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Distributed Objects
Naim R. El-Far, PhD Candidate
TA for SEG3202 Software Design and Architecture with N. El-Kadri (Summer 2005)
Tutorial 3 of 4 – 10/6/2005
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About this Material
Some of the slides in today’s presentation are based on slides by Dr. J. S. Chase of the Duke University (http://www.cs.duke.edu/~chase/), Dr. J. Ng of the University of Hong Kong (http://www.comp.hkbu.edu.hk/~jng), and public domain slides from Rensselaer Polytechnic Institute (www.rpi.edu/dept/cct/public/j2ee).
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Outline of Today’s Presentation
A look at distributed objects Java Remote Method Invocation (RMI) Java 2 Enterprise Edition (J2EE)
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What’s in a Name?
Distributed Objects:
Software modules (objects) that are designed to work together but reside in multiple computer systems throughout an organization. A program in one machine sends a message to an object in a remote machine to perform some processing. The results are sent back to the local machine.
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“The Network is the Computer”
SomeClass AnotherClass
method call
returned object
computer 1 computer 2
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What exactly do we mean by “objects”? Objects are units of data with the following properties:
– typed and self-contained• Each object is an instance of a type that defines a set of methods
(signatures) that can be invoked to operate on the object.
– encapsulated• The only way to operate on an object is through its methods; the
internal representation/implementation is hidden from view.
– dynamically allocated/destroyed• Objects are created as needed and destroyed when no longer
needed, i.e., they exist outside of any program scope.
– uniquely referenced• Each object is uniquely identified during its existence by a
name/OID/reference/pointer that can be held/passed/stored/shared.
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Why are objects useful?
The properties of objects make them useful as a basis for defining persistence, protection, and distribution.– Objects are self-contained and independent.
• Objects are a useful granularity for persistence, caching, location, replication, and/or access control.
– Objects are self-describing.• Object methods are dynamically bound, so programs can import
and operate on objects found in shared or persistent storage.
– Objects are abstract and encapsulated.• It is easy to control object access by verifying that all clients
invoke the object’s methods through a legal reference.• Invocation is syntactically and semantically independent of an
object’s location or implementation.
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Distributing Objects
“Trick”: Extend the object name space outside of a
process and across a distributed system. Extend the object name space across
secondary storage. Define RPC services as objects
(namespacing, wrapping). Make object references unforgeable and
reject invocation attempts with invalid references
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Issues Vital to Distributed Object Systems1. Can we use distributed objects as a basis for interoperability among
software modules written in different languages?2. Can objects interact across systems with different data formats?3. Can objects interact securely across mutually distrusting nodes
and/or object infrastructures by different vendors?
4. How can we find objects in the presence of node failures?
5. What should we do about pending activities in failed nodes/objects?
6. How can we recover object state after failures?
7. Can we safely execute object invocations from nodes with intermittent connectivity?
8. What about long-term storage of objects, and invocation of stored objects that are not currently active?
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Distributed Object Technologies
1. Remote Method Invocation (RMI)• API and architecture for distributed Java objects
2. Microsoft Component Object Model (COM/DCOM)• binary standard for distributed objects for Windows platforms
3. CORBA (Common Object Request Broker Architecture)
• platform and location transparency for sharing well-defined objects across a distributed computing platform
4. Enterprise Java Beans (EJB)• CORBA-compliant distributed objects for Java, built using RMI
5. Web services and SOAP• protocols and standards used for exchanging data between
applications
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Introduction to Java RMI
Java RMI allowed programmer to execute remote function class using the same semantics as local functions calls.
Local Machine (Client)
SampleServer remoteObject;int s;…
s = remoteObject.sum(1,2);
System.out.println(s);
Remote Machine (Server)
public int sum(int a,int b) { return a + b;}
1,2
3
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The General RMI Architecture
Local/Remote machines
Registry Skeleton Stub
RMI Server
skeleton
stub
RMI Client
Registry
bind
lookupreturn call
Local Machine
Remote Machine
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The Stub and Skeleton
A client invokes a remote method, the call is first forwarded to stub. The stub is responsible for sending the remote call over to the server-side skeleton
The stub opening a socket to the remote server, marshaling (encoding in standard, independent format) the object parameters and forwarding the data stream to the skeleton.
A skeleton contains a method that receives the remote calls, unmarshals the parameters, and invokes the actual remote object implementation.
Stu
b
RMI Client RMI Server
skeleton
return
call
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Steps for Developing an RMI System1. Define the remote interface
2. Develop the remote object by implementing the remote interface
3. Develop the client program
4. Compile the Java source files
5. Generate the client stubs and server skeletons
6. Start the RMI registry
7. Start the remote server objects
8. Run the client
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Step 1: Defining the Remote Interface
To create an RMI application, the first step is the defining of a remote interface between the client and server objects.
/* SampleServer.java */import java.rmi.*;
public interface SampleServer extends Remote{ public int sum(int a,int b) throws RemoteException;}
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Step 2: Develop the Remote Object and its InterfaceThe server is a simple unicast remote server.
Create server by extending java.rmi.server.UnicastRemoteObject.
/* SampleServerImpl.java */
import java.rmi.*;
import java.rmi.server.*;
import java.rmi.registry.*;
public class SampleServerImpl extends UnicastRemoteObject implements SampleServer
{
SampleServerImpl() throws RemoteException
{
super();
}
…
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Step 2: Develop the remote object and its interface
Implement the remote methods
/* SampleServerImpl.java */
…
public int sum(int a,int b) throws RemoteException
{
return a + b;
}
}
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Step 2: Develop the remote object and its interface
/* SampleServerImpl.java */ public static void main(String args[]) { try {
//set the security manager System.setSecurityManager(new RMISecurityManager());
//create a local instance of the object SampleServerImpl Server = new SampleServerImpl();
//put the local instance in the registry Naming.rebind("SAMPLE-SERVER" , Server);
System.out.println("Server waiting....."); } catch (java.net.MalformedURLException me) { System.out.println("Malformed URL: " + me.toString()); } catch (RemoteException re) { System.out.println("Remote exception: " + re.toString()); } }
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Step 3: Develop the client program In order for the client object to invoke methods on the server, it
must first look up the name of server in the registry. You use the java.rmi.Naming class to lookup the server name.
The server name is specified as URL in the form (rmi://host:port/name )
Default RMI port is 1099. The name specified in the URL must exactly match the name
that the server has bound to the registry. In this example, the name is “SAMPLE-SERVER”
The remote method invocation is programmed using the remote interface name (remoteObject) as prefix and the remote method name (sum) as suffix.
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Step 3: Develop the client programimport java.rmi.*;
import java.rmi.server.*;
public class SampleClient
{
public static void main(String[] args)
{
// set the security manager for the client
System.setSecurityManager(new RMISecurityManager());
//get the remote object from the registry
try
{
System.out.println("Security Manager loaded");
String url = "//localhost/SAMPLE-SERVER";
SampleServer remoteObject = (SampleServer)Naming.lookup(url);
System.out.println("Got remote object");
System.out.println(" 1 + 2 = " + remoteObject.sum(1,2) );
}
catch (RemoteException exc) {
System.out.println("Error in lookup: " + exc.toString()); }
catch (java.net.MalformedURLException exc) {
System.out.println("Malformed URL: " + exc.toString()); }
catch (java.rmi.NotBoundException exc) {
System.out.println("NotBound: " + exc.toString());
}
}
}
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Step 4 & 5: Compile the Java source files & Generate the client stubs and server skeletons
Once the interface is completed, you need to generate stubs and skeleton code. The RMI system provides an RMI compiler (rmic) that takes your generated interface class and procedures stub code on its self.
javac SampleServer.java
javac SampleServerImpl.java
rmic SampleServerImpl
javac SampleClient.java
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Step 6: Start the RMI registry
RMI Registry must be started manually. The rmiregistry us uses port 1099 by default. You can also
bind rmiregistry to a different port by indicating the new port number as : rmiregistry <new port>
rmiregistry (Unix)
start rmiregistry (Windows)
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Steps 7 & 8: Start the remote server objects & Run the client Once the Registry is started, start the
server. Set the security policy
java –Djava.security.policy=policy.all SampleServerImpl
java –Djava.security.policy=policy.all SampleClient
More at http://www.neward.net/ted/Papers/JavaPolicy/
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ExternalApplication
Java Technologies
J2EE Framework
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J2EE Container
Component Based Architecture
Applications are built from components– Java class files
– Configuration files (usually XML)
– Data files (html, images, directories,…)
Components are packaged into archives for deployment– jar : Java Application Resource
– war : Web Application Resource
– ear : Enterprise Application Resource
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N-tier application
View application server as a data store Leverage data abstraction
Today
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N-Tier Application
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N-Tier Application
OVER
LDAP
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N-Tier Application
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N-Tier Application
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N-Tier Application
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N-Tier Complexity
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Business Logic
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Business Logic
datadata
PROCESS
OUTPUT
INPUT
datadata
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N-Tier Complexity
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N-Tier Complexity
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N-Tier Complexity
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N-Tier Complexity
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N-Tier Complexity
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N-Tier Complexity
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ServletsJSP/XML/XSLT
SessionBeans
EntityBeans
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Java Technologies
ExternalApplication
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EJB
Enterprise Java Bean
Bean – originally a Java class with get() and set() methods– e.g.: getFirstName(), setFirstName()
EJBs come in 3 flavors:
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EJB – Entity Bean
Entity Bean
Represent actual data items (e.g. rows in a result set)
Two forms of Entity Bean– Container managed persistence:
DB interactions handled by the J2EE environment– Bean managed persistence: requires that the bean carries out DB interactions
itself
May be called across network connection (RMI)
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Session Bean
Model a process or task
Represent resources private to the client
May update shared data
Two forms of Session Bean
– Stateful: state maintained between method calls
– Stateless
One client per session bean instance
EJB – Session Bean
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Used in conjunction with Java Messaging System
Activated on JMS message arrival
No state maintained between activations
APPLICATION
message queue
EJB – Message Driven Bean
*****
*
*session facade
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Conclusion
Applying engineering principles to software
Need Scalability Distribution, modularity, OO, etc
Distributed Objects – the main idea Distributed Object technologies: Java
RMI, J2EE, and next time … .NET and CORBA