Changing the way of designing distributed applications Communication oriented design: FIRST design the communication protocol for the distributed system and then the program is developed accordingly Application oriented design: Develop the application as if everything were locally and then divide the application in modules which will run in remote machines The first approach is harder to design but simpler to implement The second one is better when there is a framework supporting the implementation of such

Alternative Technologies Development of networks => development of distributed systems Development of middleware (libraries, tools, etc..) supporting easy programming of distributed systems Based on TCP/IP protocols Higher level programming language for distributed systems The distributed case is not a problem anymore

Remote Procedure Calls Motivation: NFS development (SUN) A client application can call a procedure (function) in a server application running on another computer as it were locally implemented Handle over parameters, receiving results in an appropiate format (integer, string, float,..) eXternal Data Representation

Remote Procedure Calls The client stops until the procedure call returns Call(parameters) Receive results RPC Server RPC Client Server framework: provided by the system

The Interface File Specifies the protocol of the remote procedure: name, required parameters (number and type), result (type). It is called the interface file because it holds the information the client needs to use Uses Interface for compiling Implements interface RPC Server RPC Client Interface definition file

Remote Objects This paradigm was rapidly replaced by the remote object paradigm An application can invoke a method of an object located in another JVM The interface file remains the key to describe the object protocol to the clients RemoteObjectServer Invoke method Receive result

Files Involved Obtain reference to remote object Apply method as usually Receive results as usually Client program Server program Define a particular class for implementing remote objects and implements the interface Create a remote-able object Make it publicly available Define the methods (only the header) Which will be able to called remotely implements Use for compiling Interface

An Example: Remote Date Server The only method will be Date getDate(), the server will answer with the local Date Remote Server getDate() Tue Jun 12 17:20:24

The interface file import java.rmi.*; import java.util.Date; public interface RemoteDate extends Remote { public Date getDate() throws RemoteException; } It must import java.rmi.* It must extend Remote Every declared method should throw a RemoteException

Define a class for implementing remote date objects The remote Object Class definition Remote Interface RemoteObject Implements Remote Extends DateServer.java

The Client Program import java.rmi.*; import java.rmi.server.*; import java.util.Date; public class DateClient { public static void main( String args[] ) { try { RemoteDate dateobj = (RemoteDate)Naming.lookup( "rmi://"+args[0]+"/DateServer" ); Date datum = dateobj.getDate(); System.out.println( “Server Date : " + datum ); } catch ( Exception e ){ System.out.println(e); } }

The Stub and Skel Files Communication is implemented by the stub and skel files They are generated by compiling the class files of the remote object implementation with the rmic command Client Stub Remote Object Server Skel

The name registry server A server to register and make public remote objects Server register the object by this server, clients ask this server for a reference to a certain object It should run at the server‘s computer and have access (like any java program) to the stub file. It must be started before the remote object is registered Client Remote Object Server rmiregistry

Which file where ? A client needs the stub file and the interface file The server needs the Stub & Skel file Client Remote Object Server DateClient.class RemoteDate.class DateServer_stub.class DateServer.class RemoteDate.class DateServer_stub.class DateServer_skel.class

How to generate stub & skel The compiled implementation class should be compiled (again) with the rmic command DateClient.java RemoteDate.java DateClient.class DateServer.class DateServer_stub.class DateServer_skel.class DateServer.java RemoteDate.class javac rmic

Starting rmiregistry from program It is possible to start a registry server from the program A number server will illustrate this and the concurrency problem Remote Number Server Client getNumber() 1 2 3

The RMI-based Chat The client must also receive messages from the server It also implements a remote object, which is passed as parameter !!! The server does not need to locate the client Remote Object Client Remote Object Server addClient(this) sendMessage(text) newMessage(text)

A Remote file server Access to files Client -open file read/write -close file -Read line -Write line What happens if: -open a file which does not exists -Read a line from a file not opened -Other exceptions

Automatic distribution (stub) The stub can be distributed automatically but then the code needs to include a security policy statement A security policy file must be provided, which must be specified in the command line When starting the server, a URL for retrieving the stub file must be specified java –Djava.security.policy=policy.txt -Djava.rmi.server.codebase= ClientProgram See examples PideNumero ReparteNumeros

Automatic distribution (stub) The downloading of the stub class is made via URL A “web-server” must be running at the server side We can use a small server to server only class files for this purpose ClassFileServer (extends ClassServer) Steps : –Download RFSClient.cass, RFSInterface.class, policy.txt –Server start ClassFileServer port path –Server start Remote Object server –Client contacts server (with policy and codebase parameters

Activation of the server Sometimes it is not convenient to have all server objects running waiting for a client RMI provides an activation schema for remote object servers There is only one “server” running (the rmideamon), who will “wake up” the server when a client requests a service It is necessary then to write and run a set-up program, which will register the “sleeping” server with the rmid For the client, there is no difference

Steps for writing an activable server Rewrite the server so it extends the activable class´instead of RemoteUnicastObject import java.rmi.activation.*; public class MyRemoteClass extends Activable implement MyInterface Remove the original constructor and write one which receives two parameters: public MyRemoteClass(ActivationID id, MarshalledObject data) throws RemoteException { super(id, 0); } Compile with javac and rmic Write and compile the set-up program which will register the activable class to the rmi daemon See under kapitel 8/ rmid

Steps for running this Start the ClassFileServer and rmiregistry Start the rmid rmid –J-Djava.security.policy=policy.txt Run the Setup program Java -Djava.security.policy=policy.txt – Djava.rmi.server.codebase= Setup Run the client Java -Djava.security.policy=policy.txt – Djava.rmi.server.codebase= client Look where the output of the server program is given The code of the client does not change. Activation is strictly a server-side implementation decision.