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Creating a Distributed System with RMI. B.Ramamurthy. Remote Method Invocation. Remote Method Invocation (RMI) is Java’s implementation of object-to-object communication among Java objects to realize a distributed computing model.

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remote method invocation
Remote Method Invocation
  • Remote Method Invocation (RMI) is Java’s implementation of object-to-object communication among Java objects to realize a distributed computing model.
  • RMI allows us to distribute our objects on various machines, and invoke methods on the objects located on remote sites.

B.Ramamurthy

rmi based distributed system

5.

3.

3.

2.

XYZ

Implementation

XYZ

Client

Stub

Stub

1.

uses

implements

XYZ

interface

Client Host

Server Host

RMI-based Distributed System

4.

B.Ramamurthy

steps in rmi based application
Steps in RMI-based Application

1. Design the interface for the service.

2. Implement the methods specified in the interface.

3. Generate the stub and the skeleton.

4. Register the service by name and location.

5. Use the service in an application.

B.Ramamurthy

compile and register commands
Compile and Register Commands

rmiregistry

Finds object by name

Stores object by name

rmic

Compile

5.

3.

3.

2.

XYZ

Client

Stub

XYZ

Implementation

Stub

1.

uses

implements

XYZ

interface

Client Host

Server Host

B.Ramamurthy

more details
More Details
  • Once the object (or service) is registered, a client can look up that service.
  • A client (application) receives a reference that allows the client to use the service (call the method).
  • Syntax of calling is identical to a call to a method of another object in the same program.

B.Ramamurthy

parameter marshalling
Parameter Marshalling
  • Transfer of parameters (or marshalling) is done by the RMI.
  • Complex objects are streamed using Serialization.
  • RMI model of networking for distributed system involves only Java.
  • No need to learn IDL or any other language.

B.Ramamurthy

case study
Case Study
  • Take a look at:
  • http://download.oracle.com/javase/6/docs/technotes/guides/rmi/hello/hello-world.html

B.Ramamurthy

defining remote interface
Defining Remote Interface

import java.rmi.*;

// the interface extends Remote interface

// any class implementing Remote can be accessed remotely security permitting

public interface XYZ extends Remote

{ // specify methods that can be called remotely

// each method “throws RemoteException”

}

B.Ramamurthy

remoteexception
RemoteException
  • Any time you depend on outside entities there is a potential for problems in communication, networking, server crash etc.
  • Any exception due to these should be handled by the services.
  • This feature imparts robustness to the application.
  • Java mandates this feature for any RMI service.

B.Ramamurthy

implementing the remote interface
Implementing the Remote Interface

import java.rmi.*;

import java.rmi.server.*;

import java.net.*;

// others as needed

XYZImpl

implements TemperatureServer {

  • The main method instantiates an object for the service, and registers it with rmiregistry.

B.Ramamurthy

server object name
Server Object Name
  • Syntax for the server object name is:

//host:port/remoteObjectName

  • Default port number for rmiregistry is 1099
  • For local host the object name:

//localhost/XYZServer

For a remote host

//127.0.0.1/XYZServer

B.Ramamurthy

name binding
Name Binding
  • rebind method binds a server’s object name to the object’s name as it is in the registry.
  • Clients use the name in the registry.
  • There is also a bind() method.
  • But rebind is better since it binds the most recently registered object.

B.Ramamurthy

the client
The Client

import java.rmi.*;

// import other packages

  • constructor takes care of lookup of remote object and access.

B.Ramamurthy

client details
Client Details
  • The name of the server object along with the IP of the remote location is used in Naming class’s lookup method to get an object reference.
  • This object reference is then used for remote method calls.
  • Observe that there is no difference between the local and remote call.

B.Ramamurthy

preparing the application
Preparing the Application

1. Compile all the class using javac.

2. Then start the registry (this will be running as a daemon)

rmiregistry &

B.Ramamurthy

preparing the application17
Preparing the Application

4. Run the server which will register with the RMI registry.

Java XYZServerImpl &

5. Run the client.

Java XYZClient parameters&

B.Ramamurthy

inside rmi
Inside RMI
  • http://download.oracle.com/javase/6/docs/
  • Basic RMI classes: /usr/java1.1/src/java/rmi
    • java.rmi.registry.*
    • java.rmi.Naming class (static/class methods)
    • java.rmi.Remote interface (marker interface)
    • java.rmi.server.*
    • Default RMI port 1099
    • Both lookup from local and remote are acceptable.

B.Ramamurthy

implementation of rmi 5 2 5
AccessException.java

RemoteException.java

AlreadyBoundException.java

ConnectException.java

ServerException.java

ConnectIOException.java

ServerRuntimeException.java

MarshalException.java

StubNotFoundException.java

UnexpectedException.jav

ServerError.java

UnknownHostException.java

NoSuchObjectException.java

UnmarshalException.java

NotBoundException.java

RMISecurityException.java

RMISecurityManager.java

Remote.java

MarshalledObject.java

Naming.java

activation

dgc

Registry

server

Implementation of RMI (5.2.5)

B.Ramamurthy

the role of proxy and skeleton stub in remote method invocation
The role of proxy and skeleton (stub) in remote method invocation

server

client

remote

skeleton

object B

object A

proxy for B

Request

& dispatcher

for B’s class

Reply

servant

Communication

Communication

Remote reference

Remote

module

reference module

module

module

Object A invokes a remote object in Object B for which it holds a remote object

reference.

“System Model”

B.Ramamurthy

rmi internals communication module
RMI Internals: Communication Module
  • Carries out request-reply protocol;
  • On the client side {message type, message id, remote reference to object} are gathered and sent out. At most once invocation semantics;
  • On the server side, it gets local reference for remote reference from remote reference module, invokes a dispatcher with this reference.

B.Ramamurthy

rmi internals remote reference module
RMI Internals: Remote Reference module
  • Responsible for translating between local and remote object references and for creating remote object references.
  • A remote object table has a mapping between local and remote references. A table at server (entry for object ref for B) and a table at client (entry for object ref for proxy B).

B.Ramamurthy

rmi internals remote references
RMI Internals: Remote References
  • Action of remote reference module: See RemoteRef.java interface
    • When a remote object is to be passed as argument or result for the first time, the remote ref is asked to create a remote ref object which is added to the table.
    • When a remote object reference arrives in a request or reply, the remote ref module is asked for corresponding local object ref, which may either a proxy or remote object. If it is not in the table RMI runtime creates it and asks remote ref module to add it to the table.

B.Ramamurthy

rmi software
RMI software
  • Layer of software between application level objects and communication and remote reference modules: “Middleware”
  • Proxy: provides remote access transparency. One proxy for every remote object in the client.
  • Dispatcher: A server has one dispatcher and RemoteStub for each class representing a remote object.
    • It receives request message from comm. module
    • It used MessageId to select appropriate method in RemoteStub.
    • RemoteStub and dispatcher use same MessageId.
  • RemoteStub: A class of remote object has a that stands for the remote interface. All the access dependencies are hidden in this class. A remote object has a servant that directly implements the methods. Java 5 creates this dynamically.
  • Binder: binds textual names to remote object references. RMIRegistry is a binder; Naming class; see fig.5.13 We will discuss this next.
  • Server Threads: one thread per invocation
  • Distributed garbage collection: See Andrew Birell’s paper [1995].

B.Ramamurthy

reference
Reference
  • Java.rmi The remote method invocation guide, by E. Pitt and K. Mcniff, Addison Wesley, ISBN: 0-201-700043-3, 2001

B.Ramamurthy

simple rmi system model
Simple RMI System Model

Host M/C

JVM

Registry

Client M/C

server

JVM

server

ClientApp

Remote interface

B.Ramamurthy

registry singleton per system during production
Registry (singleton – per system - during production)

Name

Mortgage

Tempertur

Reference

B.Ramamurthy

semantics of remote method invocation
Semantics of remote method invocation
  • A remote method has to be declared to throw a remote exception.
  • Clients of a remote method should try, catch and deal with remote exceptions.
  • Arguments of object type to the remote method are passed by deep copy and not by reference
  • Any results of object type returned by deep copy, not by reference
  • Any exception (object) thrown is also returned as deep copy
  • Java.lang.Object is specialized for remote objects and remote references
  • “At most once” semantics: idempotent: very important for transactional systems
  • Remote objects are subject to distributed garbage collection via reference counting, before local garbage collection is applied.
  • To be accessible, a remote object must implement “remote” interface (or the interface marked as Remote by the marker interface) and be exported to the RMI system.
  • Execution of remote object is asynchronous but synchronicity is realized using the underlying network.

B.Ramamurthy

serialization
Serialization
  • Serialization is a the operation of encoding objects into stream of bytes: bytification.
  • De-serialization is taking the bytes and constructing objects out of the byte stream.
  • Is used to marshal and unmarshal arguments and results.
  • If serialization is not addressed your distributed system will not work: whether it be your mobile systems or land networked system. A stumbling block for many who are new to DS.

B.Ramamurthy

serialization contd
Serialization (contd.)
  • In order for an object to be passed around in a DS it must implement Serializable interface (another marker interface: Remote was the first one we saw)
  • For performance reasons you may want to specify the serialization version (though not required) serialVersionUID

B.Ramamurthy

serialization contd not uml diagram
Serialization (contd.) (not UML diagram)

A

A

A

A

B

B

B

C

C

C

A

Incorrectly de-serialized

B

Once an object is serialized,

serialized object is reused

C1

C2

B.Ramamurthy

reference32
Reference
  • http://download.oracle.com/javase/6/docs/technotes/guides/rmi/index.html

B.Ramamurthy

rmi package registry
RMI package: Registry
  • Naming is class that provided lookup, bind, unbind, rebind and list for remote object references: higher level
  • Registry for a Registry interface: with the above methods at a lower level: you have to locate the registry object and then invoke the methods

rmi://host:port/name

Fully qualified identification/reference for a remote object.

B.Ramamurthy

registry
Registry
  • You can create a registry
  • Start a registry
  • Get registry location: LocateRegistry class
  • Then do binding, lookup etc.

B.Ramamurthy

unicastremoteobject
UnicastRemoteObject
  • UnicastRemoteObject is a base for your remote object that provides simple transient point-point RMI servers.
  • There are other features that we have not covered such as security model using policy files that is in the demo that we posted in handout#1

B.Ramamurthy

rmi internals use of reflection
RMI Internals: Use of Reflection
  • What is reflection? See Reflection package
  • Reflection enables Java code to discover information about the fields, methods and constructors of loaded classes, and
  • To use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.
  • http://download.oracle.com/javase/tutorial/reflect/index.html
  • Reflection feature allowed for dynamic creation of stubs in Java 2 version onwards.
  • Read more about reflection model in computing.

B.Ramamurthy

rmi internals distributed garbage collection
RMI Internals: Distributed Garbage Collection
  • Based on reference counts.
  • Local garbage collectors and a distributed support.
  • Each server holds the list of processes that hold remote object references: for example, B.Holders
  • When a client C first receives a remote reference to a particular remote object, say B, it makes a addRef(B) invocation to server of that remote object and then creates proxy; server adds C to B.Holders.
  • When client C’s garbage collector finds that proxy is no longer reachable (ref count), it makes a removeRef(B) invocation to server and then deletes proxy; the server removes C from B.Holders.
  • When B.Holders is empty, server’s local garbage collector will reclaim the space occupied B unless there are any local holders.
  • These extra calls for updates occur during proxy creation and deletion and do not affect normal opertion.
  • Tolerates communication failures: addRef() and removeRef() are idempotent: effects of N > 0 identical requests is the same as for a single request.
  • If addRef() fails with an exception, proxy is not created, removeRef() is transmitted; removeRef() failures are dealt with by “leases” (Jini kind).

B.Ramamurthy

a little bit of reflection
A Little bit of Reflection
  • Method class, invoke method
  • Invoke method requires two parameters: first the object to receive invocation, second an array of Object parameters.
  • Invoke executes the method on the object and returns result as Object.
  • Method M;
  • Object result = M.invoke(String, Args);

B.Ramamurthy

using reflection in rmi
Using Reflection in RMI
  • Proxy has to marshal info. about a method and its arguments into a request message.
  • For a method it marshals an object of class Method into the request. It then adds an array of objects for the method’s arguments.
  • The dispatcher unmarshals the Method object and its arguments from request message.
  • The remote object reference is obtained from remote ref. table.
  • The dispatcher then calls the “invoke” method on the object reference and array of arguments values.
  • After the method execution the dispatcher marshals the result or any exceptions into the reply message.

B.Ramamurthy

summary
Summary
  • We discussed designing a distributed system using RMI
  • We also looked at RMI internal
  • We also learned about marker interface, distributed garbage collection, object marshalling, registry, server-port binding, Naming class of RMI, …

B.Ramamurthy