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Remote Method Invocation for Effective Client-Server Communication

Learn about RMI, sockets, RPC, and how to implement remote method calls for efficient communication between distributed systems. Explore concepts like stubs, marshalling, and object serialization to enhance your remote procedure calls setup.

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Remote Method Invocation for Effective Client-Server Communication

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  1. Remote Method Invocation (RMI)

  2. Client-Server Communication • Sockets • Remote Procedure Calls • Remote Method Invocation (Java)

  3. Sockets • A socket is defined as an endpoint for communication. • Concatenation of IP address and port • The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 • Communication consists between a pair of sockets. • Considered a low-level form of communication between distributed processes. • Sockets allow only an unstructured stream of bytes to be exchanged. It is the responsibility of the client or server application to impose a structure on the data.

  4. Socket Communication

  5. Remote Procedure Calls • Remote procedure call (RPC) abstracts procedure calls between processes on networked systems. • Stub – client-side proxy for the actual procedure on the server. Server has a similar stub as well. • The client-side stub locates the server and marshals the parameters. • The server-side stub receives this message, unpacks the marshaled parameters, and performs the procedure on the server. • External data representation (XDR) I.e most-significant (big-endian), least-significant(little-endian)

  6. Execution of RPC

  7. Remote Method Invocation • Remote Method Invocation (RMI) is a Java mechanism similar to RPCs. • RMI allows a Java program on one machine to invoke a method on a remote object.

  8. Marshalling Parameters

  9. Remote Method Invocation • RMI and RPC differs in two ways: • RPCs support procedural programming whereby only remote procedures or functions may be called. RMI is object based: It supports invocation of methods on remote objects. • The parameters to remote procedures are ordinary data structures in RPC; with RMI it is possible to pass objects as parameters to remote methods. • If the marshaled parameters are local (non remote) objects, they are passed by copy using a technique known as object serialization. • Object serialization allowed the state of an object to be written toa byte stream.

  10. Introduction to RMI • Remote Method Invocation (RMI) • Allows remote method calls • Objects in different programs can communicate • Method calls appear same as those in same program • Based on Remote Procedure Calls (RPC) • Developed in 1980's • Allows procedural program (like C) to call function on another computer • Performs networking and marshalling of data (packaging arguments and return values) • Not compatible with objects • Interface Definition Language required - describe functions • RMI is Java's implementation of RPC

  11. Introduction to RMI • RMI • Register method as remotely accessible • Client can look up method and receive a reference • Use reference to call method • Syntax same as a normal method call • Marshalling of data • Can transfer objects as well • Class ObjectOutputStream converts Serializable object into stream of bytes • Transmit across network • Class ObjectInputStream reconstructs object • No Interface Definition Language needed • Use Java's own interface

  12. Case Study: Creating a Distributed System with RMI • RMI example • Downloads weather information from National Weather Service website http://iwin.nws.noaa.gov/iwin/us/traveler.html • Note: Format of website changed several times, if example does not work do the appropriate modifications. • Store information on a server • Request information through remote method calls

  13. Case Study: Creating a Distributed System with RMI

  14. Case Study: Creating a Distributed System with RMI • Four major steps • Define remote interface • Describes client/server communication • Define server application to implement remote interface • Same name as remote interface, ends with Impl • Define client application that uses remote interface reference • Interacts with server implementation • Compile and execute server and client

  15. Defining the Remote Interface • First step • Define remote interface that describes remote methods • Client calls remote methods, server implements them • To create a remote interface • Define interface that extends interface Remote (java.rmi) • Tagging interface - no methods to define • An object of a class that implements interface Remote directly or indirectly is a remote object and can be accesses from any JVM. • Each method in Remote interface must throw RemoteException • Potential network errors

  16. Defining the Remote Interface • Interface TemperatureServer • Extends Remote • Describes method getWeatherInfo

  17. 1 // Fig. 20.1: TemperatureServer.java 2 // TemperatureServer interface definition Interface Remote in java.rmi 3import java.rmi.*; 4 5 public interface TemperatureServer extends Remote { Methods in Remote interface (is a relationship) must be able to throw a RemoteException. 6 public WeatherInfo[] getWeatherInfo() 7 throws RemoteException; 8 } 1. import 1.1 extendsRemote 2. getWeatherInfo 2.1 throwsRemoteException

  18. Implementing the Remote Interface • Define TemperatureServerImpl • Implements Remote interface TemperatureServer • Client interacts with TemperatureServerImpl object • Uses array of WeatherInfo objects to store data • Copy sent to client when calls getWeatherInfo

  19. 37 URL url = new URL( 38 "http://iwin.nws.noaa.gov/iwin/us/traveler.html" ); 18 public class TemperatureServerImpl extends UnicastRemoteObject 19 implements TemperatureServer { 22 public TemperatureServerImpl() throws RemoteException Implementing the Remote Interface • UnicastRemoteObject • Provides functionality for remote objects • Constructor exports object so it can receive remote calls • Wait for client on anonymous port number • Subclass constructors must throw RemoteExceptions • URL object • Contains URL for Traveler's Forecast web page • Throws MalformedURLException

  20. 40 BufferedReader in = 41 new BufferedReader( 42 new InputStreamReader( url.openStream() ) ); Implementing the Remote Interface • Open connection to file specified by URL • Method openStream (class URL) • Opens network connection using Http protocol • If successful, InputStream object returned (else IOException) • InputStreamReader • Translates bytes to Unicode characters • BufferedReader • Buffers characters • Method readLine • Returns one line as a String

  21. 44 String separator = "</PRE><HR> <BR><PRE>"; 47 while ( !in.readLine().startsWith( separator ) ) 48 ; // do nothing 51 String s1 = 52 "CITY WEA HI/LO WEA HI/LO"; 66 inputLine = in.readLine(); // get first city's info Implementing the Remote Interface • Sentinel String to find relevant part of HTML code • readLine until sentinel found • A string used as column head • Second "WEA HI/LO" is for next day, we do not use • Locate column head and get first city's info

  22. 70 WeatherInfo w = new WeatherInfo( 71 inputLine.substring( 0, 16 ), 72 inputLine.substring( 16, 22 ), 73 inputLine.substring( 23, 29 ) ); 75 cityVector.addElement( w ); // add to Vector 84 weatherInformation[ i ] = 85 ( WeatherInfo ) cityVector.elementAt( i ); 88 in.close(); // close connection to NWS server Implementing the Remote Interface • WeatherInfo objects • City name, temperature, description of weather • Method substring to extract data from line • Store all WeatherInfo objects in a Vector • Store data in WeatherInfo array • elementAt returns Object (must be cast) • Close connection

  23. 116 String serverObjectName = "//localhost/TempServer"; Implementing the Remote Interface • Name of server object • Used by clients to connect • //host:port/remoteObjectName • host - computer running registry for remote objects • Where remote object executes • port - port number of registry on host (1099 default) • remoteObjectName - client uses to locate object • Registry managed by rmiregistry (located at host and port) • Remote objects register with it, clients use it to locate service • localhost (same computer) • Same as IP 127.0.0.1

  24. 112 TemperatureServerImpl temp = 113 new TemperatureServerImpl(); 116 String serverObjectName = "//localhost/TempServer"; 117 Naming.rebind( serverObjectName, temp ); Implementing the Remote Interface • static method rebind (class Naming) • Binds object to rmiregistry • Named //localhost/TempServer • Name used by client • rebind replaces any previous objects with same name • Method bind does not

  25. 1 // Fig. 20.1: TemperatureServer.java 2 // TemperatureServer interface definition 3 import java.rmi.*; 4 5 public interface TemperatureServer extends Remote { 6 public WeatherInfo[] getWeatherInfo() Allows objects to be exported. 7 throws RemoteException; 8 } 9 TemperatureServer interface. 10 // Fig. 20.2: TemperatureServerImpl.java Superclass constructor exports objects, and this constructor must be able to throw RemoteException. 11 // TemperatureServerImpl definition 12 import java.rmi.*; 13 import java.rmi.server.*; 14 import java.util.*; 15 import java.io.*; 16 import java.net.*; 17 18public class TemperatureServerImpl extends UnicastRemoteObject 19 implements TemperatureServer { 20 private WeatherInfo weatherInformation[]; 21 22 public TemperatureServerImpl() throws RemoteException 23 { 24 super(); 25 updateWeatherConditions(); 26 } 27 1. Interface ------------------ 1. extends UnicastRemote Object, implements TemperatureServer 1.1 Constructor

  26. 34 "Updating weather information..." ); 35 36 // Traveler's Forecast Web Page URL of web site (URL object). 37 URL url = new URL( 38 "http://iwin.nws.noaa.gov/iwin/us/traveler.html" ); 39 40 BufferedReader in = 41 new BufferedReader( Open connection to file. InputStreamReader formats it to Unicode characters, and BufferedReader buffers the characters. 42 new InputStreamReader( url.openStream() ) ); 43 readLine until separator found. 44 String separator = "</PRE><HR> <BR><PRE>"; 45 46 // locate first horizontal line on Web page 47 while ( !in.readLine().startsWith( separator ) ) 48 ; // do nothing 49 50 // s1 is the day format and s2 is the night format 51 String s1 = 52 "CITY WEA HI/LO WEA HI/LO"; 53 String s2 = 54 "CITY WEA LO/HI WEA LO/HI"; 55 String inputLine = ""; 56 28 // get weather information from NWS 29 private void updateWeatherConditions() 30 throws RemoteException 31 { 32 try { 33 System.err.println( 2. updateWeather Conditions 2.1 URL 2.2 BufferedReader 2.3 readLine

  27. 67 68 while ( !inputLine.equals( "" ) ) { Create WeatherInfo object, add data (substring), add to Vector. Loop until blank line reached. 69 // create WeatherInfo object for city 70 WeatherInfo w = new WeatherInfo( 71 inputLine.substring( 0, 16 ), 72 inputLine.substring( 16, 22 ), 73 inputLine.substring( 23, 29 ) ); 74 Create WeatherInfo array, cast Vector elements. 75 cityVector.addElement( w ); // add to Vector 76 inputLine = in.readLine(); // get next city's info 77 } 78 79 // create array to return to client 80 weatherInformation = 81 new WeatherInfo[ cityVector.size() ]; 82 83 for ( int i = 0; i < weatherInformation.length; i++ ) 84 weatherInformation[ i ] = 85 ( WeatherInfo ) cityVector.elementAt( i ); 86 57 // locate header that begins weather information 58 do { 59 inputLine = in.readLine(); 60 } while ( !inputLine.equals( s1 ) && 61 !inputLine.equals( s2 ) ); 62 63 64 Vector cityVector = new Vector(); 65 66 inputLine = in.readLine(); // get first city's info 2.4 Locate header 2.5 Loop 2.5.1 WeatherInfo 2.5.2 readLine 2.6 WeatherInfo array

  28. 100 // implementation for TemperatureServer interface method 101 public WeatherInfo[] getWeatherInfo() 102 { 103 return weatherInformation; 104 } Return the WeatherInfo array. 105 106 public static void main( String args[] ) throws Exception 107 { 108 System.err.println( 109 "Initializing server: please wait." ); 110 111 // create server object 112 TemperatureServerImpl temp = 113 new TemperatureServerImpl(); 114 87 System.err.println( "Finished Processing Data." ); 88 in.close(); // close connection to NWS server 89 } 90 catch( java.net.ConnectException ce ) { 91 System.err.println( "Connection failed." ); 92 System.exit( 1 ); 93 } 94 catch( Exception e ) { 95 e.printStackTrace(); 96 System.exit( 1 ); 97 } 98 } 99 2.7 close 3. getWeatherInfo 4. main 4.1 temp

  29. Name of server object. rebind binds object to rmiregistry. 115 // bind TemperatureServerImpl object to the rmiregistry 116 String serverObjectName = "//localhost/TempServer"; 117 Naming.rebind( serverObjectName, temp ); 118 System.err.println( 119 "The Temperature Server is up and running." ); 120 } 121 } 4.2 serverObjectName 4.3 rebind

  30. 1 / Fig. 20.3: WeatherInfo.java This allows objects to be passed as a stream of bytes. 2 // WeatherInfo class definition 3 import java.rmi.*; 4 import java.io.Serializable; 5 6public class WeatherInfo implements Serializable { 7 private String cityName; 8 private String temperature; 9 private String description; 10 11 public WeatherInfo( String city, String desc, String temp ) 12 { 13 cityName = city; 14 temperature = temp; 15 description = desc; 16 } 17 18 public String getCityName() { return cityName; } 19 20 public String getTemperature() { return temperature; } 21 22 public String getDescription() { return description; } 23 } 1. Class WeatherInfo implements Serializable 1. Instance variables 1.1 Constructor 2. Get methods

  31. Define the client • Next step • Client code to get weather info from TemperatureServerImpl • Calls getWeatherInfo through RMI • Graphically display weather info • Class WeatherItem (extends JLabel) stores info about each city • Display name, High/low, and image (depending on conditions)

  32. 34 WeatherInfo weatherInfo[] = mytemp.getWeatherInfo(); 22 private void getRemoteTemp( String ip ) 26 String serverObjectName = "//" + ip + "/TempServer"; 30 TemperatureServer mytemp = ( TemperatureServer ) 31 Naming.lookup( serverObjectName ); Define the client • Can specify IP address at command line (more later) • static method lookup (class Naming) • Returns reference to Remote object • Cast to TemperatureServer • Reference may be used as normal • Only difference that copy of array returned

  33. 68 public static void main( String args[] ) 69 { 70 TemperatureClient gt = null; 40 JPanel p = new JPanel(); 74 if ( args.length == 0 ) 75 gt = new TemperatureClient( "localhost" ); 76 else 77 gt = new TemperatureClient( args[ 0 ] ); 50 for ( int i = 0; i < w.length; i++ ) { 51 w[ i ] = new WeatherItem( weatherInfo[ i ] ); 52 p.add( w[ i ] ); 53 } Define the client • Add WeatherItems • Initialize with WeatherInfo • main • Passes command line argument (ip) to constructor • localhost default

  34. 18 static { 19 backgroundImage = new ImageIcon( "images/back.jpg" ); 20 weatherImages = 21 new ImageIcon[ weatherImageNames.length ]; 22 23 for ( int i = 0; i < weatherImageNames.length; ++i ) 24 weatherImages[ i ] = new ImageIcon( 25 "images/" + weatherImageNames[ i ] + ".jpg" ); 26 } Define the client • Class WeatherItem • extends JLabel • static initializer block • For complex initialization of static variables • backgroundImage - ImageIcon, has background • weatherImages - ImageIcon array, holds weather images

  35. 35 weatherInfo = w; 38 for ( int i = 0; i < weatherConditions.length; ++i ) 39 if ( weatherConditions[ i ].equals( 40 weatherInfo.getDescription().trim() ) ) { 41 weather = weatherImages[ i ]; 32 public WeatherItem( WeatherInfo w ) Define the client • Array of descriptions and matching array of images • weatherConditions and weatherImages • Tests WeatherInfo object, loads proper image

  36. 1 // Fig. 20.4: TemperatureClient.java 2 // TemperatureClient definition 3 import java.awt.*; 4 import java.awt.event.*; 5 import javax.swing.*; 6 import java.rmi.*; Use ip specified at command line. 7 Lookup remote object in registry. Returns Remote reference, cast to proper type. 8 public class TemperatureClient extends JFrame 9 { 10 public TemperatureClient( String ip ) 11 { 12 super( "RMI TemperatureClient..." ); 13 getRemoteTemp( ip ); 14 15 setSize( 625, 567 ); 16 setResizable( false ); 17 show(); 18 } 19 20 // obtain weather information from TemperatureServerImpl 21 // remote object 22 private void getRemoteTemp( String ip ) 23 { 24 try { 25 // name of remote server object bound to rmi registry 26 String serverObjectName = "//" + ip + "/TempServer"; 27 28 // lookup TemperatureServerImpl remote object 29 // in rmiregistry 30 TemperatureServer mytemp = ( TemperatureServer ) 31 Naming.lookup( serverObjectName ); 1. import 1.1 Constructor 2. getRemoteTemp 2.1 serverObjectName 2.2 Naming.lookup

  37. 34 WeatherInfo weatherInfo[] = mytemp.getWeatherInfo(); 35 WeatherItem w[] = Call like regular method. 36 new WeatherItem[ weatherInfo.length ]; 37 ImageIcon headerImage = 38 new ImageIcon( "images/header.jpg" ); 39 40 JPanel p = new JPanel(); 41 42 // determine number of rows for the GridLayout; 43 // add 3 to accommodate the two header JLabels 44 // and balance the columns 45 p.setLayout( 46 new GridLayout( ( w.length + 3 ) / 2, 2 ) ); 47 p.add( new JLabel( headerImage ) ); // header 1 48 p.add( new JLabel( headerImage ) ); // header 2 49 50 for ( int i = 0; i < w.length; i++ ) { 51 w[ i ] = new WeatherItem( weatherInfo[ i ] ); 52 p.add( w[ i ] ); 53 } 54 55 getContentPane().add( new JScrollPane( p ), 56 BorderLayout.CENTER ); 57 } 58 catch ( java.rmi.ConnectException ce ) { 59 System.err.println( "Connection to server failed. " + 60 "Server may be temporarily unavailable." ); 61 } 32 33 // get weather information from server 2.3 getWeatherInfo 2.4 GUI 2.4.1 WeatherItem

  38. 67 68 public static void main( String args[] ) 69 { 70 TemperatureClient gt = null; 71 72 // if no sever IP address or host name specified, args[ 0 ] is the first argument, which should be the IP address. 73 // use "localhost"; otherwise use specified host 74 if ( args.length == 0 ) 75 gt = new TemperatureClient( "localhost" ); 76 else 77 gt = new TemperatureClient( args[ 0 ] ); 78 79 gt.addWindowListener( 80 new WindowAdapter() { 81 public void windowClosing( WindowEvent e ) 82 { 83 System.exit( 0 ); 84 } 85 } 86 ); 87 } 88 } 62 catch ( Exception e ) { 63 e.printStackTrace(); 64 System.exit( 1 ); 65 } 66 } 3. main 3.1 args[ 0 ]

  39. 1 // Fig. 20.5: WeatherItem.java 2 // WeatherItem definition 3 import java.awt.*; 4 import javax.swing.*; 5 6 public class WeatherItem extends JLabel { 7 private static ImageIcon weatherImages[], backgroundImage; 8 private final static String weatherConditions[] = 9 { "SUNNY", "PTCLDY", "CLOUDY", "MOCLDY", "TSTRMS", 10 "RAIN", "SNOW", "VRYHOT", "FAIR", "RNSNOW", 11 "SHWRS", "WINDY", "NOINFO", "MISG" }; Use names in weatherImageNames array to load ImageIcons. 12 private final static String weatherImageNames[] = 13 { "sunny", "pcloudy", "mcloudy", "mcloudy", "rain", 14 "rain", "snow", "vryhot", "fair", "rnsnow", 15 "showers", "windy", "noinfo", "noinfo" }; 16 17 // static initializer block to load weather images 18 static { 19 backgroundImage = new ImageIcon( "images/back.jpg" ); 20 weatherImages = 21 new ImageIcon[ weatherImageNames.length ]; 22 23 for ( int i = 0; i < weatherImageNames.length; ++i ) 24 weatherImages[ i ] = new ImageIcon( 25 "images/" + weatherImageNames[ i ] + ".jpg" ); 26 } 27 28 // instance variables 29 private ImageIcon weather; 30 private WeatherInfo weatherInfo; 1. Class WeatherItem 1.1 static variables 1.2 Initializer block 1.3 Load ImageIcons

  40. 34 weather = null; 35 weatherInfo = w; 36 37 // locate image for city's weather condition 38 for ( int i = 0; i < weatherConditions.length; ++i ) Loop though weatherConditions and compare to getDescription. 39 if ( weatherConditions[ i ].equals( 40 weatherInfo.getDescription().trim() ) ) { 41 weather = weatherImages[ i ]; 42 break; 43 } Attach background to WeatherItem. 44 45 // pick the "no info" image if either there is no 46 // weather info or no image for the current 47 // weather condition 48 if ( weather == null ) { 49 weather = weatherImages[ weatherImages.length - 1 ]; 50 System.err.println( "No info for: " + 51 weatherInfo.getDescription() ); 52 } 53 } 54 55 public void paintComponent( Graphics g ) 56 { 57 super.paintComponent( g ); 58 backgroundImage.paintIcon( this, g, 0, 0 ); 59 31 32 public WeatherItem( WeatherInfo w ) 33 { 2. Constructor 2.1 Compare conditions 3. paintComponent 3.1 paintIcon

  41. 67 } 68 69 // make WeatherItem's preferred size the width and height of 70 // the background image Draw city name, high/low, and attach weather image to WeatherItem. 71 public Dimension getPreferredSize() 72 { 73 return new Dimension( backgroundImage.getIconWidth(), 74 backgroundImage.getIconHeight() ); 75 } 76 } 60 Font f = new Font( "SansSerif", Font.BOLD, 12 ); 61 g.setFont( f ); 62 g.setColor( Color.white ); 63 g.drawString( weatherInfo.getCityName(), 10, 19 ); 64 g.drawString( weatherInfo.getTemperature(), 130, 19 ); 65 66 weather.paintIcon( this, g, 253, 1 ); 3.2 drawString 3.3 paintIcon

  42. Compile and Execute the Server and the Client • Build and execute application • All pieces in place • Compile classes with javac • Remote server class (TemperatureServerImpl) compiled with rmic compiler • Makes a stub class - allows client to access remote methods and server to provide its services • Gets remote method calls, passes to RMI system, which performs networking • rmic TemperatureServerImpl

  43. Compile and Execute the Server and the Client • Start rmiregistry • Type rmiregistry at command window • No text in response

  44. Compile and Execute the Server and the Client • Must bind remote server object • Run TemperatureServerImpl application java TemperatureServerImpl • Superclass UnicastRemoteObject • Constructor exports remote object • main binds object to rmiregistry • rmiregistry provides host and port number to clients

  45. Compile and Execute the Server and the Client • Execute TemperatureClient • java TemperatureClient • If server on different machine, specify IP on command line java TemperatureClient 192.168.150.4 • Result on next slide

  46. Program Output

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