Chapter 4 slides

Slides for Chapter 4:
Interprocess Communication

From Coulouris, Dollimore, Kindberg and Blair
Distributed Systems:
Concepts and Design
Edition 5, © Addison-Wesley 2012

Figure 4.1
Middleware layers

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012

Application Program Interface

The application program interface to UDP provides a message
passing abstraction. This enables a sending process to transmit a
single message to a receiving process.
The application program interface to TCP provides the abstraction of
a two-way stream between pairs of processes. The information
communicated consists of a stream of data items with no
message boundaries.

3

The characteristics of interprocess communication

 Synchronous and asynchronous communication
 Message destinations (Internet address, local port)
 Reliability
 Ordering

4

Figure 4.2
Sockets and ports

agreed port
socket any port socket

message
client server
other ports

Internet address = 138.37.94.248 Internet address = 138.37.88.249


Java API for Internet addresses

 As the IP packets underlying UDP and TCP are sent to
Internet addresses, Java provides a class, InetAddress, that
represents Internet addresses.
 Users of this class refer to computers by Domain Name
System (DNS) hostnames.
 For example, instances of InetAddress that contain Internet
addresses can be created by calling a static method of
InetAddress, giving a DNS hostname as the argument.
 The method uses the DNS to get the corresponding Internet
address.
 InetAddress aComputer =InetAddress.getByName("bruno.dcs.qmul.ac.uk");
 This method can throw an UnknownHostException.
6

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
DatagramSocket aSocket = null;
try {
aSocket = new DatagramSocket();
byte [] m = args[0].getBytes();
InetAddress aHost = InetAddress.getByName(args[1]);
int serverPort = 6789;
//Datagram packet array of bytes containing message length of message Internet address port number
DatagramPacket request = new DatagramPacket(m, m.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()));
}catch (SocketException e){System.out.println("Socket: " + e.getMessage());
}catch (IOException e){System.out.println("IO: " + e.getMessage());}
}finally {if(aSocket != null) aSocket.close();}
} Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
}

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{
DatagramSocket aSocket = null;
try{
aSocket = new DatagramSocket(6789);
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 (IOException e) {System.out.println("IO: " + e.getMessage());}
}finally {if(aSocket != null) aSocket.close();}
}
}

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
Socket s = null;
try{
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());}}
}
}


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{
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


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

Marshalling and Unmarshalling

 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.
 Thus marshalling consists of the translation of structured data
items and primitive values into an external data
representation.
 Similarly, unmarshalling consists of the generation of primitive
values from their external data representation and the
rebuilding of the data structures.
12

Common Object Request Broker Architecture (CORBA)

 standard for software interoperability (set of common, object-
oriented interfaces that can communicate on various
platforms) .
 CORBA is most popular in communications Middleware using
an Object Request Broker ORB.
 CORBA evolved out of TCP/IP.

13

Figure 4.7
CORBA’s Common Data Representation (CDR) for constructed types


Figure 4.8
CORBA CDR message

index in notes
sequence of bytes 4 bytes on representation
0–3 5 length of string
4–7 "Smit" ‘Smith’
8–11 "h___"
12–15 6 length of string
16–19 "Lond" ‘London’
20-23 "on__"
24–27 1984 unsigned long

The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1984}


Marshalling in CORBA

 Marshalling operations can be generated automatically from
the specification of the types of data items to be transmitted in
a message.

16

Figure 4.9
Indication of Java serialized form

Serialized values Explanation
Person 8-byte version number h0 class name, version number

int year java.lang.String java.lang.String number, type and name of
3 name: place: instance variables
1984 5 Smith 6 London h1 values of instance variables

The true serialized form contains additional type markers; h0 and h1 are handles


Figure 4.10 XML definition of the Person structure

<person id="123456789">
<name>Smith</name>
<place>London</place>
<year>1984</year>

</person >

Figure 4.11 Illustration of the use of a namespace in the Person structure

<person pers:id="123456789" xmlns:pers = "http://www.cdk5.net/person">
<pers:name> Smith </pers:name>
<pers:place> London </pers:place >
<pers:year> 1984 </pers:year>
</person>


Figure 4.12 An XML schema for the Person structure

<xsd:schema xmlns:xsd = URL of XML schema definitions >
<xsd:element name= "person" type ="personType" />
<xsd:complexType name="personType">
<xsd:sequence>
<xsd:element name = "name" type="xs:string"
<xsd:element name = "place" type="xs:string"
<xsd:element name = "year" type="xs:positive
</xsd:sequence>
<xsd:attribute name= "id" type = "xs:positiveInteger"
</xsd:complexType>
</xsd:schema>


Figure 4.13
Representation of a remote object reference

 When a client invokes a method in a remote object, an invocation
message is sent to the server process that hosts the remote
object.
 This message needs to specify which particular object is to have
its method invoked. A remote object reference is an identifier for
a remote object that is valid throughout a distributed system.
 A remote object reference is passed in the invocation message
to specify which object is to be invoked.

32 bits 32 bits 32 bits 32 bits
interface of
Internet address port number time object number remote object


Multicast Communication

 A multicast operation is an operation that sends a single
message from one process to each of the members of a group of
processes, usually in such a way that the membership of the
group is transparent to the sender.

 Multicast messages provide a useful infrastructure for
constructing distributed systems with the following
characteristics:
 Fault tolerance based on replicated services
 Discovering services in spontaneous networking
 Better performance through replicated data
 Propagation of event notifications


IP multicast – An implementation of multicast communication

 IP multicast is built on top of the Internet Protocol (IP). Note
that IP packets are addressed to computers – ports belong to
the TCP and UDP levels.

 IP multicast allows the sender to transmit a single IP packet to
a set of computers that form a multicast group.

 The sender is unaware of the identities of the individual
recipients and of the size of the group. A multicast group is
specified by a Class D Internet address – that is, an address
whose first 4 bits are 1110 in IPv4. (remember that we need
Multicast routers and Multicast address allocation).
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Figure 4.14
Multicast peer joins a group and sends and receives datagrams

import java.net.*;
import java.io.*;
public class MulticastPeer{
// args give message contents & destination multicast group (e.g. "228.5.6.7")
MulticastSocket s =null;
try {
InetAddress group = InetAddress.getByName(args[1]);
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


Figure 4.14
continued

// get messages from others in group
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 (IOException e){System.out.println("IO: " + e.getMessage());}
}finally {if(s != null) s.close();}
}
}


Network virtualization

 Network virtualization is concerned with the construction of
many different virtual networks over an existing network such
as the Internet.
 Each virtual network can be designed to support a particular
distributed application.
 For example, one virtual network might support multimedia
streaming, as in BBC iPlayer, and coexist with another that
supports a multiplayer online game, both running over the
same underlying network.

26

Overlay networks

 An overlay network is a virtual network consisting of nodes
and virtual links, which sits on top of an underlying network
(such as an IP network) and offers something that is not
otherwise provided:
 a service that is tailored towards the needs of a class of application or
a particular higher-level service – for example, multimedia content
distribution;
 more efficient operation in a given networked environment – for
example routing in an ad hoc network;
 an additional feature – for example, multicast or secure
communication.

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Figure 4.15
Types of overlay

table continues on the next slide
28

Figure 4.15 (continued)
Types of overlay

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Figure 4.16
Skype overlay architecture

 Skype is a peer-to-peer application offering
Voice over IP (VoIP).
 It also includes instant messaging, video
conferencing and interfaces to the standard
telephony service through SkypeIn and
SkypeOut. The software was developed by
Kazaa in 2003 and hence shares many of
the characteristics of the Kazaa peer-to-
peer file-sharing application.
 It is widely deployed, with an estimated 370
million users as of the start of 2009.
 Skype is an excellent case study of the use
of overlay networks in real-world (and large-
scale) systems.

30

Figure 4.17
Point-to-point communication in Message Passing Interface (MPI)

31

Figure 4.18
Selected send operations in MPI

32

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