This document summarizes remote procedure calls (RPC), remote method invocation (RMI), and event-based distributed programming models. It discusses RPC and RMI architectures including client stubs, server skeletons, and dispatchers. Middleware provides location and protocol transparency. Interfaces specify accessible methods. The document also covers a Java RMI case study of a shared whiteboard application that uses callbacks to notify clients of updates.

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. Middleware layers
Applications
Middleware
RMI, RPC and events
Request reply protocol layers
External data representation
Operating System

5. Interfaces
• Interface
– Specifies accessible procedures and variables
– Inner alteration won’t affect the user of the
interface
• Interface in distributed system
– Can’t access variables directly
– Input argument and output argument
– Pointers can’t be passed as arguments or returned
results

6. Interface cases
• RPC’s Service interface
– specification of the procedures of the server, defining the
types of the input and output arguments of each procedure
• RMI’s 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

7. CORBA IDL example
struct Person {
string name;
string place;
long year;
CORBA has a struct
} ;
interface PersonList {
remote interface
remote interface defines
methods for RMI
readonly attribute string listname;
void addPerson(in Person p) ;
void getPerson(in string name, out Person p);
long number();
};
• Remote interface:
parameters are in, out or inout
– 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

8. Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

9. Distributed object model
remote
invocation invocation
remote
invocation
local
local
invocation
local
invocation
Figure 5.3
A
B
C
D
E
F
• 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

10. Invocation semantics
• Local invocations are executed exactly once
• Remote invocations cannot achieve this. Why not?
– the Request-reply protocol can apply fault-tolerance
measures
Fault tolerance measures Invocation
semantics
Retransmit request
message
Duplicate
filtering
Re-execute procedure
or retransmit reply
No
Yes
Yes
Not applicable
No
Yes
Not applicable
Re-execute procedure
Maybe
At-least-once
Retransmit reply At-most-once

11. Remote and local method invocations
remote
invocation invocation
remote
invocation
local
local
invocation
local
invocation
A
B
C
D
E
F

12. A remote object and its remote interface
remote
interface
m1
m2
m3
m4
m5
m6
remoteobject
Data
implementation
{ of methods

13. 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)

14. The architecture of remote method invocation
skeleton
object A object B
Request
proxy for B
Reply
& dispatcher
for B’s class
Remote Communication Communication Remote reference
reference module module module module
remote
client server
Proxy - makes RMI transparent carries to client. out Class
Request-reply
implements remote interface. Marshals protocol
requests and
unmarshals results. Forwards request.
Skeleton - implements methods in remote interface.
Unmarshals requests and marshals results. Invokes
method in remote object.
translates between local and remote
object references and creates remote
object references. Uses remote object
table
RMI software - between
application level objects
and communication and
remote reference
modules
Dispatcher - gets request from communication module and
invokes method in skeleton (using methodID in message).

15. Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

16. 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
client
client process server process
Request
Reply
Communication Communication
module module dispatcher
service
client stub server stub
procedure procedure
program procedure

17. 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

18. 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

19. Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

20. 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

21. Example - dealing room system
• Requirements
– allow dealers to see the latest market price of the
stocks 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

22. Dealing room system
Dealer’s computer
External
source
Information
provider
Notification
Information
provider
Dealer
External
source
Dealer
Dealer
Dealer
Notification
Notification
Notification
Notification
Notification
Notification
Notification
Notification
Notification
Dealer’s computer
Dealer’s computer Dealer’s computer

23. Architecture for distributed event notification
• Event service: maintain a database of published events and of
subscribers’ interests
• decouple the publishers from the subscribers
Event service
object of interest
object of interest observer
subscriber
subscriber
notification
notification
object of interest observer subscriber
3.
1.
2. notification
notification

24. 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.

25. 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

26. 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

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. 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

29. Example: shared whiteboard
• Remote Interface
• Server program and Client program
• Callbacks
– A server’s 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

30. Design and implementation of Java RMI
• Java classes supporting RMI
RemoteObject
RemoteServer
UnicastRemoteObject
<servant class>
Activatable

31. Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary

32. 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

33. Files interface in Sun XDR
const MAX = 1000;
typedef int FileIdentifier;
typedef int FilePointer;
typedef int Length;
struct Data {
int length;
char buffer[MAX];
};
struct writeargs {
FileIdentifier f;
FilePointer position;
Data data;
};
struct readargs {
FileIdentifier f;
FilePointer position;
Length length;
};
program FILEREADWRITE {
version VERSION {
void WRITE(writeargs)=1; 1
Data READ(readargs)=2; 2
}=2;
} = 9999;

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;
public interface Shape extends Remote {
int getVersion() throws RemoteException;
GraphicalObject getAllState() throws RemoteException; 1
}
public 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.UnicastRemoteObject;
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()throws 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.*;
import java.util.Vector;
public class ShapeListClient{
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager());
ShapeList aShapeList = null;
try{
aShapeList = (ShapeList) Naming.lookup("//bruno.ShapeList") ; 1
Vector sList = aShapeList.allShapes(); 2
} catch(RemoteException e) {System.out.println(e.getMessage());
}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;
}
Methods added in Shapelist interface:
Int register(WhiteboardCallback callback) throws RemoteException;
Void deregister(int callbackID) throws RemoteException;