Changing the way of designing distributed applications

1. 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 a schema

2. 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

3. 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 appropriate format (integer, string, float,..) • eXternal Data Representation, serialization

4. 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

5. 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 Implements interface Uses Interface for compiling

6. 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

7. 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 • Define a particular class for implementing remote objects and implements the interface • Create a remote-able object • Make it publicly available Server program Client program

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

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

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

11. 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); } } }

12. 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 Remote Object Server Client Stub Skel

13. 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

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

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

16. 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 Server2 getNumber() 1 3 2 getNumber() getNumber() Client Client Client

17. What do I get when calling a remote object ? • A reference to the remote object • If the object has references to other not remote objects, the callers gets a COPY of these objects • If the remote object has a reference to other remote object, the caller gets a reference to these objects • See ListServer, ListClient, List Interface and MyList

18. 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 addClient(this) Remote Object Server sendMessage(String) refreshList(String[]) newMessage(String)

19. 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

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

21. 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

22. 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

23. 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 !.

24. Homework • Implement an unifyed RMI-based FFS and Directory Server based on the interfaces shown before • The remote object shuold implement the following methods: • byte[] read(FileId, i, n) : attempts to read up to n bytes from a file starting from the byte in the position i. • boolean write(FileId, i, Datos):writes a data sequence starting from the position i into the specified file • FileId create() : creates a new file (empty) and returns its FileId • boolean delete(FileId) : deletes the file • FileId lookup(Dir, File) : localises the name of the file in the directory UFID • boolean addName(Dir, Name, File) : If Name was not in the directory, the pair(Name,File) is added modifying the corresponding file • boolean unName(Dir, Name) : the pair (Name, file) is deleted from the directory • String[] getNames(Dir) : returns the list of names in the directory • It is up to you how to implement the FileId • For trying your system implement