Communication between distributed objects Remote procedure call Chapter 5: Distributed objects and remote invocation Introduction Communication between distributed objects Remote procedure call Events and notifications Java RMI case study Summary

Provide a programming model Provide transparence 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

Interface in distributed system 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

RPC’s Service interface Interface cases RPC’s Service interface specification of the procedures of the server 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 may pass objects or remote object references as arguments or returned result Interface definition languages program language, e.g. Java RMI Interface definition language (IDL), e.g. CORBA IDL, DCE IDL and DCOM IDL

Communication between distributed objects Remote procedure call Chapter 5: Distributed objects and remote invocation Introduction Communication between distributed objects Remote procedure call Events and notifications Java RMI case study Summary

Discuss RMI under following headings The object model Distributed objects The distributed object model Design issues semantics of remote invocations Implementation RMI above the request-reply protocol Distributed garbage collections

The object model Object references Interfaces Actions Objects can be accessed via object references First-class values Interfaces A definition of the signatures of a set of methods No constructers A class can implement several interfaces, e.g. Java Actions Initiated by an object invoking a method in another object Two affects Change the state of the receiver Further invocations on methods in other objects

The object model … continued Exceptions mechanism A clean way to deal with error conditions List exceptions at the method head throw user know exceptions Catch exceptions Garbage collection Freeing the space occupied by cancelled objects C++: collected by programmers Java: collected by JVM

Benefits of distributed objects Natural extension physical distribution of objects into different processes or computers in a distributed system Benefits of distributed objects Enforce encapsulation can’t access variables directly Support heterogeneous systems Local object cache Assume other architectural models Replicated objects Migrated objects

The distributed objects model Remote object reference An unique identifier in a distributed system May be passed as arguments and results of remote method invocation Remote interface remote object class implements the methods of its remote interface Actions in a distributed system may incur a chain of invocations on different computers Garbage collection Usually based on reference counting Exception notify the client and the client handle exceptions

Design Issues – Invocation semantics Choices for different delivery guarantees retry request message duplicate filtering retransmission of results Three different semantics Fault tolerance measures Invocation semantics Retransmit request message Duplicate filtering Re-execute procedure or retransmit reply No Yes Not applicable Retransmit reply At-most-once At-least-once Maybe

Different invocation semantics Maybe For invoker: executed once, or not at all ??? Suffer from: (1) message lost; (2) server crash Useful for app. in which occasional failed invocation are acceptable At least once For invoker: execute at least once, or an exception Suffer from: (1) server crash; (2) arbitrary failures for non-idempotent method At most once For invoker: receives result, or an exception Prevent: omission failures by retrying, arbitrary failures

Design Issues - Transparency What can be made transparent marshal message passing object locating and contacting What can’t be made transparent vulnerable to failure latency Current consensus transparent in syntax different in expression

Remote reference module Implementation of RMI The inner scene of RMI Communication module Request/reply between client and server Select dispatcher at server side Remote reference module Translate between local and remote object reference Create remote object reference Remote object table entries for remote objects held by the process entries for local proxies

Implementation of RMI – RMI software Proxy forward invocation to remote object one remote object one proxy Skeleton implement the method in the remote interface unmarshal the arguments in the request invoke the corresponding method in the remote object wait for the invocation complete marshal the result in the reply message Dispatcher select appropriate method in the skeleton one dispatcher and skeleton for one remote object

Implementation of RMI - execution The classes for proxies, dispatchers and skeletons generated automatically by an interface compiler, e.g. rmic Server program create and initialize at least one of the remote objects register Client program look up the remote object references invoke The binder Maintain mapping information of textual names to remote object references

Implementation of RMI - Object state Activation of remote objects to avoid resource waste, the servers can be started whenever they are needed a remote object could be active or passive Persistent object stores Persistent object an object that is guaranteed to live between activations of processes different passivate strategies at the end of a transaction when the program exit E.g., Persistent Java, PerDiS

Distributed garbage collection The aim of a distributed garbage collector Retain the object (local&remote) when it is still be referenced Collect the object when none holds reference to it Java distributed garbage collection algorithm based on reference counting server maintain processes set that hold remote object references to it client notify server to modify the process set when the process set becomes empty, server local garbage collector reclaims the space Leases in Jini lease: the granting of the use of a resource for a period of time avoid to discover whether the resource users are still interested or their programs have not exited

Communication between distributed objects Remote procedure call 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 Request Reply Communication module dispatcher service client stub server stub procedure client process server process program

XDR - Interface definition language 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

Communication between distributed objects Remote procedure call 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 asynchronous

Example - dealing room system Requirements allow dealers to see the latest market price information 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 subscriber observer object of interest Event service 3. 1. 2. notification

The roles of the participating objects The object of interest its changes of state might be of interest to other objects Event 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 Publisher an object that declares that it will generate notifications of particular types of event

Notification delivery Delivery semantics Unreliable Reliable real-time Roles for observers Forwarding send notifications to subscribers on behalf of one or more objects of interests Filtering of notifications 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

Communication between distributed objects Remote procedure call Chapter 5: Distributed objects and remote invocation Introduction Communication between distributed objects Remote procedure call Events and notifications Java RMI case study Summary

Arguments and return results of remote method 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, non-remote 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

Java classes supporting RMI Design and implementation of Java RMI Java classes supporting RMI RemoteServer UnicastRemoteObject <servant class> Activatable RemoteObject

Communication between distributed objects Remote procedure call 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 Sun RPC 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 Middleware Request reply protocol External data representation Operating System RMI, RPC and events

CORBA IDL example // In file Person.idl struct Person { string name; string place; long year; } ; interface PersonList { readonly attribute string listname; void addPerson(in Person p) ; void getPerson(in string name, out Person p); long number(); };

{ A remote object and its remote interface object Data remote implementation object { of methods

Remote and local method invocations B C D E F

The role of proxy and skeleton in remote method invocation object A object B skeleton Request proxy for B Reply Communication Remote Remote reference module reference module for B’s class & dispatcher remote client server

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 { Length length; program FILEREADWRITE { version VERSION { void WRITE(writeargs)=1; 1 Data READ(readargs)=2; 2 }=2; } = 9999;

Dealing room system Dealer’s computer Information provider Dealer External source Notification

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;

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(); 1 Naming.rebind("Shape List", aShapeList ); 2 System.out.println("ShapeList server ready"); }catch(Exception e) { System.out.println("ShapeList server main " + e.getMessage());} }

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") ; 1 Vector sList = aShapeList.allShapes(); 2 } catch(RemoteException e) {System.out.println(e.getMessage()); }catch(Exception e) {System.out.println("Client: " + e.getMessage());} }

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;