3. Remote Procedure Call

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1 3. Remote Procedure Call Master II Software Engineering Imed Bouchrika Dept of Mathematics & Computer Science University of Souk-Ahras Imed Bouchrika. Distributed Objects, Uni of Souk-Ahras

2 Addition Server using Sockets import java.io.*; import java.net.*; class Server implements Runnable { Socket connectionsocket; public Server(Socket s){ try{ System.out.println("Client Got Connected " ); connectionsocket=s; }catch(exception e){e.printstacktrace();} } public void run(){ try{ BufferedReader reader = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); BufferedWriter writer= new BufferedWriter(new OutputStreamWriter(connectionSocket.getOutputStream())); writer.write("*** Welcome to the Calculation Server (Addition Only) ***\r\n"); writer.write("*** Please type in the first number and press Enter : \r\n"); writer.flush(); String data1 = reader.readline().trim(); writer.write("*** Please type in the second number and press Enter : \r\n"); writer.flush(); String data2 = reader.readline().trim(); int num1=integer.parseint(data1); int num2=integer.parseint(data2); int result=num1+num2; System.out.println("Addition operation done " ); writer.write("\r\n=== Result is : "+result); writer.flush(); connectionsocket.close(); }catch(exception e){e.printstacktrace();} } public static void main(string argv[]) throws Exception { System.out.println("Threaded Server is Running " ); ServerSocket mysocket = new ServerSocket(5555); while(true) { Socket sock = mysocket.accept(); Server server=new Server(sock); Thread serverthread=new Thread(server); serverthread.start(); } } } 2

3 Local Invocation of Methods? A simple class Addition with a method to add two numbers. Public class Addition{ public Addition(){ //Constructor } } public int add(int a, int b){ return a+b; } public static void main(string [] args){ Addition local = new Addition(); int result=local.add(9,10); System.out.println( The result is : +result); } 3

4 Local Invocation of Methods? To Invoke the Add method for the Addition Object Public class Hello{ } public static void main(string [] args){ Addition local = new Addition(); int result=local.add(9,10); System.out.println( The result is : +result); } Where shall we put the Addition Class? 4

5 Local Invocation of Methods? The method add is called locally : Within the same machine that has the implementation code. Local Invocation involves: Parameters passing Local variables Return Data. 5

6 Local Invocation of Methods? What if we want to call or invoke the method ADD Remotely ( From a different machine )? From a machine that does not have the implementation code for the add operation. Using Sockets? With Proxies at the both ends of the Server and Client? Would be extremely difficult to maintain for complex and large systems. The solution is to use: Remote Procedure Call (RPC) 6

7 Remote Procedure Call : RPC In 1984, Birrell and Nelson devised a mechanism to allow programs to call procedures on other machines. Process on machine A can call a procedure on machine B. The process on A is suspended and execution continues on B. When B returns, the return value is passed to A and A continues execution. This mechanism is called Remote Procedure Call (RPC). To the programmer, the goal is that it should appear as if a normal procedure call is taking place. 7

8 Remote Procedure Call : RPC Call procedure and wait for reply Process A request Process B Receive request and start process execution Resume execution Blocking state reply Send reply and wait for next execution Executing state RPC Slide 8

9 Remote Procedure Call : RPC Remote Invocation Remote Procedure Call ( RPC ) Is a way of communication in which a computer program can invoke a subroutine or function To execute on a remote machine and returns the results to the invoker if any. The programmer usually write the same code as for the local case whilst all the complexity is hidden and managed by the middleware 9

10 Remote Procedure Call : RPC The RPC mechanism in short : Create stub ( Proxy, Broker...) functions to make it appear to the user that the call is local Stub function contains ONLY the function s interface The core of the functionalities or Business logic, still resides on the server side. 10

11 Marshalling For simple communication, we can send a string of commands : plain formatted ASCII text. Example : GET HTTP/1.0 /index.html If the application is complex and object oriented? Problem with structures of data or objects We need to push the objects through the connection pipe in a format that the connection can handle. The solution is called : Marshalling 11

12 Marshalling of Data Marshalling is a process that converts a complex object into a byte of stream and then converts it back into an object using the reverse process. In Java, This is implemented using Serialization For RPC Marshalling is needed for passing methods arguments as well as returned data through the network stream and vice versa. 12

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23 Addition Example via RPC 2-8

24 Remote Procedure Call : RPC Benefits of RPC Writing applications is simplified RPC hides all network code into stub functions Application programmers don t have to worry about details Sockets, port numbers, byte ordering 24

25 Programming with RPC Language support Most programming languages (C, C++, Java,...) have no concept of remote procedure calls Language compilers will not generate client and server stubs Common solution: Use a separate compiler to generate stubs (pre-compiler) In Java, we use rmic for generating stubs(not true Now) 25

26 Issues with RPC Parameter passing Pass by value Easy: just copy data to network message Pass by reference Makes no sense without shared memory Representing data Remote machine may have: Different byte ordering Different sizes of integers and other types Different floating point representations Different character sets... 26

27 Issues with RPC Delivery guarantees Retry request message: whether to retransmit the request message until either a reply or the server is assumed to have failed; Duplicate filtering : when retransmission are used, whether to filter out duplicates at the server; Retransmission of replies: whether to keep a history of reply messages to enable lost replies to be retransmitted without re-executing the server operations. 27

28 RPC Call Semantics This ensure the Remote Procedure is called exactly once if the server does not crash during execution of the Remote Procedure. When the server crashes during the Remote Procedure execution, the partial execution may lead to erroneous results. In this case, we want the effect that the Remote Procedure has not been executed at all. We consider usually the following errors: Lost request message Lost reply message Server crash Client crash 28

29 RPC Call Semantics Maybe Call Semantics After a RPC time-out (or a client crashed and restarted), the client is not sure if the Remote Procedure may or may not have been called. This is the case when no fault tolerance is built into RPC mechanism. maybe semantics is not desirable. 29

30 RPC Call Semantics at-least-once Call Semantics With this call semantics, the client can assume that the Remote Procedure is executed at least once (on return from the Remote Procedure). Can be implemented by retransmission of the (call) request message on time-out. Acceptable only if the server s operations are idempotent. That is : f(x) = f(f(x)). 30

31 RPC Call Semantics at-most-once Call Semantics When a RPC returns, it can be assumed that the remote procedure has been called exactly once or not at all. Implemented by the server's filtering of duplicate requests and caching of replies (in reply history). 31

32 Examples for RPC Java RMI CORBA DCOM 32

33 User Interface for Chat Master II Software Engineering Imed Bouchrika Dept of Mathematics & Computer Science University of Souk-Ahras Imed Bouchrika. Distributed Objects, Uni of Souk-Ahras

34 Java Code 34

1. Distributed Systems

1. Distributed Systems Master II Software Engineering Dr. Imed Bouchrika Dept of Mathematics & Computer Science University of Souk-Ahras imed@imed.ws Imed Bouchrika. Distributed Objects, Uni of Souk-Ahras