Tuesday, December 16, 2008

1. Tuesday, December 16, 2008 The practical scientist is trying to solve tomorrow's problem on yesterday's computer. Computer scientists often have it the other way around. - Numerical Recipes, C Edition

2. Distributed ComputingClass: BSIT-8Instructor: Raihan Ur Rasool Chapter 04: Inter-process Communication

3. Chapter 3 – SELF STUDY • Networking issues for distributed systems • Types of network • Network Principles • Protocol layers • Internetworking • Routing • Internet protocols • Case Studies • Ethernet • MobileLAN • ATM

4. Objectives of the lecture • To study the general characteristics of interprocess communication and the particular characteristics of both datagram and stream communication in the Internet. • To be able to write Java and C applications that use the Internet protocols and Java serialization. • To be aware of the design issues for Request-Reply protocols and how collections of data objects may be represented in messages (RMI and language integration are left until Chapter 5). • To be able to use the Java API to IP multicast and to consider the main options for reliability and ordering in group communication.

5. Chapter 4: Interprocess Communication • Introduction • The API for the Internet protocols • External data representation and marshalling • Client-Server communication • Group communication • Case study: interprocess communication in UNIX • Summary

6. Introduction • Middleware layer • TCP ( UDP) from a programmers point of view • TCP : two way stream • UDP : datagram, message passing • Java interface, UNIX Socket • marshalling and demarshalling • data form translation • Client-Server communication • Request-Reply protocols • Java RMI, RPC • Group communication

7. Middleware layer (1)

8. Middleware layer (2) • An adapted reference model for networked communication. 2-5

9. Middleware layer (3) • A software layer that • masks the heterogeneity of systems • provides a convenient programming abstraction • provides protocols for providing general-purpose services to more specific applications, e.g. naming, security, transaction, persistent storage and event notification • authentication protocols • authorization protocols • distributed commit protocols • distributed locking protocols • high-level communication protocols • remote procedure calls (RPC) • remote method invocation (RMI)

10. Middleware (4) • General structure of a distributed system as middleware. 1-22

11. Middleware programming models • Remote Calls • remote Procedure Calls (RPC) • distributed objects and Remote Method Invocation (RMI) • eg. Java RMI • Group multicast protocols • Common Object Request Broker Architecture (CORBA) • Other programming models • remote event notification • remote SQL access • distributed transaction processing

12. Where are we ? • Introduction • The API for the Internet protocols • External data representation and marshalling • Client-Server communication • Group communication • Case study: interprocess communication in UNIX • Summary

13. The characteristics of inter-process communication (1) • Producer—Consumer and wait • Send and receive • Synchronous and asynchronous • a queue associated with message destination, Sending process add message to remote queue • Receiving process remove message from local queue • Synchronous: • send and receive are blocking operations • Asynchronous: • send is nonblocking, • receive could be blocking or nonblocking • (receive notification by interrupt) • Java supports multiple threads in a single process • Non-blocking communication appears to be more efficient, but it involves extra complexity in the receiving process: flow control

14. The characteristics of interprocess communication (2) • Message destinations: • Internet address + local port • Port has one receiver but many senders • service name: names into server locations at run time • location independent resource identifiers, e.g. in Mach • Reliability • validity: messages are guaranteed to be delivered despite a reasonable number of packets being dropped or lost • Integrity: messages arrive uncorrupted and without duplication • Ordering • the messages be delivered in sender order • The order in which they were transmitted by the sender. • The delivery out of sender’s order is regarded as failure

15. agreed port any port socket socket message client server other ports Internet address = 138.37.94.248 Internet address = 138.37.88.249 Socket • Endpoint for communication between processes • Both forms of communication (UDP and TCP ) use the socket abstraction • Originated from BSD Unix, present in Linux, Windows NT and Macintosh OS etc • bound to a local port (216 possible port number) and one of the Internet address • a process cannot share socket with other processes on the same computer

16. Sockets • The application level API to TCP & UDP • Allows users to write application “protocol objects” which sit on top of TCP and UDP. • Execution of TCP/UDP/IP are in kernel space. Sockets provide the bridge to user space. • Java provides 3 types of sockets • DatagramSocket – for UDP • ServerSocket – for TCP server • Socket – endpoint of a TCP stream App 1 App 2 App 3 socket socket socket UDP TCP IP More on Sockets:http://java.sun.com/docs/books/tutorial/networking/sockets/definition.html

17. UDP datagram communication • UDP datagrams are sent without acknowledgement or retries • Issues relating to datagram communication • Message size: not bigger than 64k in size, otherwise truncated on arrival • blocking: non-blocking sends (message could be discarded at destination if there is not a socket bound to the port ) and blocking receives (could be timeout) • Timeout:receiver set on socket –server blocks forever • Receive from any: not specify an origin for messages, but could be set to receive from or send to particular remote port by socket connection operation

18. UDP datagram communication • Failure model • omission failure: message be dropped due to checksum error or no buffer space at sender side or receiver side (send-omission, receive-omission) • ordering: message be delivered out of sender order • application maintains the reliability of UDP communication channel by itself (ACK) • DNS & VOIP are implemented over UDP • UDP datagram do not suffer from overheads associated with guaranteed message delivery

19. Java API for UDP datagrams • DatagramPacket • DatagramSocket • send and receive : transmit datagram between a pair of sockets • setSoTimeout : receive method will block for the time specified and then throw an InterruptedIOexception • connect: connect to a particular remote port and Internet address • Examples • acceptable to services that are liable to occasional omission failures, e.g. DNS

20. Datagram Sockets • Creating a datagram socket • DatagramSocket soc = new DatagramSocket(port-no.) • Creating a datagram packect • DatagramPacket p = new DatagramPacket(byte[] data, int len, InetAddress dest, int dest-port); • Sending a packet • soc.send( p ) • Receiving a packet • q = new DatagramPacket(buff, len);soc.receive( q ); IP header: source-ip-address destination-ip-address protocol = 17 (udp) UDP header: source port-no destination port-no Application Data

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

22. 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();} } }

23. TCP stream communication (1) • The API to the TCP • provide the abstraction of a stream of bytes to which data may be written and from which data may be read • Hidden network characteristics • message sizes (how much data to write/read) (Application) • lost messages (ACK) • flow control (match the speed) • message duplication and ordering (identifier with each IP Packet) • message destinations (establish connection) Once connection is established, no need of address & ports • While connection establishing: client/server • Client  Connect request, Server Accept request, becomes Peers • Client: creates stream socket, bind to a port, and makes a connect request to server • Server: creates a listening socket, bind to a server port, and waits for clients

24. TCP stream communication (2) • issues related to stream communication • Matching of data items: agree to the contents of the transmitted data (int & double, write/ read, ) • Blocking: send blocked until the data is written in the receiver’s buffer, receive blocked until the data in the local buffer becomes available • Threads: server create a new thread for every connection • failure model • integrity and validity have been achieved by checksum, sequence number, timeout and retransmission in TCP protocol • connection could be broken due to unknown failures • Can’t distinguish between network failure and the destination process failure • Can’t tell whether its recent messages have been received or not

25. Server side Client Side 1. Contact server Server socket client socket 3. Establish stream communication 2. Create new Stream socket New socket TCP byte stream sockets • Server socket – waits for connections • soc = new ServerSocket( port-no ); • Socket newsoc = soc.accept( ); • Client socket – connects to server • client = new Socket(server-addr, server-port); • When connected, get streams • InputStream in = client.getInputStream( );OutputStream out = client.getOutputStream( )

26. Java API for TCP Streams • ServerSocket • accept: listen for connect requests from clients • Socket • constructor • not only create a socket associated with a local port, but also connect it to the specified remote computer and port number • getInputStream • getOutputStream

27. 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());}} } }

28. TCP server makes a connection for each client and then echoes the client’s request (1) import java.net.*; import java.io.*; public class TCPServer { public static void main (String args[]) { try{ intserverPort = 7896; ServerSocketlistenSocket = 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

29. TCP Server (2) class Connectionextends 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*/}} } }

30. Where are we ? • Introduction • The API for the Internet protocols • External data representation and marshalling • Client-Server communication • Group communication • Case study: interprocess communication in UNIX • Summary

31. External data representation and marshalling introduction [data structures and a sequence of bytes] • Why does the communication data need external data representation and marshalling? [data structures must be flattened] • Different data format on different computers, e.g., big-endian/little-endian integer order, ASCII (Unix) / Unicode character coding • How to enable any two computers to exchange data values? • The values be converted to an agreed external format before transmission and converted to the local form on receipt • The values are transmitted in the sender’s format, together with an indication of the format used, and the receipt converts the value if necessary

32. External data representation and marshalling introduction • Why does the communication data need external data representation and marshalling? • Different data format on different computers, e.g., big-endian/little-endian integer order, ASCII (Unix) / Unicode character coding • How to enable any two computers to exchange data values? • The values be converted to an agreed external format before transmission and converted to the local form on receipt • The values are transmitted in the sender’s format, together with an indication of the format used, and the receipt converts the value if necessary • External data representation • An agreed standard for the representation of data structures and primitive values • Marshalling • The process of taking a collection of data items and assembling them into a form suitable for transmission in a message • Usage: for data transmission or storing in files • Three alternative approaches to EDP • CORBA’s common data representation / Java’s object serialization & XML

33. Reading Assignment • CORBA’s Common Data Representation (CDR) • Java Object Serialization • Extensible Markup language (XML) • Next Time: • Recap • Client-server communication 