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EEC-681/781
Distributed Computing Systems
Lecture 5
Wenbing Zhao
Department of Electrical and Computer Engineering
Cleveland State University
[email protected]
2
Outline
• Case study: Java RMI
– Material taken from
http://java.sun.com/developer/onlineTraining/rmi/RMI.html
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Java RMI Architecture
• Design goal: to create a Java distributed object
model that integrates naturally into the Java
programming language and the local object model
• The definition of a remote service is coded using a Java
interface
• The implementation of the remote service is coded in a
class
• Interfaces define behavior and classes define
implementation
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Java RMI Architecture Layers
• Stub and Skeleton layer: intercepts method calls made
by the client to the interface and redirects these calls to a
remote RMI service
• Remote reference layer: to interpret and manage
references made from clients to the remote service objects
• Transport layer: provides basic connectivity, as well as
some firewall penetration strategies
RMI
System
Client Program
Server Program
Stubs & Skeletons
Stubs & Skeletons
Remote Reference Layer
Remote Reference Layer
Transport Layer
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Stub and Skeleton Layer
• Java RMI follows the Proxy pattern
– The stub class plays the role of the proxy
– The remote service implementation class plays the role
of the RealSubject
<<Interface>>
Subject
Request()
RealSubject
Proxy
Request()
Request()
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Remote Reference Layer
• Defines and supports the invocation
semantics of the RMI connection
– This layer provides a RemoteRef object that
represents the link to the remote service
implementation object
– The stub objects use the invoke()method in
RemoteRef to forward the method call
– The RemoteRef object understands the
invocation semantics for remote services
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Transport Layer
• The Transport Layer makes network connection
between JVMs
• Java Remote Method Protocol (JRMP): Java
RMI’s wire protocol (proprietary) on top of TCP/IP
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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RMI Registry
• RMI Registry (rmiregistry): a simple naming
service that comes with Java RMI
– It runs on each machine that hosts remote service
objects, by default on port 1099
– It accepts registration request only from the local RMI
servers
– It accepts queries for services anywhere
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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RMI Registry
• To access the remote server from client side:
– Query a registry by invoking the lookup()method on
the static Naming class
– The method lookup()accepts a URL that specifies
the server host name and the name of the desired
service
• rmi://<host_name>[:<name_service_port>]/<service_name>
– The method returns a remote reference to the service
object
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
10
Steps to Create a Remote Service
• First create a local object that implements that
service
• Next, export that object to Java RMI. When the
object is exported, Java RMI creates a listening
service that waits for clients to connect and
request the service
• After exporting, register the object in the Java
RMI Registry under a public name
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Using Java RMI
• A working Java RMI system is composed of
several parts:
–
–
–
–
–
Interface definitions for the remote services
Implementations of the remote services
Stub and Skeleton files
A server to host the remote services
An RMI Naming service that allows clients to find the
remote services
– A class file provider (an HTTP or FTP server)
– A client program that needs the remote services
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Steps to Build a System
• Design your system
• Write and compile Java code for interfaces
• Write and compile Java code for implementation
classes
• Generate Stub and Skeleton class files from the
implementation classes
• Write Java code for a remote service host program
• Develop Java code for Java RMI client program
• Install and run Java RMI system
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Service Interface
public interface Calculator extends java.rmi.Remote {
public long add(long a, long b) throws
java.rmi.RemoteException;
public long sub(long a, long b) throws
java.rmi.RemoteException;
public long mul(long a, long b) throws
java.rmi.RemoteException;
public long div(long a, long b) throws
java.rmi.RemoteException;
}
To compile it:
> javac Calculator.java
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Implementation of Remote Service
public class CalculatorImpl
extends java.rmi.server.UnicastRemoteObject
implements Calculator {
// Implementations must have an explicit constructor //
in order to declare the RemoteException exception
public CalculatorImpl() throws
java.rmi.RemoteException {
super();
}
public long add(long a, long b)
throws java.rmi.RemoteException {
return a + b;
}
…
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Stubs and Skeletons
• To generate the stub and skeleton files,
invoke the RMI compiler, rmic
• The compiler runs on the remote service
implementation class file
> rmic CalculatorImpl
• After you run rmic you should find the file
CalculatorImpl_Stub.class
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Host Server
• Remote RMI services must be hosted in a server
process
import java.rmi.Naming;
public class CalculatorServer {
public CalculatorServer() {
try {
Calculator c = new CalculatorImpl();
Naming.rebind("rmi://localhost:1099/CalculatorService", c);
} catch (Exception e) {
System.out.println("Trouble: " + e);
}
}
public static void main(String args[]) {
new CalculatorServer();
}
}
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Client
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.net.MalformedURLException;
import java.rmi.NotBoundException;
public class CalculatorClient {
public static void main(String[] args) {
try {
Calculator c = (Calculator) Naming.lookup(
"rmi://localhost/CalculatorService");
System.out.println( c.sub(4, 3) );
System.out.println( c.add(4, 5) );
System.out.println( c.mul(3, 6) );
System.out.println( c.div(9, 3) );
}
…
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
18
Running the RMI System
• Make sure you change to the directory that
contains the classes you have written
• Launch a terminal, start Java RMI registry
> rmiregistry
• Launch another terminal, start the server
> java CalculatorServer
• Launch the 3rd terminal, start the client
> java CalculatorClient
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Parameter Passing in Java RMI
• When Java RMI calls involve passing
parameters or accepting a return value
– How does RMI transfer these between JVMs?
– What semantics are used?
– Does RMI support pass-by-value or pass-byreference?
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Parameter Passing in Java RMI
• Primitive parameters: pass by value
– For both input parameter and return type,
Java RMI makes a copy of a primitive data
type and send it to the destination
• Object parameters: pass by value
– The object to be passed, together with all the
objects it references, are serialized and
copied over
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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Parameter Passing in Java RMI
• Remote object parameters: pass by
reference
– A client can obtain a reference to a remote
object through the Java RMI Registry program
– A client can obtain a remote reference as a
result of making a remote method call
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
Distributed Garbage Collection in
Java RMI
• Java takes care of memory management
• Distributed garbage collection is needed in Java
RMI
– Many challenges to do so. Most prominent problem is
that a client might quit without notice
– Solution: lease based. The resource is granted to a
client for certain period of time. It is the client’s
responsibility to renew the lease. The resource is
reclaimed if lease is not renewed
Fall Semester 2008
EEC-681: Distributed Computing Systems
Wenbing Zhao
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