3. Remote Procedure Call - EJB Tutorial · 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 2014-2015 http://www.imed.ws

3. Remote Procedure Call

Master II – Software Engineering

Imed BouchrikaDept of Mathematics & Computer Science

University of Souk-Ahras

[email protected]

Imed Bouchrika. Distributed Objects, Uni of Souk-Ahras 2014-2015 http://www.imed.ws

Addition Server using Socketsimport 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(); } }

}

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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);

}

}

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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);

}

}

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Where shall we put the Addition Class ?


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


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)

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Remote Procedure Call : RPCIn 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

Imed Bouchrika. Distributed Objects, Uni of Souk-Ahras 2014-2015 http://www.imed.ws RPC Slide 8

Blocking state

Process A Process B

request

reply

Executing state

Call procedure and wait for reply

Receive request and start process execution

Send reply and wait for next execution

Resume execution

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

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Remote Procedure Call : RPCThe 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.

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MarshallingFor 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

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Marshalling of DataMarshalling 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.

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2-8

Addition Example via RPC


Remote Procedure Call : RPCBenefits 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


Programming with RPCLanguage 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)

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Issues with RPCParameter 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

....

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Issues with RPCDelivery 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.

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RPC Call SemanticsThis 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 crash28


RPC Call SemanticsMaybe 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.

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RPC Call Semanticsat-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)).

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RPC Call Semanticsat-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).

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Examples for RPCJava RMI

CORBA

DCOM

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Imed Bouchrika. Distributed Objects , Uni of Souk-Ahras 2014-2015 http://www.imed.ws

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Master II – Software Engineering

Imed BouchrikaDept of Mathematics & Computer Science

University of Souk-Ahras

[email protected]


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3. Remote Procedure Call - EJB Tutorial · 3. Remote Procedure Call Master II –Software Engineering Imed Bouchrika Dept of Mathematics & Computer Science University of Souk-Ahras