1. Slides for Chapter 4: Interprocess Communication
From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design
Edition 3, © Addison-Wesley 2001

2. 4.2 The API for the Internet protocols
OUTLINE
4.1 Introduction
4.2 The API for the Internet protocols
4.3 External data representation and marshalling
4.4 Client-server communication
4.5 Group communication
4.6 Case study : interprocess communication in UNIX
4.7 Summary
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

3. Figure 4.1 Middleware layers
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

4. Introduction
Middleware is concerned with Integrating communication into a program language paradigm
Remote Method Invocation (RMI)
Allow object to invoke a method in an object in a remote process
Ex: CORBA ; Java RMI
Remote procedure calling (RPC )
Allow a client to call a procedure in a remote server
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

5. Chapter3: Chapter4: Introduction
discusses Internet transport-level protocols TCP & UDP without saying how middleware and application programs could use these protocols
Chapter4:
characteristics of interprocess communication
discusses UDP&TCP from a programmer’s point of view
Failure model
Case study : UNIX socket interface to UDP &TCP
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

6. Introduction
The application program interface to UDP provides a message passing abstraction-the simplest form of interprocesses communication
Enables a sending process to transmit a single message to receiving process
Independent packet Containing above messages called datagrams
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

7. The role of a producer-consumer form a pare of processes
Introduction
Streams provides to TCP provides the abstraction of two-way stream between pares of processes
The role of a producer-consumer form a pare of processes
1st section: produce data items
2nd section: consume them
3rd section: translate into a form suitable for sending in message over the network (distributed system)
4th & 5th section : design a suitable protocols to support client-server and group communication
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

8. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

9. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

10. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

11. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

12. 4.2 The API for the Internet protocols
4.2.1The characteristics of interprocess communication Message passing between a pair of processes can be support by two message communication operations: send and receive
Synchronous and asynchronous communication
Massage destinations
Reliability
Ordering
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

13. Figure 4.2 Sockets and ports
message
agreed port
any port
socket
Internet address =
Internet address =
other ports
client
server
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

14. 216 of possible port numbers
4.2.2 Socket
Both forms(UDP,TCP)of communication use socket abstraction provides an endpoint for communication between processes
Figure 4.2
216 of possible port numbers
Java API for Internet address : Java provide a class :InetAdress
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

15. 4.2.3 UDP datagram communication
Message size
Blocking
Timeout
Receive from any
Failure model
Omission failure
Ordering
Use of UDP
Java API for UDP datagrams
Java provides datagram communicatuon : DatagramPacket & DatagramSocket
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

16. Figure 4.3 UDP client sends a message to the server and gets a reply
import java.net.*;
import java.io.*;
public class UDPClient{
public static void main(String args[]){
// args give message contents and server hostname
try {
DatagramSocket 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);
byte[] buffer = new byte[1000];
DatagramPacket reply = new DatagramPacket(buffer, buffer.length);
aSocket.receive(reply);
System.out.println("Reply: " + new String(reply.getData()));
aSocket.close();
}catch (SocketException e){System.out.println("Socket: " + e.getMessage());
}catch (IOException e){System.out.println("IO: " + e.getMessage());} }
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

17. Figure 4.4 UDP server repeatedly receives a request and sends it back to the client
import java.net.*;
import java.io.*;
public class UDPServer{
public static void main(String args[]){
try{
DatagramSocket aSocket = new DatagramSocket(6789);
byte[] buffer = new byte[1000];
while(true){
DatagramPacket request = new DatagramPacket(buffer, buffer.length);
aSocket.receive(request);
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());}
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

18. 4.2.4 TCP stream communication
Hidden by the stream abstraction:
Message sizes
Lost message
Flow control
Message destinations
Paragraphs address some outstanding issues related to stream communication
Matching of data items
Blocking
Treads
Failure model
Use of TCP
HTTP
FTP
SMTP
Java API for TCP streams
ServerSocket
Socket
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

19. Figure 4.5 TCP client makes connection to server, sends request and receives reply
import java.net.*;
import java.io.*;
public class TCPClient {
public static void main (String args[]) {
// arguments supply message and hostname of destination
try{
int serverPort = 7896;
Socket 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 4.3
String data = in.readUTF();
System.out.println("Received: "+ data) ;
s.close();
}catch (UnknownHostException e){
System.out.println("Sock:"+e.getMessage());
}catch (EOFException e){System.out.println("EOF:"+e.getMessage());
}catch (IOException e){System.out.println("IO:"+e.getMessage());} }
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

20. Figure 4.6 TCP server makes a connection for each client and then echoes the client’s request
import java.net.*;
import java.io.*;
public class TCPServer {
public static void main (String args[]) {
try{
int serverPort = 7896;
ServerSocket listenSocket = new ServerSocket(serverPort);
while(true) {
Socket clientSocket = listenSocket.accept();
Connection c = new Connection(clientSocket); }
} catch(IOException e) {System.out.println("Listen :"+e.getMessage());}
// this figure continues on the next slide
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

21. Figure 4.6 continued class Connection extends Thread {
DataInputStream in;
DataOutputStream out;
Socket clientSocket;
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();
out.writeUTF(data);
clientSocket.close();
} catch(EOFException e) {System.out.println("EOF:"+e.getMessage());
} catch(IOException e) {System.out.println("IO:"+e.getMessage());}
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

22. 4.3 External data representation and marshalling
An agreed standard for the standard for the representation of data structures and primitive values is called an external data representation
Marshalling is the process of taking a collection of data items and assembling them into a form suitable for transmission in a message
Unmarshalling is the process of disassembling them on arrival to produce an equivalent collection of data items at the destination
CORBA’s Common Data Representation (CDR)
Java object serialization
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

23. Figure 4.7 CORBA CDR for constructed typespr s n ta t i o q ue ce l g th ( u si ed ) fo ll ow b el m nt r d ri ch a ra c te rs
n o ca al so h av w de
rs) rr ay le
s i
r (
o l en
h s ci f ie
d b
eca us is x ru ct
n t he or
r o la at co mp v
s a re pe
y t
r d ec ar ni
g f we
e s
cte
d m mb er
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

24. Figure 4.8 CORBA CDR message
The flattened form represents a
Person
struct with value: {‘Smith’, ‘London’, 1934}
0–3
4–7
8–11
12–15
16–19
20-23
24–27 5
"Smit"
"h___" 6
"Lond"
"on__"
1934
index in
sequence of bytes
4 bytes
notes
on representation
length of string
‘Smith’
‘London’
unsigned long
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

25. Figure 4.9 Indication of Java serialized form
The true serialized form contains additional type markers; h0 and h1 are handles
Serialized values
Person 3
1934
8-byte version number
int year
5 Smith
java.lang.String
name:
6 London h0
place: h1
Explanation
class name, version number
number, type and name of
instance variables
values of instance variables
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

26. Figure 4.10 Representation of a remote object reference
4.3.3 Remote object references
A remote object reference is an identifier for a remote object that is valid through a distributed system
Figure 4.10 Representation of a remote object reference
Internet address
port number
time
object number
interface of
remote object
32 bits
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

27. 4.4 Client-server communication
This form of communication is designed to support the roles and message exchanges in typical client-server interactions
The request-reply protocol
doOperation
getRequest
sendReply
Failure model of request-reply protocol
Use UDP
Omission failure
Guaranteed
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

28. Figure 4.11 Request-reply communication
Client
Server
Request
doOperation
message
getRequest
select object
(wait)
execute
method
Reply
message
sendReply
(continuation)
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

29. Figure 4.12 Operations of the request-reply protocol
public byte[] doOperation (RemoteObjectRef o, int methodId, byte[] arguments)
sends a request message to the remote object and returns the reply.
The arguments specify the remote object, the method to be invoked and the arguments of that method.
public byte[] getRequest ();
acquires a client request via the server port.
public void sendReply (byte[] reply, InetAddress clientHost, int clientPort);
sends the reply message reply to the client at its Internet address and port.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

30. Figure 4.13 Request-reply message structure
messageType
requestId
objectReference
methodId
arguments
int (0=Request, 1= Reply)
int
RemoteObjectRef
int or Method
array of bytes
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

31. Figure 4.14 RPC exchange protocolsm e M es
sag s
nt b y C li nt S r ve R qu t pl A ck no w
ledg
e re
ply
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

32. Figure 4.15 HTTP request message
GET //
HTTP/ 1.1
URL or pathname
method
HTTP version
headers
message body
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

33. Figure 4.16 HTTP reply message
200 OK
resource data
HTTP version
status code
reason
headers
message body
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

34. 4.5 Group communication
Def: An operation that sends a single message from one process to each of the members of a group of processes
Multicast message provide a useful infrastructure for constructing distributed system with the following characteristics
Fault tolerance based on replicated services
Finding the discovery servers in spontaneous networking
Better performance through
Replicated data
Propagation of event notifications
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

35. 4.5.1 IP multicast-an implementation of group communication
IP multicast:IP multicast is built on top of the Internet Protocol,IP.
Failure model for multicast datagrams: same as UDP datagram
Java API to IP multicast: class MulticastSocket
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

36. Figure 4.17 continued // get messages from others in group
byte[] buffer = new byte[1000];
for(int i=0; i< 3; i++) {
DatagramPacket messageIn =
new DatagramPacket(buffer, buffer.length);
s.receive(messageIn);
System.out.println("Received:" + new String(messageIn.getData())); }
s.leaveGroup(group);
}catch (SocketException e){System.out.println("Socket: " + e.getMessage());
}catch (IOException e){System.out.println("IO: " + e.getMessage());}
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

37. Figure 4.17 Multicast peer joins a group and sends and receives datagrams
import java.net.*;
import java.io.*;
public class MulticastPeer{
public static void main(String args[]){
// args give message contents & destination multicast group (e.g. " ")
try {
InetAddress group = InetAddress.getByName(args[1]);
MulticastSocket s = new MulticastSocket(6789);
s.joinGroup(group);
byte [] m = args[0].getBytes();
DatagramPacket messageOut =
new DatagramPacket(m, m.length, group, 6789);
s.send(messageOut);
// this figure continued on the next slide
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

38. 4.5.2 Reliability and ordering of multicast
Some examples of the effects of reliability and ordering
Fault tolerance based on replicated services
Finding the discovery servers in spontaneous networking
Better performance through replicated data
Propagation of event notifications
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

39. Figure 4.18 Sockets used for datagrams
ServerAddress and ClientAddress are socket addresses
Sending a message
Receiving a message
bind(s, ClientAddress)
sendto(s, "message", ServerAddress)
bind(s, ServerAddress)
amount = recvfrom(s, buffer, from)
s = socket(AF_INET, SOCK_DGRAM, 0)
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

40. Figure 4.19 Sockets used for streams
Requesting a connection
Listening and accepting a connection
s = socket(AF_INET, SOCK_STREAM,0)
s = socket(AF_INET, SOCK_STREAM,0)
bind(s, ServerAddress);
listen(s,5);
connect(s, ServerAddress)
sNew = accept(s, ClientAddress);
write(s, "message", length)
n = read(sNew, buffer, amount)
ServerAddress and ClientAddress are socket addresses
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000

41. 4.7 Summary
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Addison-Wesley Publishers 2000