Communication between distributed objects

1
Chapter 5 Distributed objects and remote
invocation

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

2
Middleware

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

3
Distributed programming model

• Remote procedure call (RPC)
• 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

4
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

5
Interface cases

• RPCs Service interface
• specification of the procedures of the server,
  defining the types of the 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,
  defining the types of them.
• may pass objects or remote object references as
  arguments or returned result
• Interface definition languages
• program language, e.g. Java RMI
• Interface definition languages (IDLs), are
  designed to allow objects implemented in
  different languages to invoke one another.
• e.g. CORBA IDL (n1), DCE IDL and DCOM IDL

6
CORBA IDL example
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()

• Remote interface
• specifies the methods of an object available for
  remote invocation
• an interface definition language (or IDL) is used
  to specify remote interfaces. E.g. the above in
  CORBA IDL.
• Java RMI would have a class for Person, but CORBA
  has a struct

7
Chapter 5 Distributed objects and remote
invocation

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

8
Distributed object model
Figure 5.3

• each process contains objects, some of which can
  receive remote invocations, others only local
  invocations
• those that can receive remote invocations are
  called remote objects
• objects need to know the remote object reference
  of an object in another process in order to
  invoke its methods.
• the remote interface specifies which methods can
  be invoked remotely

9
Invocation semantics

• Local invocations are executed exactly once
• Remote invocations cannot achieve this. Why not?
• the Request-reply protocol can apply
  fault-tolerance measures

10
Invocation semantics failure model

• Maybe, At-least-once and At-most-once can suffer
  from crash failures when the server containing
  the remote object fails.
• Maybe - if no reply, the client does not know if
  method was executed or not
• omission failures if the invocation or result
  message is lost
• At-least-once - the client gets a result (and the
  method was executed at least once) or an
  exception (no result)
• arbitrary failures. If the invocation message is
  retransmitted, the remote object may execute the
  method more than once, possibly causing wrong
  values to be stored or returned.
• if idempotent operations are used, arbitrary
  failures will not occur
• At-most-once - the client gets a result (and the
  method was executed exactly once) or an exception
  (instead of a result, in which case, the method
  was executed once or not at all)

11
The architecture of remote method invocation
RMI software - between application level objects
and communication and remote reference modules
12
Chapter 5 Distributed objects and remote
invocation

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

13
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

14
Sun RPC case study

• 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

15
Sun RPC case study continued

• 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

16
Chapter 5 Distributed objects and remote
invocation

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

17
Event-notification model

• Idea
• one object react to a change occurring in another
  object
• Event examples
• modification of a document
• an electronically tagged book being at a new
  location
• 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 components in a DS that were not
  designed to interoperate can be made to work
  together
• Asynchronous prevent publishers needing to
  synchronize with subscribers

18
Example - dealing room system

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

19
Architecture for distributed event notification

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

20
The roles of the participating objects

• The object of interest
• its changes of state might be of interest to
  other objects
• Event
• An event occurs at an object of interest as 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.
• Avoid over-complicating the object of interest.
• Publisher
• an object that declares that it will generate
  notifications of particular types of event. May
  be an object of interest or an observer.

21
Notification delivery

• Delivery semantics
• Unreliable, e.g. deliver the latest state of a
  player in a Internet game
• Reliable, e.g. dealing room
• real-time, e.g. a nuclear power station or a
  hospital patient monitor
• Roles for observers
• Forwarding
• send notifications to subscribers on behalf of
  one or more objects of interests
• Filtering of notifications according to some
  predicate
• Patterns of events
• Notification mailboxes
• notification be delayed until subscriber being
  ready to receive

22
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

23
Chapter 5 Distributed objects and remote
invocation

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

24
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

25
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

26
Design and implementation of Java RMI

• Java classes supporting RMI

27
Chapter 5 Distributed objects and remote
invocation

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

28
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
• Example whiteboard based on Java RMI
• Sun RPC
• Event-notification
• Publish/subscribe
• Event service
• Example dealing room

29
Middleware layers
30
A remote object and its remote interface
31
Remote and local method invocations
32
Files interface in Sun XDR
33
Dealing room system
34
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.
35
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
36
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 ...
37
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())
38
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())
39
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