SUMMARY INTRODUCTION REMOTE METHOD INVOCATION

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SUMMARY REMOTE METHOD INVOCATION PROGRAMMAZIONE CONCORRENTE E DISTR. Università degli Studi di Padova Dipartimento di Matematica Corso di Laurea in Informatica, A.A. 2015 2016 rcardin@math.unipd.

applications to call object methods located remotely A server program creates some remote objects A client program obtains a remote reference and then invokes methods on them Distributed object

1 SUMMARY REMOTE METHOD INVOCATION PROGRAMMAZIONE CONCORRENTE E DISTR. Università degli Studi di Padova Dipartimento di Matematica Corso di Laurea in Informatica, A.A rcardin@math.unipd.it Introduction Remote interfaces Dynamic code loading Serialization Security manager Exporting remote objects Compiling Running Distributed garbage collection 2 INTRODUCTION INTRODUCTION RMI allows applications to call object methods located remotely A server program creates some remote objects A client program obtains a remote reference and then invokes methods on them Distributed object application Sharing resources, processing load across systems RMI provides mechanism by which the server and the client communicate Using low level communication (i.e. sockets) is harder and less performing Serialization has a cost and its difficult to share some state Distributed object applications need to do the following Locate remote objects Application can register its remote objects with RMI's simple naming facility, the RMI registry Communicate with remote objects Details of communication between remote objects are handled by RMI Load class definitions for objects that are passed around RMI provides mechanisms for loading an object's class definitions as well as for transmitting an object's data 3 4

INTRODUCTION CREATING DISTRIBUTED APPLICATIONS Client side 3 Server side Developing a distributed application using RMI involves these general steps: Designing and implementing the components 5 6 1

Starting the application Registry, server, client,.

2 INTRODUCTION CREATING DISTRIBUTED APPLICATIONS Client side 3 Server side Developing a distributed application using RMI involves these general steps: Designing and implementing the components Defining the remote interfaces Implementing the remote objects Implementing the clients Compiling sources 4 2 Making classes network accessible Typically a web server is used for this purpose Starting the application Registry, server, client, CREATING DISTRIBUTED APPLICATIONS REMOTE INTERFACES Distributed application are made up of remote interfaces and classes Objects with methods that can be invoked across Java virtual machines are called remote objects. A remote interfaces has the following characteristics Extends the interface java.rmi.remote Each method of the interface declares java.rmi.remoteexception in its throws clause Remote objects are treated like an object reference A local representation (remote stub) is sent to other JVMs Implementation of the Remote proxy pattern Any other object is copied before being sent 7 8

REMOTE INTERFACES REMOTE INTERFACES Each interface defines a specific protocol It s important to clearly separate methods interfaces from their implementation // The interface implements java.rmi.

remoteexception public interface Compute extends Remote { <T> T executetask(task<t> t) throws RemoteException; } Methods of an interface that extends Remote can be invoked from another JVM Remote

3 REMOTE INTERFACES REMOTE INTERFACES Each interface defines a specific protocol It s important to clearly separate methods interfaces from their implementation // The interface implements java.rmi.remote and its unique method // throws a java.rmi.remoteexception public interface Compute extends Remote { <T> T executetask(task<t> t) throws RemoteException; } Methods of an interface that extends Remote can be invoked from another JVM Remote methods MUST throw a RemoteException A checked exception used to indicate either a communication failure or a protocol error Remote interfaces are defined server side A remote interface must be implemented The implementing class can declare more methods, but these will not be seen by the clients public class ComputeEngine implements Compute { // Implement at least remote methods } Parameters of remote methods follow these rules: Remote objects are passed by reference. Changes made remotely are reflected in the original remote object A stub is a proxy to the remote object that communicates with the original remote object Local objects are passed by copy, using serialization Changes are not propagated back in any way 9 10 REMOTE INTERFACES DYNAMIC CODE LOADING Interfaces that are not marked as remote must be serializable Type that are not marked as remote are passed by copy, using serialization Use java.io.serializable has a marked interface // This is not a remote intefaces, but it is used in the protocol // defined by a remote method. It s implementation must implement // also the Serializable interface public interface Task<T> { T execute(); } The implementation will be dinamically downloaded by RMI into the host JVM This is possibile beacuse all objects are written in Java 11 12

SERIALIZATION SECURITY MANAGER Representation of an object as a sequence of bytes, that includes data as well as type The process is JVM independent Serialization process uses the stream API

If a field is not, it must be marked as transient Transient fields are not serialized and are lost intentionally A transient variable cannot be final or static Serialization is used by RMI, EJB and

4 SERIALIZATION SECURITY MANAGER Representation of an object as a sequence of bytes, that includes data as well as type The process is JVM independent Serialization process uses the stream API ObjectInputStream and ObjectOutputStream types Deserialization is the opposite process The type must implement java.io.serializable All fields in the class must be serializable. If a field is not, it must be marked as transient Transient fields are not serialized and are lost intentionally A transient variable cannot be final or static Serialization is used by RMI, EJB and so on Defines the outer boundaries of the sandbox for downloaded (untrusted) code It descends from class java.lang.securitymanager Security policies specify actions that are unsafe Accept or open a socket from/to a host and port Modify a thread (change its priority,...) Interact with file system Interact with system properties Exit an application For every unsafe action there is a corresponding checkxxx method Actions not allowed throw a SecurityException SECURITY MANAGER SECURITY MANAGER RMI programs must install a security manager Otherwise RMI will not download classes if (System.getSecurityManager() == null) { System.setSecurityManager(new SecurityManager()); } Only one security manager can be installed By default, an application has no security manager installed Policies are specified using *.policy files grant codebase "file:/home/ann/src/" { permission java.security.allpermission; }; Server and client application must specify their policy file Default file: java.home/lib/security/java.policy Use -Djava.security.policy property specify a file 15 16

EXPORTING REMOTE OBJECTS EXPORTING REMOTE OBJECTS Remote objects need to be exported to the RMI runtime Exporting means create stub objects that expose the remote methods to clients Stub is the

exportObject(engine, 0); The stub implements the remote interface It is a remote proxy to the remote object 2 arg to exportobject is the listening port for incoming connection 17 18 EXPORTING REMOTE

5 EXPORTING REMOTE OBJECTS EXPORTING REMOTE OBJECTS Remote objects need to be exported to the RMI runtime Exporting means create stub objects that expose the remote methods to clients Stub is the client placeholder for the remote object Compute engine = new ComputeEngine(); // Listening to port 0 means to listen on every port Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0); The stub implements the remote interface It is a remote proxy to the remote object 2 arg to exportobject is the listening port for incoming connection EXPORTING REMOTE OBJECTS EXPORTING REMOTE OBJECTS RMI Registry: a remote map for other remote objects, indexed by names The server register a remote object (stub), the client retrieve it using the same name The java.rmi.naming type provides methods for storing and obtaining references to remote objects // Name is a string that represents an URL without the scheme // //host:port/name Naming.bind(name, object) Available methods are bind, unbind and rebind The name argument specifies the host and port of the registry The object represents the remote object (stub) to expose to clients RMI Registry The creation and lookup of a new Registry can be done using the type LocateRegistry A registry is a remote object binded to a fixed name This type offers also an API to interact with the registry // Retrieves the default registry listening on port 1099 Registry registry = LocateRegistry.getRegistry(); // Asks the registry to bind a stub to a name registry.rebind(name, stub); For security reasons, an application can bind an object with a registry running on the same host Using the lookup method, a client can retrieve a stub // The name was the same used to bind the object Compute comp = (Compute) registry.lookup(name); 19 20

EXPORTING REMOTE OBJECTS COMPILING 2 A client looks up from the RMI registry a stub binded to a name Compilation of an RMI program requires a series of different steps Create an archive (JAR) with

6 EXPORTING REMOTE OBJECTS COMPILING 2 A client looks up from the RMI registry a stub binded to a name Compilation of an RMI program requires a series of different steps Create an archive (JAR) with remote interfaces This archive will be shared across server and clients The above jar needs to be shared with the registry javac compute/compute.java compute/task.java jar cvf compute.jar compute/*.class 3 The client calls remote methods, passing serializable type as arguments 1 A server bind a stub to a specific name in a RMI registry Definitions that can be automatically downloaded have to be placed in a public folder For example, use an HTTP web server for this purpose Build server classes javac -cp c:\home\ann\public_html\classes\compute.jar engine\computeengine.java COMPILING RUNNING Compilation of an RMI program requires a series of different steps Build client classes The Task interface needs to be public accessible The client needs also the jar of remote interfaces javac -cp /home/jones/public_html/classes/compute.jar client/computepi.java client/pi.java cp client/pi.class /home/jones/public_html/classes/client Defining security policies for server and clients Create a *.policy file for each actor, specifying which actions are allowed to be performed by the downloaded code grant codebase "file:/home/ann/src/" { permission java.security.allpermission; }; Starting the server components Start the RMI registry, given with the JDK platform start rmiregistry 1099 :: By default the registry run on port 1099 Start the server program Use java.rmi.server.codebase to specify the location of remote interfaces archive Use java.rmi.server.hostname to specify the host for stubs exported from the server JVM By default RMI uses the the IP address of the host Use java.security.policy to specify the policy file java -cp c:\home\ann\src;c:\classes\compute.jar -Djava.rmi.server.codebase= file:/c:/classes/compute.jar -Djava.rmi.server.hostname=mycomputer.example.com -Djava.security.policy=server.policy engine.computeengine 23 24

RUNNING RUNNING Starting the client component Start the client program Use java.rmi.server.codebase to specify the location of client s interfaces implementation Use java.security.

$c:\home\jones\src;c:\classes\compute.jar -Djava.rmi.server.codebase=file:/c:/classes/ -Djava.security.policy=client.policy client.computepi mycomputer.example.$ all live references within each JVM The first reference to a remote object sends a referenced message to the server for that object.

all live references within each JVM The first reference to a remote object sends a referenced message to the server for that object.

7 RUNNING RUNNING Starting the client component Start the client program Use java.rmi.server.codebase to specify the location of client s interfaces implementation Use java.security.policy to specify the policy file The host name of the server can be supplied as an argument of the program The registry search the stub implementation into the web server java -cp c:\home\jones\src;c:\classes\compute.jar -Djava.rmi.server.codebase=file:/c:/classes/ -Djava.security.policy=client.policy client.computepi mycomputer.example.com 45 The implementation of client interfaces are loaded by the server from the public codebase RUNNING DISTRIBUTED GARBAGE COLLECTION RMI uses a reference counting algorithm Keeps track of all live references within each JVM The first reference to a remote object sends a referenced message to the server for that object. Reference count is incremented As live references are found to be unreferenced in local JVM, the count is decremented. When the last reference has been discarded, an unreferenced message is sent When a remote object is not referenced by any client, the RMI runtime refers to it using a weak reference Due to connection delays, it is possible that premature collection of remote object will occur A reference to this object will thrown a RemoteException 27 28

EXAMPLES https://github.com/rcardin/pcd snippets REFERENCES The Java Tutorials: RMI https://docs.oracle.com/javase/tutorial/rmi/ The Java Tutorials: Security Manager https://docs.oracle.com/javase/tutorial/essential/environment/sec urity.

8 EXAMPLES snippets REFERENCES The Java Tutorials: RMI The Java Tutorials: Security Manager urity.html Java security: How to install the security manager and customize your security policy java/javasecurity how to install the security manager and customize yoursecurity policy.html Default Policy Implementation and Policy File Syntax olicyfiles.html REFERENCES Java RMI : What is the role of the stub skeleton that are generated by the rmic compiler rmi what isthe role of the stub skeleton that are generated by the rmic Java Serialization Garbage Collection of Remote Objects 31

A Typical RMI Application

A Typical RMI Application Client and Server run on different machines Remote Object(s) registered in rmiregistry by Server Remote Object(s) look d up by Client When necessary, code transferred from web