Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Library (computing) wikipedia , lookup
Name mangling wikipedia , lookup
Java (programming language) wikipedia , lookup
Design Patterns wikipedia , lookup
Object-oriented programming wikipedia , lookup
Java performance wikipedia , lookup
Class (computer programming) wikipedia , lookup
Distributed Data Management Architecture wikipedia , lookup
Application Interface Specification wikipedia , lookup
Team Foundation Server wikipedia , lookup
C Sharp syntax wikipedia , lookup
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 RPC Client RPC Server Call(parameters) Receive results 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 RPC Client Interface definition file RPC Server Uses Interface for compiling Implements interface 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 Interface Define the methods (only the header) Which will be able to called remotely implements Use for compiling • • • Obtain reference to remote object Apply method as usually Receive results as usually Client program • • • Define a particular class for implementing remote objects and implements the interface Create a remote-able object Make it publicly available Server program An Example: Remote Date Server • The only method will be Date getDate(), the server will answer with the local Date Remote Server getDate() 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 Remote Extends RemoteObject Remote Interface Implements Extends The remote Object Class definition 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 rmiregistry Client Remote Object Server Which file where ? • A client needs the stub file and the interface file • The server needs the Stub & Skel file Client DateClient.class RemoteDate.class DateServer_stub.class Remote Object Server 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 RemoteDate.java DateServer.java DateClient.java javac javac javac RemoteDate.class DateServer.class rmic DateClient.class DateServer_skel.class DateServer_stub.class 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 getNumber() 1 getNumber() Client 3 2 Client getNumber() Client 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 addClient(this) Remote Object Client sendMessage(text) newMessage(text) Remote Object Server A Remote file server Access to files -open file read/write -close file -Read line -Write line Client 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=http://hostname/filelocation 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=http://hostname:port/ Setup • Run the client Java -Djava.security.policy=policy.txt – Djava.rmi.server.codebase=http://hostname:port/ 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.