© Mourad Elhadef, Ph. D. CSI408-Distributed Information Systems Chapters 4 & 5 Interprocess...

© Mourad Elhadef, Ph. D.

CSI408-Distributed Information Systems

Chapters 4 & 5 Interprocess Communication

Presentation of the characteristics of protocols for communication between processes:

Client-Server or group communication, RPC, RMI, …

Communication between distributed objects and remote invocation

© Mourad Elhadef, Ph. D.CSI408-Distributed Information Systems 4&5-2

2nd Part: Types of Interprocess Communication (Client-Sever, Group)

Themes

1st Part: Means of Interprocess Communication (RPC, RMI, …)


Layered Protocols

Means of Interprocess Communication

RMI (Remote Method Invocation)

Introduction

Readings


Request-Reply ProtocolMarshalling and external data

representation

RMI, RPC and events

Operating System

UDP and TCP

Applications, services

Introduction (1)

Middleware layers


Introduction (2)

Characteristics of middleware layers: Location transparency (definition)

Independent: protocols that support the middleware abstractions are independent of the underlying transport protocols (example)

Heterogeneous hardware: hide the difference due to hardware architectures (big-endian, little-endian, …) (mean)

Use of several programming languages: allow distributed applications to use more than one programming language (example)


RMI/INTERFACE:Import java.rmi.*;package EXAMPLES;public interface RAND_VERS extends Remote{ public void INITIALIZE_RANDOM(long arg) throws RemoteExceptions; public double GET_NEXT_RANDOM(void) throws RemoteExceptions;};

RPC/IDL :program RAND_PROG{ version RAND_VERS { void INITIALIZE_RANDOM(long) = 1; double GET_NEXT_RANDOM(void) = 2; } = 1;} = 0x31111111;

CORBA/IDL :module EXAMPLES{ interface RAND_PROG { void INITIALIZE_RANDOM(in long arg); double GET_NEXT_RANDOM(void); };};

RPC/IDL:program RAND_PROG{ version RAND_VERS { void INITIALIZE_RANDOM(long) = 1; double GET_NEXT_RANDOM(void) = 2; } = 1;} = 0x31111111;

Introduction (3)

Definition of remote

procedure using

different IDL

CORBA/IDL:module EXAMPLES{ interface RAND_PROG { void INITIALIZE_RANDOM(in long arg); double GET_NEXT_RANDOM(void); };};


Introduction (4)

Characteristics of interprocess communication:

Types of communication:

Synchronous: the sending and receiving processes synchronize at every message

Asynchronous: no synchronization between the sending and receiving processes

Send and Receive: blocking operations

Send: non-blocking operation

Receive: non-blocking or blocking operation


Introduction (5)

Characteristics (continued): Message destinations:

Messages sent to <Internet address, Local port > Processes may use multiple ports from which to

receive messages

Communication reliability: Integrity (definition)

Validity (definition)

Ordering: some applications require that messages be delivered in sender order


Introduction (6)

Characteristics (continued) : Sockets: used by UDP and TCP as endpoint for

communication between processes Must be bound to a local port and one of the Internet

addresses Each socket is associated to one protocol: UDP or TCP

216 possible ports


Layered Protocols (1)

Multiple layers intervene during interprocess communication

Internetwork layers (Internet)


Layered Protocols (2)

Types of communication:

UDP Datagram Communication

TCP Stream Communication


UDP Datagram Communication Datagrams transmitted by UDP: without

acknowledgments or retries

Blocking: UDP use a non-blocking send and a blocking receive

Failure model: UDP suffers from Omission failures: messages dropped (causes)

Ordering: messages can sometimes be delivered out of sender order

UDP advantage: do not suffer from the overheads associated with guaranteed message delivery


Datagram Communication: Java API (1)

Manipulating Internet addresses: Class InetAddress: represents Internet addresses

(example)

InetAddress ( string address ): multiple forms of the constructor exist

short int getPort () const bool setPort ( short int port ) string getAddress () const bool setAddress ( string address ) bool setAddress ( unsigned long int address ) string getHostName () string getHostByIP ( string ip ) string getIPByName ( string host ) …


Sender:

DatagramPacket( byte buffer[], int length, InetAddress address, int

port)

Number of bytes to readbuffer for holding the incoming datagram


Manipulating datagrams: Class DatagramPacket : creates packets containing data to be

transmitted or received and the address of destination or source of the datagrams Port

NumberInternet Address

Message’s length

Message’s bytesDatagram packet

Constructors: Receiver: DatagramPacket(byte buffer[], int length)

UDP Port

Data packet Length of data packet

Destination address



Manipulating datagrams (continued): main methods of the class DatagramPacket

InetAddress getAddress ()

int getPort ()

void setAddress(InetAddress iaddr)

byte[] getData ()

void setData(byte ibuf[])

void setPort(int iport)

int getLength ()

void setLength(int ilength)


Constructors:

DatagramSocket () throws SocketException

DatagramSocket (int port) throws SocketException

DatagramSocket(int port, InetAddress laddr) throws

SocketException


Manipulating sockets: Class DatagramSocket: represents a socket for sending

and receiving datagram packets



Manipulating sockets (continued): main methods of the class DatagramSocket void send(DatagramPacket data) throws IOException

void receive(DatagramPacket data) throws IOException void setSoTimeout(int timeout) throws SocketException Enable/disable SO_TIMEOUT with the specified timeout. 0 = no timeout

void close ()

int getLocalPort ()

int getSoTimeout() throws SocketException

void connect (InetAddress address, int port)

void disconnect()



Example: Client sends a request import java.net.*; import java.io.*; public class UDPClient{ public static void main(String args[]) { // args[0] = message to be sent to the server; args[1] = IP address of the server

DatagramSocket aSocket = null; try {

aSocket = new DatagramSocket(); byte [] m = args[0].getBytes();

InetAddress aHost = InetAddress.getByName(args[1]);int serverPort = 6789;DatagramPacket request = new DatagramPacket(m,

args[0].length(), aHost, serverPort);aSocket.send(request);



Example (continued): Client waits for a reply

byte[] buffer = new byte[1000];DatagramPacket reply = new DatagramPacket(buffer,

buffer.length);aSocket.receive(reply);System.out.println("Reply: " + new String(reply.getData()));

}catch (SocketException e){System.out.println("Socket: " +

e.getMessage()); }catch (IOException e){System.out.println("IO: " +

e.getMessage()); }finally {if(aSocket != null) aSocket.close();}}}



Example (continued): Server waits continually for a request

import java.net.*; import java.io.*; public class UDPServer{ public static void main(String args[]) { DatagramSocket aSocket = null; try{

aSocket = new DatagramSocket(6789); byte[] buffer = new byte[1000]; while(true){ DatagramPacket request = new DatagramPacket(buffer,

buffer.length); aSocket.receive(request);



Example (continued): Server sends the response to the client

DatagramPacket reply = new DatagramPacket( request.getData(), request.getLength(),

request.getAddress(), request.getPort());

aSocket.send(reply);} }catch (SocketException e){System.out.println("Socket: " +

e.getMessage()); }catch (IOException e) {System.out.println("IO: " + e.getMessage()); }finally {if(aSocket != null) aSocket.close();}}}


TCP Stream Communication (1)

Use of input or output streams

Characteristics: Message sizes (size)

Lost messages: use of an acknowledgment scheme

Flow control: TCP protocol attempts to match the

speeds of the processes that read and write to a stream (how?)

Message duplication and ordering: (How?)

Message destinations: (how?)



Characteristics (continued):

Matching of data items: processes need to agree as to the contents of the data transmitted over a stream (example)

Blocking: (When?)

Use of threads: (How?)

Server: accepts a connection

A thread is created for the new client



Failure model: must satisfy

Integrity property: (How?)

Validity property: (How?)

TCP limits: does not guarantee to deliver messages in the face of all possible difficulties


Stream Communication: Java API (1)

Client: Class Socket: used by a client to establish a connection

with the server

public Socket (String Host, int Port) throws UnknownHost…

public Socket (InetAddress Host, int Port) throws Unknown…

public Socket (InetAddress Host, int Port, InetAddress LocalAddress, int LocalPort) throws UnknownHost…

…

Constructors: creates a TCP socket by specifying the DNS hostname and a port of a server



Client (continued): main methods of class Socket

close() throws IOException InetAddress getInetAddress () int getPort () InetAddress getLocalAddress () int getLocalPort () int getSoTimeout() throws SocketException void setSoTimeout(int timeout) throws SocketException InputStream getInputStream() throws IOException OutputStream getOutputStream() throws IOException …


import java.net.*; import java.io.*;public class Ports1 { public static void main(String[] args) { Socket s; String host = "localhost"; if(args.length > 0) host=args[0] ; for(int i = 0 ; i < 1024; i++) {

try { s=new Socket(host,i); System.out.println("Server " + " attached to port "+ i);} catch (ConnectException e) { ; } catch (UnknownHostException e) {

System.err.println(e+" --> "+i) ;} catch (IOException e)

{ System.err.println(e); }}}}


Example 1: list all services of ports 0-1023 on a local machine

arlequin{elhadef}102: java Ports1Server attached to port 7 Server attached to port 9 Server attached to port 13 Server attached to port 19 Server attached to port 22 Server attached to port 23 Server attached to port 37 Server attached to port 79 Server attached to port 111 Server attached to port 512 Server attached to port 513 Server attached to port 514 Server attached to port 515

Execution results:



Example 2: manipulating InetAddress addressesimport java.net.*; import java.io.*;public class Ports2 { public static void main(String[] args) { Socket s; String host = "www.dmi.usherb.ca"; if(args.length > 0) hote=args[0] ; try {

InetAddress address = InetAddress.getByName(host); for(int i = 1 ; i < 1024; i++) { try { s=new Socket(address,i);

System.out.println("Server attached to port "+ i + " of " + address);

} catch (ConnectException e) { ; } catch (IOException e) { System.err.println(e); }}

} catch (UnknownHostException e) { System.err.println(e) ; }}}



Example 2: manipulating InetAddress addressesarlequin{elhadef}120: java Ports2Server attached to port 7 of www.dmi.usherb.ca/132.210.40.10Server attached to port 9 of www.dmi.usherb.ca/132.210.40.10Server attached to port 13 of www.dmi.usherb.ca/132.210.40.10Server attached to port 15 of www.dmi.usherb.ca/132.210.40.10Server attached to port 19 of www.dmi.usherb.ca/132.210.40.10Server attached to port 25 of www.dmi.usherb.ca/132.210.40.10Server attached to port 37 of www.dmi.usherb.ca/132.210.40.10Server attached to port 53 of www.dmi.usherb.ca/132.210.40.10Server attached to port 79 of www.dmi.usherb.ca/132.210.40.10Server attached to port 80 of www.dmi.usherb.ca/132.210.40.10Server attached to port 109 of www.dmi.usherb.ca/132.210.40.10Server attached to port 110 of www.dmi.usherb.ca/132.210.40.10Server attached to port 111 of www.dmi.usherb.ca/132.210.40.10Server attached to port 113 of www.dmi.usherb.ca/132.210.40.10Server attached to port 143 of www.dmi.usherb.ca/132.210.40.10

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Example 3: manipulating socket’s attributesimport java.net.*; import java.io.*;public class Attributes { public static void main(String[] args) { Socket s; for(int i = 0 ; i < args.length ; i++) {

try { s=new Socket(args[i],80); System.out.println("\nConnect to "+ s.getInetAddress() + " on port " + s.getPort() + "\nfrom port "+ s.getLocalPort() + " of "+ s.getLocalAddress()) ; } catch (ConnectException e) { ;

} catch (IOException e) { System.err.println(e); } } }}



Manipulating data streams:

Get I/O stream: functions

public InputStream getInputStream() throws IOException publi OutputStream getOutputStream() throws IOException

Manipulate streams with the following classes that offer many functionalities

DataInputStream

DataOutputStream



Example 4: read the date and the time from the Unix’ daytime server (port 13) on the host arlequin.dmi.usherb.ca

import java.net.*; import java.io.*;public class DateAndTime { public static void main(String[] args) { Socket s; String host; DataInputStream timeStream; host="arlequin.dmi.usherb.ca"; try { InetAddress address = InetAddress.getByName(host);

try { s=new Socket(address,13); timeStream = new DataInputStream(s.getInputStream());

String DateTime= timeStream.readLine(); System.out.println("Date and time on: "+host+" are "+DateTime); } catch (ConnectException e) { System.err.println(e);

} catch (IOException e) { System.err.println(e); } } catch (UnknownHostException e) { System.err.println(e); } }}

arlequin{elhadef}131: java DateAndTime.javaDate and time on: arlequin.dmi.usherb.ca are Sun Sep 22 17:18:23 2002

Execution results:


Constructors: create a TCP socket at a server port for listening for connect requests from clients

public ServerSocket(int port) throws IOException, … public ServerSocket(int port, int backlog) throws IOExcept… public ServerSocket(int port, int backlog, InetAddress

bindAddr) throws IOException, … port: the specified port, or 0 to use any free port bindAddr: the local InetAddress the server will bind to backlog: the maximum length of the queue


Server: Class ServerSocket: used by a server to establish a connection

with a client

Method used to establish the communication: accept a connection

socket accept() throws IOException


import java.net.*; import java.io.*;public class TCPClient {public static void main (String args[]) { // arguments supply message and hostname

Socket s = null; try{ int serverPort = 7896; s = new Socket(args[1], serverPort); DataInputStream in = new DataInputStream( s.getInputStream()); DataOutputStream out =new DataOutputStream( s.getOutputStream()); out.writeUTF(args[0]); // UTF is a string encoding see Sn. 4.4

String data = in.readUTF(); // read a line of data from the stream

System.out.println("Received: "+ data) ; }catch (UnknownHostException e) {System.out.println("Socket:"+e.getMessage()); }catch (EOFException e){System.out.println("EOF:"+e.getMessage()); }catch (IOException e){System.out.println("readline:"+e.getMessage()); }finally {if(s!=null) try {s.close();

}catch (IOException e) {System.out.println ("close:" + e.getMessage());}}}}


Example 5: TCP Client connects to a server, sends a request and waits for a response


import java.net.*;import java.io.*;public class TCPServer {public static void main (String args[]) { try{

int serverPort = 7896; // the server portServerSocket listenSocket = new ServerSocket(serverPort);while(true) {

Socket clientSocket = listenSocket.accept();Connection c = new Connection(clientSocket);

} } catch(IOException e) {System.out.println("Listen socket:"+e.getMessage());}}}

class Connection extends Thread { DataInputStream in; DataOutputStream out; Socket clientSocket;


Example 5 (continued): TCP Server establishes a connection with each client and displays its request


public Connection (Socket aClientSocket) { try {

clientSocket = aClientSocket;in = new DataInputStream( clientSocket.getInputStream());out =new DataOutputStream( clientSocket.getOutputStream());this.start();

} catch(IOException e) {System.out.println("Connection:"+e.getMessage());}}

public void run(){ try { // an echo server

String data = in.readUTF(); // read a line of data from the streamout.writeUTF(data);

}catch (EOFException e){System.out.println("EOF:"+e.getMessage()); } catch(IOException e) {System.out.println("readline:"+e.getMessage()); } finally{ try {clientSocket.close();}catch (IOException e){/*close failed*/}}}}



RMI (Remote Method Invocation) (1)

Distributed object model: allow objects at different processes to communicate with each other using calls to remote methods

B and F: distant objects

C must have a reference to object E so that it can invoke

one of its methods


Before Studying RMI

External Data Representation

CORBA’s CDR

JAVA’s Object Serialization

Remote Object Reference

Remote Interface

RMI (continued)


External Data Representation and Marshalling (1)

Different representations:

Floating-point numbers: different ways

Different coding to represent characters

Different formats: Little-endian, big-endian

converted into sequences of bytes Receiving process

Rebuild the data structures

Sending processInformation to be

transmitted = data structures (set of

interconnected objects)External data Representation


External Data Representation and Marshalling (2)

Different tools:

Form suitable for transmission into a message (binary, ASCII, e.g., HTTP)

Data collection

Marshalling

Unmarshalling

Carried out by a middleware layer

CORBA’s Common Data Representation (CDR)

JAVA’s Object Serialization

SUN’s XDR Standard: exchange of messages between clients et servers SUN NFS (Network File System)


Defined with CORBA 2.0 [Object Management Group, 1998]

Binary format: arguments and results of the invocations

CORBA’s CDR (1)

Can represent all data types: short, long, unsigned short, unsigned long, float, double, char, boolean, octet, any (tout type simple or composed : string, array, struct, enumerated, union, …)

CDR (CORBA) & XDR (SUN): types not included, suppose that the sender and the receiver both know the order and the field types


Example:

CORBA’s CDR (2)

unsigned long (32 bits)

{‘Smith’, ‘London’, 1934} struct Person { string name; string place; long year;};


JAVA’s Object Serialization (1)

Binary format: messages are build from an object or hierarchy of objects

Java RMI: objects are passed as parameters or results to method invocations

Class ObjectOutputStream: writing the contents of the instance variables

Class ObjectInputStream: reconstructing the original message



Example: Class PersonPublic class Person implements Serializable { private String name; private String place; private int year; public Person (String aName,

String a Place, int aYear) {name = aName;place = aPlace;year = aYear;

} … }

Person p= new Person("Smith", "London",

1934);



Class ObjectOutputStream ObjectOutputStream(OutputStream) ObjectOutputStream(void) write(byte[]) write(int) writeByte(int) writeChar(int) writeFloat(float) writeLong(long) writeShort(int) writeUTF(String) writeBytes(String) writeObject(Object) writeDouble(double) writeBoolean(boolean) …

Person p= new Person("Smith", "London", 1934);

// SerializationObjectOutputStream SP = new ObjectOutputStream();SP.writeObject(p);

// DeserializationObjectInputStream DSP = new ObjectInputStream();Person aPerson = DSP.readObject();


Remote Object Reference

Server Process

Client ProcessInvokes one method of a remote object

Invocationmessage

Unique identifierof the process

Remote Object Reference: remote object identifier valid in a distributed system

(local) incremented when the process creates an object

Unicity: space

Unicity: time


Remote Interface (1)

Specify which method can be remotely invoked

Remote object: can invoke only the methods m1-m3 published in the remote interface

Local object: can invoke all methods m1-m6


Remote Interface (2)

Example: CORBA’s Interface Definition Language (IDL)

// In file Person.idlstruct 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();

};


No Not applicable

Not applicable MaybeCORBA


RMI invocation semantics:

Retransmit request message

Duplicate filtering

Re-execute procedure or retransmit reply

Yes Yes retransmit reply At-most-oneJava RMI, CORBA

Yes No Re-execute procedure At-least-oneSUN/RPC

Fault tolerance measures

Invocation semantics



Architecture:

RMI Client RMI Server

Stub Skeleton

Remote Reference Layer (RRL)

Transport Layer

Remote Reference Layer (RRL)

Transport Layer



Programming with RMI: steps to follow

1) Define the interfaces of all distant classes

2) Create and compile the implementations of these classes

3) Create stub and skeleton classes: RMIC

4) Create and compile a server application

5) Start RMIREGISTRY and execute the server application

6) Create and compile a client program that invokes the distant methods

7) Start client application



Case study: A bank’s application capable of managing multiples

accounts

It’ll be placed on a distant machine, interrogated and manipulated by clients

1) Define the interfaces of all distant classes Class CreditManager : search or create a bank account

Class CreditCard: make purchases, set up signature, or get the account’s state



Class CreditManager:

package credit;import credit.*;import java.rmi.*;

public interface CreditManager extends Remote {

public CreditCard findCreditAccount(String Customer) throws UnknownAccountException, RemoteException;

public CreditCard newCreditAccount(String newCustomer)

throws DuplicateAccountException, RemoteException;}



Class CreditCard:

package credit;import credit.*;import java.rmi.*;

public interface CreditCard extends Remote {

public float getCreditLine() throws RemoteException;

public void makePurchase(float amount, int signature) throws InvalidSignatureException, RemoteException,

CreditLineExceededException;

public void setSignature(int pin) throws RemoteException;}



2) Create and compile the implementations of these classes

package credit;import java.rmi.*;import java.rmi.server.*;import java.util.Hashtable;

public class CreditManagerImpl extends UnicastRemoteObject implements CreditManager {

private static transient Hashtable accounts = new Hashtable();

public CreditManagerImpl() throws RemoteException { }…



2) Create and compile the implementations of these classes (continued) public CreditCard newCreditAccount(String customerName)

throws DuplicateAccountException, RemoteException { CreditCardImpl newCard = null; if (accounts.get(customerName) == null)

newCard = new CreditCardImpl(customerName); else throw new DuplicateAccountException();

accounts.put(customerName, newCard); return newCard;} …



2) Create and compile the implementations of these classes (continued)public CreditCard findCreditAccount(String customer)

throws UnknownAccountException, RemoteException {

CreditCardImpl account = (CreditCardImpl)accounts.get(customer);

if (account == null) throw new UnknownAccountException();

elsereturn account;

}}



2) Create and compile the implementations of these classes (continued)package credit;import java.rmi.*;import java.rmi.server.*;import java.io.Serializable;

public class CreditCardImpl extends UnicastRemoteObject implements CreditCard, Serializable

{private float creditLine = 5000f;private int signature = 0;private String accountName;

…



2) Create and compile the implementations of these classes (continued)public CreditCardImpl(String customer)

throws RemoteException { accountName = customer;}

public float getCreditLine() throws RemoteException { return creditLine;}

public void setSignature(int pin) throws RemoteException { signature = pin;}…



2) Create and compile the implementations of these classes (continued)public void makePurchase(float amount, int signature)

throws InvalidSignatureException, RemoteException, CreditLineExceededException {

if (signature != this.signature) throw new InvalidSignatureException();

if (amount > creditLine) throw new CreditLineExceededException();

else creditLine -= amount;

}}


class Naming void rebind (String name, Remote obj) void bind (String name, Remote obj) void unbind (String name, Remote obj) Remote lookup(String name) String [] list()



//computerName:port/objectName

Where to find RMIregistry

RMIregistry (binder): maintains a table containing mapping from textual names to remote object references


package credit;import java.util.*;import java.rmi.*;

public class ServerBank { public static void main (String args[]) { System.setSecurityManager(new RMISecurityManager()); try {

CreditManagerImpl cmi = new CreditManagerImpl(); Naming.rebind("cardManager", cmi);

} catch (Exception e) { System.out.println(e.getMessage());

}}}





6) Create and compile a client program that invokes the distant methods

package credit;import java.rmi.*;public class ClientBank { public static void main(String args[]) {

CreditManager cm = null;CreditCard account = null;if (args.length < 3) { System.err.println("Usage : java ClientBank" +

" <server> <name1> <name2>");System.exit (1);

}…



6) Create and compile a client program that invokes the distant methods (continued)

System.setSecurityManager(new RMISecurityManager());try { String url = new String ("rmi://" + args[0] + "/cardManager"); cm = (CreditManager)Naming.lookup(url);} catch (Exception e) { System.out.println(“ Error while accessing the bank manager" + e);}try { account = cm.newCreditAccount(args[1]);} catch (Exception e) { System.out.println(“ Error during creation " + args[1] + " : " + e);} …



6) Create and compile a client program that invokes the distant methods (continued)

try { System.out.println(" Avail credit : " + account.getCreditLine()); account.setSignature(1234); account.makePurchase(100.00f, 1234); System.out.println(" Avail. credit : " + account.getCreditLine()); account.makePurchase(160.00f, 1234); System.out.println(" Avail. credit : "+ account.getCreditLine());} catch (Exception e) { System.out.println("Error " + args[1]);}try { account = cm.findCreditAccount(args[2]);} catch (Exception e) { System.out.println(“ Error while seeking for" + args[2] + e);}}



7) Start client application javac -d . -classpath . *.java rmic -d . -classpath . credit.CreditCardImpl credit.CreditManagerImpl

CreditCardImpl_Skel.class CreditCardImpl_Stub.class CreditManagerImpl_Skel.class CreditManagerImpl_Stub.class

start java -Djava.security.policy=c:\BankRMI\permit.policy -Djava.rmi.server.codebase=file:///c:/BankRMI/ -classpath . credit.ServerBank

start rmiregistry

java -Djava.security.policy=c:\BankRMI\permit.policy -Djava.rmi.server.codebase=file:///c:/BankRMI/

-classpath . credit.ClientBank 209.197.157.169 Pierre Marie



7) Start client application

Execution results: Avail. credit: 5000.0Setting up signatureFirst purchase of 100 $Avail. credit: 4900.0Second purchase of 160 $Avail. credit: 4740.0Error while seeking for Marie

credit.UnknownAccountException


Readings

Tutorial: Getting Started Using RMI, http://java.sun.com/j2se/1.3/docs/guide/rmi/getstart.doc.html

Jim Waldo, Remote procedure calls and Java Remote Method Invocation, IEEE Concurrency, 6(3), pages 5-7, Sept 1998

A.D. Birrell and B.J. Nelson, Implementing Remote Procedure Calls (RPC), ACM Trans. on Computer Systems, 1984, pp 39-54


Group Communication

Types of Interprocess Communication

Client-Server Communication

Readings


Client-Server Communication(1)

Synchronous communication: Client process is blocked until the response is received from the server

Reliable: Server’s reply = acknowledgment

Client ServerRequest

getRequestdoOperation

(Wait)

..

Request-Reply Communication

(continuation)

Execute method

Select object

ReplysendReply

..


Int : generated by doOperation and copied by the server (message number + address + port)

RemoteObjectRef : a reference to a remote object encapsulated in a packet

Interface of remote object

Object Number

TimePort Number

Internet Address

32 bits32 bits32 bits32 bits

messageType

requestId

methodId

objectReference

arguments Array of bytes


Request-Reply message structure:

Int (0=Request, 1=Reply)

Int or Method



Primitives:

public byte [] doOperation (RemoteObjectRef o, int methodId,

byte[] arguments);

public byte[] getRequest();

public void sendReply(byte[] reply, InetAddress

clientHost, int clientPort);


Failure model for the RR protocol: primitives doOperation, getRequest and sendReply suffer from

Omission Faults Message delivery not guaranteed

Solutions: Timeouts


Enable servers to recognize two identical messages

Lost reply messages: the server re-execute only idempotent operations

History: save the results of the last executed request of each client



Depends on the application, one of these protocols will be used

RPC exchange Protocols:

Messages sent by

Client Server Client

R Request

RRA Request Reply Acknowledgment reply

RR Request Reply


Group Communication (1)

Multicast communication: sends one message to each member of the group

Multicast messages provide a useful infrastructure for distributed systems with the following characteristics:

Fault tolerance based on replicated services

Replicated data: data are replicated to increase the performance of service

Propagation of event notifications (example)

Applications types: (examples)



IP multicast:

In practice only UDP is used for multicast

IP

IP multicast

UDP

Packet

Group(details)



API Java : class MulticastSocket

void joinGroup(InetAddress groupe)

void leaveGroup(InetAddress groupe)

synchronized void send(DatagramPacket dp, byte TTL) (TTL : definition)

synchronized void receive(DatagramPacket dp)

InetAddress getInterface() Retrieve the address of the network interface used for multicast packets

setInterface(InetAddress nt) Set the outgoing network interface for multicast packets on this

socket, to other than the system default

MulticastSocket() MulticastSocket(int port)

byte getTTL() void setTTL(byte TTL)



Example : a member joins the group, sends and receives datagrams

import java.net.*; import java.io.*;

public class MulticastPeer{

public static void main(String args[]){

// args[0] = message to multicast to the group

// args[1] = IP address of the group (e.g. "228.5.6.7")

MulticastSocket s =null;

try {

InetAddress group =

InetAddress.getByName(args[1]);

s = new MulticastSocket(6789);

s.joinGroup(group);



Example (continued):

byte [] m = args[0].getBytes();

DatagramPacket msgOut = new DatagramPacket(m,

m.length, group, 6789);

s.send(msgOut);

byte[] buffer = new byte[1000]; // get messages from others in group

for(int i=0; i< 3;i++) {

DatagramPacket msgIn = new DatagramPacket(buffer,

buffer.length);

s.receive(msgIn);

System.out.println("Rec. :" + new String(msgIn.getData())); }



Example (suite):

s.leaveGroup(group);

} catch (SocketException e){System.out.println("Socket: " +

e.getMessage());

} catch (IOException e){System.out.println("IO: " +

e.getMessage());

} finally {if(s != null) s.close();}

}}


Readings

MBone and Multicast Informations, http://www.infres.enst.fr/~dax/network/multicast.html

Ian Brown et al., Internet Multicast Tomorrow, Internet Protocol Journal, Vol 5(4), Dec. 2002 http://www.isoc.org/pubs/int/cisco-1-6.html

© Mourad Elhadef, Ph. D. CSI408-Distributed Information Systems Chapters 4 & 5 Interprocess...

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