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Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary
Middleware
• Layers of Middleware
• Provide a programming model
• Provide transparence
– Location
– Communication protocols
– Computer hardware
– Operating systems
– Programming languages
Distributed programming model
• Remote procedure call (RPC)
– call procedure in separate process
• Remote method invocation (RMI)
– extension of local method invocation in OO model
– invoke the methods of an object of another process
• Event-based model
– Register interested events of other objects
– Receive notification of the events at other objects
Interfaces
• Interface
– Specifies accessible procedures and variables
– Inner alteration won’t affect the user of the
interface
• Interface in distributed system
– Can’t access variables directly
– Input argument and output argument
– Pointers can’t be passed as arguments or returned
results
Interface cases
• RPC’s Service interface
– specification of the procedures of the server, defining the
types of the input and output arguments of each procedure
• RMI’s Remote interface
– Specification of the methods of an object that are available for
objects in other processes, defining the types of them.
– may pass objects or remote object references as arguments or
returned result
• Interface definition languages
– program language, e.g. Java RMI
– Interface definition languages (IDLs), are designed to allow
objects implemented in different languages to invoke one
another.
• e.g. CORBA IDL (n1), DCE IDL and DCOM IDL
CORBA IDL example
struct Person {
string name;
string place;
CORBA has a struct
remote interface
long year;
};
interface PersonList {
remote interface defines
readonly attribute string listname;
methods for RMI
void addPerson(in Person p) ;
void getPerson(in string name, out Person p);
long number();
};
parameters are in, out or inout
• Remote interface:
– specifies the methods of an object available for remote invocation
– an interface definition language (or IDL) is used to specify remote interfaces.
E.g. the above in CORBA IDL.
– Java RMI would have a class for Person, but CORBA has a struct
Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary
Distributed object model
Figure 5.3
local
remote
invocation
A
B
C
E
invocation local
invocation
local
invocation
D
remote
invocation
• each process contains objects, some of which can receive remote
invocations, others only local invocations
• those that can receive remote invocations are called remote objects
• objects need to know the remote object reference of an object in
another process in order to invoke its methods.
• the remote interface specifies which methods can be invoked remotely
F
Invocation semantics
• Local invocations are executed exactly once
• Remote invocations cannot achieve this. Why not?
– the Request-reply protocol can apply fault-tolerance
measures
Fault tolerance measures
Retransmit request
message
Duplicate
filtering
Invocation
semantics
Re-execute procedure
or retransmit reply
No
Not applicable Not applicable
Maybe
Yes
No
Re-execute procedure At-least-once
Yes
Yes
Retransmit reply
At-most-once
Invocation semantics: failure model
• Maybe, At-least-once and At-most-once can suffer from
crash failures when the server containing the remote object
fails.
• Maybe - if no reply, the client does not know if method
was executed or not
– omission failures if the invocation or result message is lost
• At-least-once - the client gets a result (and the method was
executed at least once) or an exception (no result)
– arbitrary failures. If the invocation message is retransmitted, the
remote object may execute the method more than once, possibly
causing wrong values to be stored or returned.
– if idempotent operations are used, arbitrary failures will not occur
• At-most-once - the client gets a result (and the method was
executed exactly once) or an exception (instead of a result,
in which case, the method was executed once or not at all)
The architecture of remote method invocation
server
client
object A proxy for B
Request
skeleton
& dispatcher
for B’s class
remote
object B
Reply
Communication
Remote
reference module
module
Communication Remote reference
module
module
Proxy - makes RMI transparent carries
to client.out
Class
Requestimplements remote interface. Marshals
requests and
reply protocol
unmarshals results. Forwards request.
translates between local and remote
Skeleton - implements methods in remote interface.
object references
andfrom
creates
remote module and
Dispatcher
- gets
request
communication
Unmarshals
requests
and marshals
results. Invokes
object method
references.
Uses(using
remote
object in message).
invokes
in skeleton
methodID
method in remote object.
table
RMI software - between
application level objects
and communication and
remote reference
modules
Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary
RPC is very similar to RMI
•
•
•
•
Service interface: the procedures that are available for remote calling
Invocation semantics choice: at-least-once or at-most-once
Generally implemented over request-reply protocol
Building blocks
–
–
–
–
Communication module
Client stub procedure (as proxy in RMI): marshalling, sending, unmarshalling
Dispatcher: select one of the server stub procedures
Server stub procedure (as skeleton in RMI): unmarshalling, calling, marshalling
client process
server process
Request
client stub
procedure
client
program
Communication
module
Reply
server stub
procedure
Communication
dispatcher
module
service
procedure
Sun RPC case study
• Designed for NFS
– at-least-once semantics
• XDR - Interface definition language
– Interface name: Program number, version number
– Procedure identifier: procedure number
• Rpcgen – generator of RPC components
–
–
–
–
–
client stub procedure
server main procedure
Dispatcher
server stub procedure
marshalling and unmarshalling procedure
Sun RPC case study …continued
• Binding – portmapper
– Server: register ((program number, version number), port
number)
– Client: request port number by (program number, version
number)
• Authentication
– Each request contains the credentials of the user, e.g. uid and
gid of the user
– Access control according to the credential information
Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary
Event-notification model
• Idea
– one object react to a change occurring in another object
• Event examples
– modification of a document
– an electronically tagged book being at a new location
• Publish/subscribe paradigm
– event generator publish the type of events
– event receiver subscribe to the types of events that are interest to them
– When event occur, notify the receiver
• Distributed event-based system – two characteristics
– Heterogeneous: components in a DS that were not designed to interoperate can be
made to work together
– Asynchronous: prevent publishers needing to synchronize with subscribers
Example - dealing room system
• Requirements
– allow dealers to see the latest market price of the
tocks they deal in.
• System components
– Information provider
• receive new trading information
• publish stocks prices event
• stock price update notification
– Dealer process
• subscribe stocks prices event
• System architecture
Architecture for distributed event notification
• Event service: maintain a database of published events and of
subscribers’ interests
• decouple the publishers from the subscribers
Event service
subscriber
object of interest
1.
notification
object of interest
2.
object of interest
3.
notification
observer
subscriber
notification
observer
subscriber
notification
The roles of the participating objects
• The object of interest
– its changes of state might be of interest to other objects
• Event
– An event occurs at an object of interest as the completion of a method execution
• Notification
– an object that contains information about an event
• Subscriber
– an object that has subscribed to some type of events in another object
• Observer objects
– the main purpose is to decouple an object of interest from its subscribers.
– Avoid over-complicating the object of interest.
• Publisher
– an object that declares that it will generate notifications of particular types of
event. May be an object of interest or an observer.
Notification delivery
• Delivery semantics
– Unreliable, e.g. deliver the latest state of a player in a
Internet game
– Reliable, e.g. dealing room
– real-time, e.g. a nuclear power station or a hospital patient
monitor
• Roles for observers
– Forwarding
• send notifications to subscribers on behalf of one or more objects of
interests
– Filtering of notifications according to some predicate
– Patterns of events
– Notification mailboxes
• notification be delayed until subscriber being ready to receive
Jini distributed event specification
• EventGenerator interface
– Provide register method
– Event generator implement it
– Subscriber invoke it to subscribe to the interested events
• RemoteEventListener interface
– Provide notify method
– subscriber implement it
– receive notifications when the notify method is invoked
• RemoteEvent
– a notification that is passed as argument to the notify method
• Third-party agents
– interpose between an object of interest and a subscriber
– equivalent of observer
Chapter 5: Distributed objects and remote invocation
• Introduction
• Communication between distributed
objects
• Remote procedure call
• Events and notifications
• Java RMI case study
• Summary
Java RMI introduction
• Remote object
– Must implement the remote interface
– must handle remote exceptions
• Arguments and return results of remote method
–
–
–
–
–
Must be serializable
All primitive types serializable
remote objects are serializable
File handles are unserializable
Remote objects are passed as remote object reference, nonremote serializable objects are copied and passed by value
• RMIregistry
– access by the Naming class
Example: shared whiteboard
• Remote Interface
• Server program and Client program
• Callbacks
– A server’s action of notifying clients about an event
– Implementation
• Client create a remote object
• Client pass the remote object reference to server
• Whenever an event occurs, server call client via the remote object
– Advantage
• Improve performance by avoid constant polling
• Delivery information in a timely manner
Design and implementation of Java RMI
• Java classes supporting RMI
RemoteObject
RemoteServer
Activatable
UnicastRemoteObject
<servant class>
Chapter 5: Distributed objects and remote invocation
•
•
•
•
•
•
Introduction
Communication between distributed objects
Remote procedure call
Events and notifications
Java RMI case study
Summary
Summary
• Two paradigms for distributed programming
– RMI(RPC)/Event notification: sync./async.
• RMI
– Distributed object model
• Remote interface, remote exception, naming service
– Remote invocation semantics
• Once, at-least-once, at-most-once
– Example: whiteboard based on Java RMI
• Sun RPC
• Event-notification
– Publish/subscribe
– Event service
– Example: dealing room
Middleware layers
Applications
RMI, RPC and events
Request reply protocol
External data representation
Operating System
Middleware
layers
A remote object and its remote interface
remoteobject
Data
remote
interface
{
m1
implementation
m2
m3
of methods
m4
m5
m6
Remote and local method invocations
remote
invocation
local
C
invocation
local
invocation
A
B
local
invocation
D
E
remote
invocation
F
Files interface in Sun XDR
const MAX = 1000;
typedef int FileIdentifier;
typedef int FilePointer;
typedef int Length;
struct Data {
int length;
char buffer[MAX];
};
struct writeargs {
FileIdentifier f;
FilePointer position;
Data data;
};
struct readargs {
FileIdentifier f;
FilePointer position;
Length length;
};
program FILEREADWRITE {
version VERSION {
void WRITE(writeargs)=1;
Data READ(readargs)=2;
}=2;
} = 9999;
1
2
Dealing room system
Dealer’s computer
Dealer
Dealer’s computer
External
source
Notification
Notification
Information
provider
Notification
Notification
Dealer
Notification
Notification
Notification
Dealer’s computer
Dealer’s computer
Notification
Information
provider
Notification
Notification
Dealer
Dealer
External
source
The Naming class of Java RMIregistry
void rebind (String name, Remote obj)
This method is used by a server to register the identifier of a remote object by
name, as shown in Figure 15.13, line 3.
void bind (String name, Remote obj)
This method can alternatively be used by a server to register a remote object
by name, but if the name is already bound to a remote object reference an
exception is thrown.
void unbind (String name, Remote obj)
This method removes a binding.
Remote lookup(String name)
This method is used by clients to look up a remote object by name, as shown
in Figure 15.15 line 1. A remote object reference is returned.
String [] list()
This method returns an array of Strings containing the names bound in the
registry.
Java Remote interfaces Shape and ShapeList
import java.rmi.*;
import java.util.Vector;
public interface Shape extends Remote {
int getVersion() throws RemoteException;
GraphicalObject getAllState() throws RemoteException;
}
1
public interface ShapeList extends Remote {
Shape newShape(GraphicalObject g) throws RemoteException; 2
Vector allShapes() throws RemoteException;
int getVersion() throws RemoteException;
}
Java class ShapeListServant implements interface ShapeList
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
import java.util.Vector;
public class ShapeListServant extends UnicastRemoteObject implements ShapeList {
private Vector theList;
// contains the list of Shapes
1
private int version;
public ShapeListServant()throws RemoteException{...}
public Shape newShape(GraphicalObject g) throws RemoteException {
2
version++;
Shape s = new ShapeServant( g, version);
3
theList.addElement(s);
return s;
}
public Vector allShapes()throws RemoteException{...}
public int getVersion() throws RemoteException { ... }
}
Java class ShapeListServer with main method
import java.rmi.*;
public class ShapeListServer{
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager());
try{
ShapeList aShapeList = new ShapeListServant();
Naming.rebind("Shape List", aShapeList );
System.out.println("ShapeList server ready");
}catch(Exception e) {
System.out.println("ShapeList server main " + e.getMessage());}
}
}
1
2
Java client of ShapeList
import java.rmi.*;
import java.rmi.server.*;
import java.util.Vector;
public class ShapeListClient{
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager());
ShapeList aShapeList = null;
try{
aShapeList = (ShapeList) Naming.lookup("//bruno.ShapeList") ;
Vector sList = aShapeList.allShapes();
} catch(RemoteException e) {System.out.println(e.getMessage());
}catch(Exception e) {System.out.println("Client: " + e.getMessage());}
}
}
1
2
Callback mechanism in the whiteboard system
Client created remote object:
Public interface WhiteboardCallback implements Remote{
void callback(int version) throws RemoteException;
}
Methods added in Shapelist interface:
Int register(WhiteboardCallback callback) throws RemoteException;
Void deregister(int callbackID) throws RemoteException;