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

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. 3 © Addison-Wesley Publishers 2000

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

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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

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

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

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

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. 3 © Addison-Wesley Publishers 2000

Figure 4.2 Sockets and ports message agreed port any port socket Internet address = 138.37.88.249 Internet address = 138.37.94.248 other ports client server Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

Figure 4.7 CORBA CDR for constructed types pr 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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

Figure 4.14 RPC exchange protocols m 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. 3 © Addison-Wesley Publishers 2000

Figure 4.15 HTTP request message GET //www.dcs.qmw.ac.uk/index.html 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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. "228.5.6.7") 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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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. 3 © Addison-Wesley Publishers 2000

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