Interprocess Communication
This chapter
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Concerns with the characteristics of protocols for
communication btw processes and distributed system
Java API provides both datagram and stream communication
Protocols for the representation of collections of data objects in
messages
Construction of protocols to support the two communication
patterns
Client-server communication
Group communication (multicast)
Middleware layers
Applic ations , s ervic es
RMI and RPC
T his
c hapter
reques t-reply protocol
marshall ing and external data repres entation
UDP and T CP
Middleware
layers
API for the Internet protocols
Characteristics of inter-process communication
Synchronous communication
In synchronous form of communication, the sending and
receiving process synchronize at every message.
 Asynchronous communication
In asynchronous form of communication, sending operation in
non-blocking means sending process is allowed to proceed
as soon as the message has been copied to local buffer.
Receive operation can have 2 variant, like: Blocking and
Non-blocking
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API for the Internet protocols (cont.)
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Message destination: if the client uses a fixed Internet
address to refer to a service, then that service must always run
on the same machine for its address to remain valid.
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It could be avoided by using name a name servers which will
translate name into server location
Mach Operating System provides location independent identifiers
for message destinations
Reliability: messages must arrive uncorrupted and without
duplication
Ordering: some applications require that messages should be
delivered in sender order
Sockets
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Both forms of communication uses socket abstraction
socket
any port
agreed port
socket
message
client
server
other ports
Internet address = 138.37.94.248
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Internet address = 138.37.88.249
Java API for Internet addresses
inetAddress acomputer = InetAddress.getByName(“www.cnn.com”);
UDP Datagram Communication
Some issues relating to datagram communication:

Message size: receiving process specify an array of bytes to
receive message. If size of the message is bigger than the array,
then message is truncated .
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Blocking: UDP datagram communication uses non-blocking
sends and blocking receives.
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Timeouts
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Failure model
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Omission failures
Ordering
Use of UDP
Java API for UDP datagram
UDP client sends a message and get a reply
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);
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();}
}
}
Java API for UDP datagram cont…
Java API provides datagram communication by means of 2 classes:
DatagramPacket and DatagramSocket
DatagramPacket: It Provides 2 constructors
Firstly: a constructor makes an instance out of an array of bytes
comprising a message, the length of the message and the
Internet address and local port number of the destination
socket.
Secondly: a constructor for use when receiving a message. Its
argument specify an array of bytes in which to receive the
message and the length of the array.
DatagramPacket provides getData, getPort and getAddress
methods
Java API for UDP datagram cont…
DatagramSocket : it supports sockets for sending and receiving
UDP datagrams. It provides constructor that takes a port
number as argument to use particular port. It also provides a
non-argument constructor that allows the system to choose a
free local port.
DatagramSocket provides the following methods:
Send and Receive: the argument of send is an instance of
DatagramPacket containing a message and destination; the
argument of receive is an empty DatagramPacket in which to
put the message.
setSoTimeout: allows to set the timeout
connect: used for connecting it to a particular remote port and
Internet address
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[]){
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);
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
Following characteristics of the network are hidden by the stream
abstraction:
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Message size
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Lost message
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Flow control
 Message duplication and ordering
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Message destinations
Some issues related to stream communication:
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Matching of data items
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Blocking
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Threads
Java API for TCP streams
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
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 4.3
String data = in.readUTF();
System.out.println("Received: "+ data) ;
}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());}
}finally {if(s!=null) try {s.close();}catch (IOException e){System.out.println("close:"+e.getMessage());}}
}
}
Java API for TCP streams cont…
Java interface to TCP streams is provided in the classes
ServerSocket and Socket
ServerSocket: this class is intended for use by server to create a
socket at a server port for listening for connect requests. Its
accept method gets a connect request from the queue, if the
queue is empty, it blocks until one arrives.
Socket: this class is for use by a pair of processes with a
connection. The client uses a constructor to create a socket,
specifying the DNS hostname and port of a server. This
constructor not only creates a socket associated with the local
port but also connects it to the specified remote computer and
port number.
External data representation and marshalling
The information stored in running programs is represented as data
structures – for example by set of interconnected objects –
whereas the information in messages consists of sequences of
bytes.
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One of the following methods can be used to enable any two
computers to exchange data values:
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The values are converted to an agreed external format before
transmission and converted to local form on receipt.
The values are transmitted in the sender’s format, together with an
indication of the format used, and the recipient converts the value
if necessary.
External data representation and marshalling
(cont.)
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Marshalling: is the process of taking collection of data items
and assembling them into a form suitable for transmission in a
message.
Unmarshalling: is the process of disassembling data items on
arrival to produce an equivalent collection of data items.
Two alternative approaches to external data
representation and marshalling are:

CORBA’s (common object oriented broker architecture) common
data representation (CDR); used in many programming
languages
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Java’s object serialization, used in Java only.
CORBA’s CDR (Common Data Representation)
index in
sequence of bytes
0–3
4–7
8–11
12–15
16–19
20-23
24–27
4 bytes
5
"Smit"
"h___"
6
"Lond"
"on__"
1934
notes
on representation
length of string
‘Smith’
length of string
‘London’
unsigned long
The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1934}
Java object serialization
Explanation
Serialized values
Person
8-byte version number
h0
class name, version number
3
int year
java.lang.String java.lang.String number, type and name of
name:
place:
instance variables
1934
5 Smith
6 London
h0 and h1 are handles
h1
values of instance variables