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Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All

Chapter 3 Inter-process Communication. Reference slides from Distributed systems concepts and design Distributed computing Liu , and operating system concepts Silberschatz. 1. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All

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Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All

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  1. Chapter 3 Inter-process Communication Reference slides from Distributed systems concepts and design Distributed computing Liu , and operating system concepts Silberschatz 1 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  2. Middleware layer This chapter is concerned with the characteristics of protocols for communication between processes. E.g. Java API RMI and RPC is concerned with integrating communication into a programming language paradigm. Under RMI and RPC, there are sockets, message passing, multicast support. Protocols for the representation of collections of data objects in messages are also discussed. 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  3. Synchronous and Asynchronous  Synchronous and Asynchronous communication: Sending and receiving processes may be either one of them.  Synchronous: two processes synchronize at every message. Both send and receive are blocking operations. Whenever a send issued the sending process is blocked until the receive is issued. Whenever a receive is issued by a process, it blocks until a message arrives.  Asynchronous: The send operation is non-blocking and the sending process is allowed to proceed as soon as the message has been copied to a local buffer. The receive can be either blocking or non- blocking(process can proceed and notified by polling or interrupt when background buffer filled. But no advantage due to multi-thread and overhead of implementing the mechanism). 3 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  4. Sockets and ports agreed port any port socket socket message client server other ports Internet address = 138.37.94.248 Internet address = 138.37.88.249 Both UDP and TCP use the socket as endpoints for communication between processes. A socket is associated with a Internet address and port number. Each computer has a large number 2^16 of possible port numbers. Processes may use the same socket for sending and receiving messages. 4 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  5. TCP  TCP stream of bytes communication  A dedicated point-to-point channel  Use TCP (Transmission Control Protocol) for data transmission.  For integrity, checksum is used to detect and reject corrupt packets an sequence numbers to detect and reject duplicate packets. For validity, timeouts and retransmission to deal with lost packets.  So messages are guaranteed to be delivered. Lossless and reliable. Sent and received in the same order. Use of TCP: many frequently used services run over TCP connections HTTP, FTP, Telnet, SMTP 5 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  6. UDP  UDP datagram communication: attractive due to no overheads associated with guaranteed message delivery  No dedicated point-to-point channel  Use UDP (User Datagram Protocol) for data transmission. No acknowledge and retries.  Omission failure as messages may be dropped occasionally either because of checksum error or no buffer space at source or destination  Messages can be sometimes be delivered out of sender order  So applications using UDP are left to provide their own checks to achieve quality of reliable communication that suffers from omission failures, which can be constructed by using acknowledgements.  Applications that are acceptable to have occasional omission failure and out of order. For example, DNS and Voice Over IP (VOIP) 6 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  7. Client/Server Communications: TCP After the server accepts the connection, communication between server and client is conducted the same as for I/O streams. The server must be running when a client starts. The server waits for a connection request from a client. To establish a server, you need to create a server socket and attach it to a port, which is where the server listens for connections. Server Host Client Host After a server socket is created, the server can use this statement to listen for connections. The client issues this statement to request a connection to a server. Server socket on port 8000 SeverSocket server = new ServerSocket(8000); I/O Stream A client socket Socket socket = server.accept() Client socket Socket socket = new Socket(host, 8000) 7 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  8. Data Transmission through Sockets Server Client int port = 8000; DataInputStream in; DataOutputStream out; ServerSocket server; Socket socket; int port = 8000; String host="localhost" DataInputStream in; DataOutputStream out; Socket socket; Connection Request server =new ServerSocket(port); socket=server.accept(); in=new DataInputStream (socket.getInputStream()); out=new DataOutStream (socket.getOutputStream()); System.out.println(in.readDouble()); out.writeDouble(aNumber); socket=new Socket(host, port); in=new DataInputStream (socket.getInputStream()); out=new DataOutputStream (socket.getOutputStream()); out.writeDouble(aNumber); System.out.println(in.readDouble()); I/O Streams InputStream input = socket.getInputStream(); OutputStream output = socket.getOutputStream(); 8 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  9. A TCP Client/Server Example  Problem: Write a client to send data to a server. The server receives the data, uses it to produce a result, and then sends the result back to the client. The client displays the result on the console. In this example, the data sent from the client is the radius of a circle, and the result produced by the server is the area of the circle. compute area radius Server Client area 9 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  10. A Client/Server Example, cont. compute area radius Server Client area Client Code Server Code Start Client Start Server Note: Start the server, then the client. 10 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  11. The InetAddress Class Occasionally, you would like to know who is connecting to the server. You can use the InetAddress class to find the client's host name and IP address. The InetAddress class models an IP address. You can use the statement shown below to create an instance of InetAddress for the client on a socket. InetAddress inetAddress = socket.getInetAddress(); Next, you can display the client's host name and IP address, as follows: System.out.println("Client's host name is " + inetAddress.getHostName()); System.out.println("Client's IP Address is " + inetAddress.getHostAddress()); 11 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  12. Serving Multiple Clients Multiple clients are quite often connected to a single server at the same time. Typically, a server runs constantly on a server computer, and clients from all over the Internet may want to connect to it. You can use threads to handle the server's multiple clients simultaneously. Simply create a thread for each connection. Here is how the server handles the establishment of a connection: while (true) { Socket socket = serverSocket.accept(); Thread thread = new Thread(new ThreadClass(socket)); thread.start(); } The server socket can have many connections. Each iteration of the while loop creates a new connection. Whenever a connection is established, a new thread is created to handle communication between the server and the new client; and this allows multiple connections to run at the same time. 12 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  13. Example: Serving Multiple Clients Server A serve socket on a port Server for Multiple Clients A socket for a client A socket for a client Start Server . . . Client 1 Client n Start Client Note: Start the server first, then start multiple clients. 13 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  14. DatagramPacket The DatagramPacket class represents a datagram packet. Datagram packets are used to implement a connectionless packet delivery service. Each message is routed from one machine to another based solely on information contained within the packet. java.net.DatagramPacket length: int address: InetAddress A JavaBeans property to specify the length of buffer. A JavaBeans property to specify the address of the machine where the package is sent or received. A JavaBeans property to specify the port of the machine where the package is sent or received. port: int +DatagramPacket(buf: byte[], length: int, host: InetAddress, port: int) Constructs a datagram packet in a byte array buf of the specified length with the host and the port for which the packet is sent. This constructor is often used to construct a packet for delivery from a client. Constructs a datagram packet in a byte array buf of the specified length. +DatagramPacket(buf: byte[], length: int) +getData(): byte[] +setData(buf: byte[]): void Returns the data from the package. Sets the data in the package. 14 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  15. DatagramSocket The DatagramSocket class represents a socket for sending and receiving datagram packets. A datagram socket is the sending or receiving point for a packet delivery service. Each packet sent or received on a datagram socket is individually addressed and routed. Multiple packets sent from one machine to another may be routed differently, and may arrive in any order. DatagramSocket To create a server DatagramSocket, use the constructor DatagramSocket(int port), which binds the socket with the specified port on the local host machine. Create a server DatagramSocket To create a client DatagramSocket, use the constructor DatagramSocket(), which binds the socket with any available port on the local host machine. Create a client DatagramSocket 15 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  16. Sending and Receiving a DatagramSocket Sending To send data, you need to create a packet, fill in the contents, specify the Internet address and port number for the receiver, and invoke the send(packet) method on a DatagramSocket. To receive data, create an empty packet and invoke the receive(packet) method on a DatagramSocket. Receiving 16 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  17. Datagram Programming Datagram programming is different from stream socket programming in the sense that there is no concept of a ServerSocket for datagrams. Both client and server use DatagramSocket to send and receive packets. Designate on a server DatagramServer DatagramSocket socket; socket = new DatagramSocket(8000); byte[] buf = new byte[256]; DatagramPacket receivePacket = new DatagramPacket(buf, bef.length) socket.receive(receivePacket); get data from buf or receivePacket.getData(); DatagramClient DatagramSocket socket; socket = new DatagramSocket(); byte[] buf = new byte[256]; InetAddress address = new InetAddress(serverName); DatagramPacket sendPacket = new DatagramPacket(buf, bef.length, address, 8000) fill in the contents in buf; socket.send(sendPacket); 17 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  18. Example: A Client/Server Example Already present a client program and a server program using socket streams. The client sends radius to a server. The server receives the data, uses them to find the area, and then sends the area to the client. Rewrite the program using datagram sockets. Server Code Client Code Note: Start the server, then the client. Start Server Start Client 18 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  19. Client/Server Communications  Both forms of communication UDP and TCP use the socket abstraction, which provides endpoint for communication between processes.  Each socket is associated with a particular protocol either UDP or TCP. Each Internet host has 2^16 (65,535) logical ports. Each port is identified by a number between 1 and 65535, and can be allocated to a particular process. Port numbers between 1 and 1023 are reserved for processes which provide well-known services such as finger, FTP, HTTP, and email. 19 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  20. Ports Assignment Assignment of some well-known ports Protocol Port Service echo 7 IPC testing daytime 13 provides the current date and time ftp 21 file transfer protocol telnet 23 remote, command-line terminal session smtp 25 simple mail transfer protocol time 37 provides a standard time finger 79 provides information about a user http 80 web server RMI Registry 1099 registry for Remote Method Invocation web server which supports servlets, JSP, or ASP special web server 8080 20 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  21. Case Studies: Distributed TicTacToe Games Server Session 1 ... Session N Player 1 Player 2 ... Player 1 Player 2 TicTacToeServer Run Server TicTacToeClient Run Client 21 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  22. Distributed TicTacToe Game Player 1 Server Player 2 1. Initialize user interface. Create a server socket. 1. Initialize user interface. 2. Request connection to the server and know which token to use from the server. Accept connection from the first player and notify the player is Player 1 with token X. Accept connection from the second player and notify the player is Player 2 with token O. Start a thread for the session. 2. Request connection to the server and know which token to use from the server. Handle a session: 3. Get the start signal from the server. 1. Tell player 1 to start. 4. Wait for the player to mark a cell, send the cell's row and column index to the server. 2. Receive row and column of the selected cell from Player 1. 3. Receive status from the server. 3. Determine the game status (WIN, DRAW, CONTINUE). If player 1 wins, or drawn, send the status (PLAYER1_WON, DRAW) to both players and send player 1's move to player 2. Exit. . 4. If CONTINUE, notify player 2 to take the turn, and send player 1's newly selected row and column index to player 2. 5. Receive status from the server. 4. If WIN, display the winner. If player 1 wins, receive player 1's last move, and break the loop. 6. If WIN, display the winner; if player 2 wins, receive the last move from player 2. Break the loop 5. If DRAW, display game is over, and receive player 1's last move, and break the loop. 7. If DRAW, display game is over; break the loop. 5. Receive row and column of the selected cell from player 2. 6. If CONTINUE, receive player 1's selected row and index and mark the cell for player 1. 6. If player 2 wins, send the status (PLAYER2_WON) to both players, and send player 2's move to player 1. Exit. 7. Wait for the player to move, and send the selected row and column to the server. 7. If CONTINUE, send the status, and send player 2's newly selected row and column index to Player 1. 8. If CONTINUE, receive player 2's selected row and column index and mark 22 the cell for player 2. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  23. Distributed TicTacToe, cont. JFrame TicTacToeServer HandleASession TicTacToeConstants Similar as in Example 12.7 JApplet TicTacToeClient Cell Runnable TicTacToeServer HandleASession TicTacToeClient +main(args: String[]): void -player1: Socket -player2: Socket -cell char[][] -continueToPlay: boolean +run(): void -isWon(): boolean -isFull(): boolean -sendMove(out: DataOuputStream, row: int, column: int): void -myTurn: boolean -myToken: char -otherToken: char -cell: Cell[][] -continueToPlay: boolean -rowSelected: int -columnSelected: int -isFromServer: DataInputStream -osToServer: DataOutputStream -waiting: boolean +run(): void -connectToServer(): void -recieveMove(): void -sendMove(): void -receiveInfoFromServer(): void -waitForPlayerAction(): void TicTacToeConstants +PLAYER1=1: int +PLAYER2 = 2: int +PLAYER1_WON = 1: int +PLAYER2_WON = 2: int +DRAW = 3: int +CONTINUE = 4: int 23 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  24. Synchronization Revisit  Message passing may be either blocking or non-blocking  Blocking is considered synchronous  Blocking send has the sender block until the corresponding receive is issued  Blocking receive has the receiver block until a message arrives  Non-blocking is considered asynchronous  Non-blocking sender proceeds after send operation (message is copied to local buffer). So the message transmission can run parallel with the sender program  Non-blocking receiving process proceeds after receive operation, which provides a buffer to be filled in the background. But it must separately receive notification that its buffer has been filled by polling or interrupt  In multiple thread process, blocking receive has no disadvantage. Simple synchronization and less complexity. So today system do not generally provide non- blocking receive. 24 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  25. Buffering  Queue of messages attached to the link; implemented in one of three ways 1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous) 2. Bounded capacity – finite length of n messages Sender must wait if link full 3. Unbounded capacity – infinite length Sender never waits Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  26. Remote Procedure Calls  Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.  Stubs – client-side proxy for the actual procedure on the server.  The client-side stub locates the server and marshalls the parameters.  The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server. 26 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  27. Marshalling Parameters Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  28. Remote Method Invocation  Remote Method Invocation (RMI) is a Java mechanism similar to RPCs.  RMI allows a Java object on one machine to invoke a method on a remote object. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  29. External Data Representation and marshalling  Data transmitted on the network is a binary stream.  Because different computers may have different internal storage format for the same data type, an external representation of data may be necessary.  Data marshalling is the process of translating/ converting the data to an external representation.  Data unmarshalling is the process of rebuilding of data structure.  Three approaches to external data representation schemes are: a. CORBA’ ’s common data representation: can represent all data types for argument and return values for RMI in CORBA for a variety of programming languages. b. Java’ ’s object serialization: flatterning of an object into a serial form that can be transmitted in a message and stored on disk. It is for use only by Java. Types information is included. c. XML (Extensible Markup Language): define a self-describing textual format for structured data for web use. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  30. Example: Person Object  Struct Person {  String name;  String place;  Unsigned long year; }; 30 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  31. CORBA Common Data Representation (CDR) message notes on representation length of string index in sequence of bytes 4 bytes 5 "Smit" "h___" 6 "Lond" "on__" 1934 0–3 4–7 8–11 12–15 16–19 20-23 24–27 ‘Smith’ length of string ‘London’ unsigned long The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1934} Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  32. Java serialized form public class Person implements Serializable { private String name; private String place; private int year; public Person() { } } Explanation Serialized values 8-byte version number java.lang.String name: Person h0 class name, version number java.lang.String place: h1 number, type and name of instance variables int year 3 1934 5 Smith 6 London values of instance variables The true serialized form contains additional type markers; h0 and h1 are handles Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  33. XML definition of the Person structure <person id="123456789"> <name>Smith</name> <place>London</place> <year>1934</year> <!-- a comment --> </person > Elements: portion of character data surrounded by start and end tag. Hierarchical structure is possible with element enclosing. Attributes: start tag may include associated attribute names and values. E.g. id=“123456789” Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  34. Example: Passing Objects in Network Programs Write a program that collects student information from a client and send them to a server. Passing student information in an object. Client Server student object student object out.writeObject(student) in.readObject() out: ObjectOutputStream in: ObjectInputStream socket.getOutputStream socket.getInputStream socket socket Student Class Network Student Sever Student Client Start Server Start Client Note: Start the server first, then the client. 34 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  35. Secure Sockets  Secure sockets perform encryption on the data transmitted.  The JavaTMSecure Socket Extension (JSSE) is a Java package that enables secure Internet communications.  It implements a Java version of SSL (Secure Sockets Layer) and TLS (Transport Layer Security) protocols  It includes functionalities for data encryption, server authentication, message integrity, and optional client authentication.  Using JSSE, developers can provide for the secure passage of data between a client and a server running any application protocol. 35 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

  36. Secure Sockets  Import javax.net.ssl;  Class SSLServerSocket is a subclass of ServerSocket, and inherits all its methods.  Class SSLSocket is a subclass of Socket, and inherits all its methods.  Example code available:  http://java.sun.com/javase/6/docs/technotes/guides /security/jsse/JSSERefGuide.html#UnsecureSecure 36 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

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