Invocation

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Objectives of the lecture

• Essence of distributed commuting
• To study communication between distributed
  objects and the integration of remote method
  invocation into a programming language.
• To be able to use Java RMI to program
  applications with distributed objects and how to
  use some aspects of reflection
• To study the extension of the event-based
  programming model to apply to distributed
  event-based programs.

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Revision

• Two alternative building blocks
• Datagram Socket based on UDP, efficient but
  suffer from failures
• Stream Socket based on TCP, reliable but
  expensive
• Marshalling
• CORBAs CDR and Java serialization
• Request-Reply protocol
• Base on UDP or TCP
• Group communication
• IP multicast is a simple multicast protocol

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

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Middleware

• Layers of Middleware
• Provide a programming model
• Provide transparency
• Location
• Communication protocols
• Computer hardware
• Operating systems
• Programming languages

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Middleware layers
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Distributed programming model

• Remote procedure call (RPC)
• Birrel and Nelson (1984)
• call procedure in separate process
• Remote method invocation (RMI)
• extension of local method invocation in OO model
• invoke the methods of an object of another
  process
• Event-based model
• Register interested events of other objects
• Receive notification of the events at other
  objects

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Conventional Procedure Call

1. Parameter passing in a local procedure call the
   stack before the call to read
2. The stack while the called procedure is active

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Client and Server Stubs

• Principle of RPC between a client and server
  program.

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Steps of a Remote Procedure Call

1. Client procedure calls client stub in normal way
2. Client stub builds message, calls local OS
3. Client's OS sends message to remote OS
4. Remote OS gives message to server stub
5. Server stub unpacks parameters, calls server
6. Server does work, returns result to the stub
7. Server stub packs it in message, calls local OS
8. Server's OS sends message to client's OS
9. Client's OS gives message to client stub
10. Stub unpacks result, returns to client

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Passing Value Parameters (1)

• Steps involved in doing remote computation
  through RPC

2-8
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Passing Value Parameters (2)

1. Original message on the Pentium
2. The message after receipt on the SPARC
3. The message after being inverted. The little
   numbers in boxes indicate the address of each byte

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Interfaces

• Interface
• Specifies accessible procedures and variables
• Inner alteration wont affect the user of the
  interface
• Interface in distributed system
• Cant access variables directly
• Input argument and output argument
• Pointers cant be passed as arguments or returned
  results

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Interface cases

• RPCs Service interface
• specification of the procedures of the server
• input and output arguments of each procedure
• RMIs Remote interface
• Specification of the methods of an object that
  are available for objects in other processes
• may pass objects or remote object references as
  arguments or returned result
• Interface definition languages
• program language, e.g. Java RMI
• Interface definition language (IDL), e.g. CORBA
  IDL, DCE IDL and DCOM IDL

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CORBA IDL example
// In file Person.idl struct Person string
name string place long year interface
PersonList readonly attribute string
listname void addPerson(in Person p) void
getPerson(in string name, out Person p) long
number()
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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

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Discuss RMI under following headings

• The object model
• Distributed objects
• The distributed object model
• Design issues
• semantics of remote invocations
• Implementation
• RMI above the request-reply protocol
• Distributed garbage collections

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The object model

• Object references
• Objects can be accessed via object references
• First-class values
• Interfaces
• A definition of the signatures of a set of
  methods
• No constructers
• A class can implement several interfaces, e.g.
  Java
• Actions
• Initiated by an object invoking a method in
  another object
• Two affects
• Change the state of the receiver
• Further invocations on methods in other objects

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The object model

• Exceptions mechanism
• A clean way to deal with error conditions
• List exceptions at the method head
• throw user know exceptions
• Catch exceptions
• Garbage collection
• Freeing the space occupied by cancelled objects
• C collected by programmers
• Java collected by JVM

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Distributed objects

• Natural extension
• physical distribution of objects into different
  processes or computers in a distributed system
• Benefits of distributed objects
• Enforce encapsulation
• cant access variables directly
• Support heterogeneous systems
• Assume other architectural models then
  client/server
• Replicated objects
• Migrated objects

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Remote and local method invocations
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The distributed objects model

• Remote object reference
• A unique identifier in a distributed system
• May be passed as arguments and results of remote
  method invocation
• Remote interface
• remote object class implements the methods of its
  remote interface
• Actions in a distributed systems
• may incur a chain of invocations on different
  computers
• Garbage collection
• Usually based on reference counting
• Exception
• notify the client and the client handle exceptions

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A remote object and its remote interface
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Design Issues Invocation semantics

• Choices for different delivery guarantees
• retry request message
• duplicate filtering
• retransmission of results
• Three different semantics

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Different invocation semantics

• Maybe
• For invoker executed once, or not at all ???
• Suffer from (1) message lost (2) server crash
• Useful for app. in which occasional failed
  invocation are acceptable
• At least once
• For invoker execute at least once, or an
  exception
• Suffer from (1) server crash (2) arbitrary
  failures for non-idempotent method
• At most once
• For invoker receives result, or an exception
• Prevent omission failures by retrying, arbitrary
  failures

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Design Issues - Transparency

• What can be made transparent
• marshaling
• message passing
• object locating and contacting
• What cant be made transparent
• vulnerable to failure
• latency
• Current consensus(Jim Waldo research paper)
• Syntax of remote invocation should be same as of
  local invocation
• Difference in remote and local objects should be
  expressed in their interfaces

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Implementation of RMI

• The inner scene of RMI
• Communication module
• Request/reply between client and server
• As discussed in chapter 4, figure 4.13
• Communication module at Server side Select
  dispatcher at server
• Remote reference module
• Translate between local and remote object
  reference
• Create remote object reference
• Remote object table
• entries for remote objects held by the process
• entries for local proxies
• Actions of RRM
• When a remote object is to be passed as argument
  or result for the first time the remote reference
  module is asked to create a remote object
  reference, which it adds to its table
• When a remote object reference arrives in a
  request or reply message, the remote reference
  module is asked for the corresponding local
  object reference, which may either refer to proxy
  or remote object

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Request-reply message structure (fig 4.13)
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The role of proxy and skeleton in remote method
invocation
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Implementation of RMI RMI software

• Proxy
• forward invocation to remote object
• one remote object one proxy
• implement the method in the remote interface
• It marshals a reference to the target object ,
  its own methodId and its arguments into request
  message and send it to the target, awaits the
  reply and message, unmarshals it and returns the
  results to invoker
• Skeleton
• implement the method in the remote interface
• unmarshal the arguments in the request
• invoke the corresponding method in the remote
  object
• wait for the invocation complete
• marshal the result in the reply message
• Dispatcher
• Receive the request from communication module and
  select appropriate method in the skeleton
• one dispatcher and skeleton for one remote object

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Implementation of RMI - execution

• The classes for proxies, dispatchers and
  skeletons
• generated automatically by an interface compiler,
  e.g. rmic
• Server program
• create and initialize at least one of the remote
  objects
• register
• Client program
• look up the remote object references
• Invoke
• Factory methods
• Remote object interface cannot have constructors
• Remote objects are either created in
  initialization section or in remote methods
  designed for that purpose, called factory methods

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Implementation of RMI - Object state

• The binder
• A service that maintains mapping information of
  textual names to remote object references
• Activation of remote objects
• to avoid resource waste, the servers can be
  started whenever they are needed- FTP service
• Activators process that start server processes
  to host remote objects.
• a remote object could be active or passive
• Passive object consist of two parts
• Implementation of its methods
• Its state in marshaled form
• Activation Making a new object from
  corresponding passive object by creating a new
  instance of its class and initializing its
  variables from the stored state
• Java RMI uses one activator on each server
  computer , which is responsible for activating
  objects on that computer.

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Implementation of RMI - Object state (cont..)

• Persistent object stores
• Persistent object
• an object that is guaranteed to live between
  activations of processes
• Persistent Object Store
• Manage and store the state of persistent objects
• different passivate strategies
• at the end of a transaction
• when the program exit
• E.g., Persistent Java, PerDiS

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Distributed garbage collection

• The aim of a distributed garbage collector
• Retain the object (localremote) when it is still
  be referenced
• Collect the object when none holds reference to
  it
• Java distributed garbage collection algorithm
• based on reference counting
• Whenever a remote object enters a process, a
  proxy is created and will stay there for as long
  as needed. The process where the actual object
  lives (server) should be informed, then later
  when there is no longer a proxy at the client,
  server should be informed
• Distributed Garbage collection works in
  corporation with local garbage collectors
• server maintain processes set that hold remote
  object references to it for each of its remote
  objects
• client notify server to modify the process set
• when the process set becomes empty, server local
  garbage collector reclaims the space

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Distributed garbage collection (cont..)

• Leases in Jini
• lease the granting of the use of a resource for
  a period of time
• avoid to discover whether the resource users are
  still interested or their programs have not
  exited

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

37
RPC is very similar to RMI

• Service interface the procedures that are
  available for remote calling
• Invocation semantics choice at-least-once or
  at-most-once
• Generally implemented over request-reply protocol
• Building blocks
• Communication module
• Client stub procedure (as proxy in RMI)
  marshalling, sending, unmarshalling
• Dispatcher select one of the server stub
  procedures
• Server stub procedure (as skeleton in RMI)
  unmarshalling, calling, marshalling

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

39
Event-notification model

• Idea
• one object react to a change occurring in another
  object
• Event examples
• modification of a document
• A new doc added in an electronically tagged book
• Publish/subscribe paradigm
• event generator publish the type of events
• event receiver subscribe to the types of events
  that are interest to them
• When event occur, notify the receiver
• Distributed event-based system two
  characteristics
• Heterogeneous
• asynchronous

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Example - dealing room system

• Requirements
• allow dealers to see the latest market price
  information
• System components
• Information provider
• receive new trading information
• publish stocks prices event
• stock price update notification
• Dealer process
• subscribe stocks prices event
• System architecture

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Dealing room system
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Architecture for distributed event notification

• Event service maintain a database of published
  events and of subscribers interests
• decouple the publishers from the subscribers

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The roles of the participating objects

• The object of interest
• its changes of state might be of interest to
  other objects
• Event
• the completion of a method execution
• Notification
• an object that contains information about an
  event
• Subscriber
• an object that has subscribed to some type of
  events in another object
• Observer objects
• the main purpose is to decouple an object of
  interest from its subscribers
• Publisher
• an object that declares that it will generate
  notifications of particular types of event

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Notification delivery

• Delivery semantics
• Unreliable
• Reliable
• real-time
• Roles for observers
• Forwarding
• send notifications to subscribers on behalf of
  one or more objects of interests
• Filtering of notifications
• Patterns of events
• Notification mailboxes
• notification be delayed until subscriber being
  ready to receive

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Jini distributed event specification

• EventGenerator interface
• Provide register method
• Event generator implement it
• Subscriber invoke it to subscribe to the
  interested events
• RemoteEventListener interface
• Provide notify method
• subscriber implement it
• receive notifications when the notify method is
  invoked
• RemoteEvent
• a notification that is passed as argument to the
  notify method
• Third-party agents
• interpose between an object of interest and a
  subscriber
• equivalent of observer

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• RPC Case Study
• Java RMI case study
• Summary

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Sun RPC case study (1)

• Designed for NFS
• at-least-once semantics
• XDR - Interface definition language
• Interface name Program number, version number
• Procedure identifier procedure number
• Rpcgen generator of RPC components
• client stub procedure
• server main procedure
• Dispatcher
• server stub procedure
• marshalling and unmarshalling procedure

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Sun RPC case study (2)

• Binding portmapper
• Server register ((program number, version
  number), port number)
• Client request port number by (program number,
  version number)
• Authentication
• Each request contains the credentials of the
  user, e.g. uid and gid of the user
• Access control according to the credential
  information

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

50
Java RMI introduction

• Remote object
• Must implement the remote interface
• must handle remote exceptions
• Arguments and return results of remote method
• Must be serializable
• All primitive types serializable
• remote objects are serializable
• File handles are unserializable
• Remote objects are passed as remote object
  reference, non-remote serializable objects are
  copied and passed by value
• RMIregistry
• access by the Naming class

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Example shared whiteboard

• Remote Interface
• Server program and Client program
• Callbacks
• A servers action of notifying clients about an
  event
• Implementation
• Client create a remote object
• Client pass the remote object reference to server
• Whenever an event occurs, server call client via
  the remote object
• Advantage
• Improve performance by avoid constant polling
• Delivery information in a timely manner

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Design and implementation of Java RMI

• Java classes supporting RMI

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Chapter 5 Distributed objects and remote
invocation

• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

54
Summary

• Two paradigms for distributed programming
• RMI(RPC)/Event notification sync./async.
• RMI
• Distributed object model
• Remote interface, remote exception, naming
  service
• Remote invocation semantics
• Once, at-least-once, at-most-once
• Sun RPC
• Event-notification
• Publish/subscribe
• Event service
• Example dealing room

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Files interface in Sun XDR
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The Naming class of Java RMIregistry
void rebind (String name, Remote obj) This
method is used by a server to register the
identifier of a remote object by name, as shown
in Figure 15.13, line 3. void bind (String
name, Remote obj) This method can alternatively
be used by a server to register a remote object
by name, but if the name is already bound to a
remote object reference an exception is
thrown. void unbind (String name, Remote obj)
This method removes a binding. Remote
lookup(String name) This method is used by
clients to look up a remote object by name, as
shown in Figure 15.15 line 1. A remote object
reference is returned. String list() This
method returns an array of Strings containing the
names bound in the registry.
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Java Remote interfaces Shape and ShapeList
import java.rmi. import java.util.Vector publi
c interface Shape extends Remote int
getVersion() throws RemoteException GraphicalObj
ect getAllState() throws RemoteException 1 pu
blic interface ShapeList extends Remote Shape
newShape(GraphicalObject g) throws
RemoteException 2 Vector allShapes() throws
RemoteException int getVersion() throws
RemoteException
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Java class ShapeListServant implements interface
ShapeList
import java.rmi. import java.rmi.server.UnicastR
emoteObject import java.util.Vector public
class ShapeListServant extends UnicastRemoteObject
implements ShapeList private Vector
theList // contains the list of Shapes 1
private int version public ShapeListServant()thr
ows RemoteException... public Shape
newShape(GraphicalObject g) throws
RemoteException 2 version Shape s
new ShapeServant( g, version) 3
theList.addElement(s)
return s public Vector allShapes()throws
RemoteException... public int getVersion()
throws RemoteException ...
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Java class ShapeListServer with main method
import java.rmi. public class
ShapeListServer public static void main(String
args) System.setSecurityManager(new
RMISecurityManager()) try ShapeList
aShapeList new ShapeListServant() 1
Naming.rebind("Shape List", aShapeList
) 2 System.out.println("ShapeList server
ready") catch(Exception e)
System.out.println("ShapeList server main "
e.getMessage())
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Java client of ShapeList
import java.rmi. import java.rmi.server. impor
t java.util.Vector public class
ShapeListClient public static void
main(String args) System.setSecurityManager(ne
w RMISecurityManager()) ShapeList aShapeList
null try aShapeList (ShapeList)
Naming.lookup("//bruno.ShapeList") 1 Vector
sList aShapeList.allShapes() 2
catch(RemoteException e) System.out.println(e.get
Message()) catch(Exception e)
System.out.println("Client "
e.getMessage())
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Callback mechanism in the whiteboard system
Client created remote object Public interface
WhiteboardCallback implements Remote void
callback(int version) throws RemoteException M
ethods added in Shapelist interface Int
register(WhiteboardCallback callback) throws
RemoteException Void deregister(int callbackID)
throws RemoteException
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Assignment 6
Q.1 From the book Distributed Operating Systems,
A. S. Tanenbaum, Summarize section 2.4 in you
own words Q.2 Read the following paper and
summarizes in your own words. Finally give your
own comments on the issues discussed in the
paper. A Note on Distributed Computing, Jim
Waldo Sun Microsystems site http//research.sun
.com/techrep/1994/smli_tr-94-29.pdf After
carefully reading and understanding the article
you have too write one page overview of this note.
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Assignment 7
Q.1 5.1 Q.2 5.3 Q.3 5.4 Q.4 5.5
64
Assignment 8
Q.1 5.12 Q.2 5.13 Q.3 5.17 Reading 5.3
RPC 5.5 Java RMI case study