Teaching material based on Distributed Systems Concepts and

Teaching material based on Distributed Systems: Concepts and Design, Edition 3, Addison-Wesley 2001. Copyright © George Coulouris, Jean Dollimore, Tim Kindberg 2001 email: authors@cdk 2. net This material is made available for private study and for direct use by individual teachers. It may not be included in any product or employed in any service without the written permission of the authors. Viewing: These slides must be viewed in slide show mode. Distributed Systems Course Topics in distributed objects Ideas needed to understand CORBA Revise 1. 4 heterogeneity 4. 3 external data representation 5. 4 distributed objects & RMI 5. 5 Java RMI case study

Heterogeneity Applies to all of the following: – networks w Internet protocols mask the differences between networks – computer hardware w e. g. data types such as integers can be represented differently – operating systems w e. g. the API to IP differs from one OS to another – programming languages w data structures (arrays, records) can be represented differently – implementations by different developers w they need agreed standards so as to be able to interwork Middleware provides a programming abstraction and masks the heterogeneity of networks etc. • 2

Middleware programming models Distributed objects and remote object invocation is the model explained in Chapter 5 – illustrated by Java RMI CORBA is in Chapter 17. We will study it shortly. – it provides remote object invocation between a client program written in one language and a server program written in another language – our book uses Java CORBA to illustrate the use of CORBA – another language commonly used in CORBA is C++ Other programming models – remote event notification – remote SQL access – distributed transaction processing • 3

External data representation This was presented in Chapter 4. It masks the differences due to different computer hardware. CORBA CDR – only defined in CORBA 2. 0 in 1998, before that, each implementation of CORBA had an external data representation, but they could not generally work with one another. That is: w the heterogeneity of hardware was masked w but not the heterogeneity due to different programmers (until CORBA 2) – CORBA CDR represents simple and constructed data types (sequence, string, array, struct, enum and union) w note that it does not deal with objects – it requires an IDL specification of data to be serialised Java object serialisation – represents both objects and primitive data values – it uses reflection to serialise and deserialise objects– it does not need an IDL specification of the objects • 4

CORBA IDL example struct Person { Figure 5. 2 string name; CORBA has a struct string place; remote interface long year; }; interface Person. List { remote interface defines readonly attribute string listname; methods for RMI void add. Person(in Person p) ; void get. Person(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 • 5

Distributed object model Figure 5. 3 local remote invocation A B C local E invocation local invocation D remote invocation F 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 need to know the remote object reference of an object in another process in order to invoke its methods. How do they get it? the remote interface specifies which methods can be invoked remotely • 6

Invocation semantics Local invocations are executed exactly once Remote invocations cannot achieve this. Why not? – the Request-reply protocol can apply fault-tolerance measures Figure 5. 5 Fault tolerance measures Retransmit request message Duplicate filtering Invocation semantics Re-execute procedure or retransmit reply No Not applicable Yes No Re-execute procedure At-least-once Yes Retransmit reply At-most-once 7 Maybe •

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) • 8

The architecture of remote method invocation Figure 5. 6 server client object A proxy for B skeleton & dispatcher for B’s class Request remote object B Reply Communication Remote reference module Communication module Proxy - makes RMI transparent to client. implements carries. Class out Requestremote interface. Marshals requests andprotocol unmarshals reply results. Forwards request. translates local and remote objectinterface. Skeleton - between implements methods in remote references creates remote object Unmarshals requests and marshals results. Invokes Dispatcher - and gets request from communication module and references. Uses remote method in remote object invokes in skeleton (usingtable method. ID in message). 9 Remote reference module RMI software - between application level objects and communication and remote reference modules •

Representation of a remote object reference 32 bits Internet address port number 32 bits time 32 bits object number Figure 4. 10 interface of remote object a remote object reference must be unique in the distributed system and over time. It should not be reused after the object is deleted. Why not? the first two fields locate the object unless migration or re-activation in a new process can happen the fourth field identifies the object within the process its interface tells the receiver what methods it has (e. g. class Method) a remote object reference is created by a remote reference module when a reference is passed as argument or result to another process – it will be stored in the corresponding proxy – it will be passed in request messages to identify the remote object whose method is to be invoked • 10

Shared whiteboard example (p 194) In the RMI and CORBA case studies, we use a shared whiteboard as an example – this is a distributed program that allows a group of users to share a common view of a drawing surface containing graphical objects, each of which has been drawn by one of the users. The server maintains the current state of a drawing and it provides operations for clients to: – add a shape, retrieve a shape or retrieve all the shapes, – retrieve its version number or the version number of a shape • 11

Java Remote interfaces Shape and Shape. List • Note the interfaces and arguments • Graphical. Object is a class that implements Serializable. Figure 5. 11 import java. rmi. *; import java. util. Vector; public interface Shape extends Remote { int get. Version() throws Remote. Exception; Graphical. Object get. All. State() throws Remote. Exception; } public interface Shape. List extends Remote { Shape new. Shape(Graphical. Object g) throws Remote. Exception; Vector all. Shapes() throws Remote. Exception; int get. Version() throws Remote. Exception; • } 12

Java class Shape. List. Server with main method Probably skip this one import java. rmi. *; Figure 5. 13 public class Shape. List. Server{ public static void main(String args[]){ System. set. Security. Manager(new RMISecurity. Manager()); try{ Shape. List a. Shape. List = new Shape. List. Servant(); Naming. rebind("Shape List", a. Shape. List ); System. out. println("Shape. List server ready"); }catch(Exception e) { System. out. println("Shape. List server main " + e. get. Message()); } } } 13 1 2

Java class Shape. List. Servant implements interface Shape. List Probably skip this one import java. rmi. *; Figure 5. 14 import java. rmi. server. Unicast. Remote. Object; import java. util. Vector; public class Shape. List. Servant extends Unicast. Remote. Object implements Shape. List { private Vector the. List; // contains the list of Shapes 1 private int version; public Shape. List. Servant()throws Remote. Exception{. . . } public Shape new. Shape(Graphical. Object g) throws Remote. Exception { 2 version++; Shape s = new Shape. Servant( g, version); 3 the. List. add. Element(s); return s; } public Vector all. Shapes()throws Remote. Exception{. . . } public int get. Version() throws Remote. Exception {. . . } } 14

Java client of Shape. List Probably skip this one Figure 5. 15 import java. rmi. *; import java. rmi. server. *; import java. util. Vector; public class Shape. List. Client{ public static void main(String args[]){ System. set. Security. Manager(new RMISecurity. Manager()); Shape. List a. Shape. List = null; try{ a. Shape. List = (Shape. List) Naming. lookup("//bruno. Shape. List") ; Vector s. List = a. Shape. List. all. Shapes(); } catch(Remote. Exception e) {System. out. println(e. get. Message()); }catch(Exception e) {System. out. println("Client: " + e. get. Message()); } } } 15 1 2

Summary Heterogeneity is an important challenge to designers : – Distributed systems must be constructed from a variety of different networks, operating systems, computer hardware and programming languages. w The Internet communication protocols mask the difference in networks and middleware can deal with the other differences. External data representation and marshalling – CORBA marshals data for use by recipients that have prior knowledge of the types of its components. It uses an IDL specification of the data types – Java serializes data to include information about the types of its contents, allowing the recipient to reconstruct it. It uses reflection to do this. RMI – each object has a (global) remote object reference and a remote interface that specifies which of its operations can be invoked remotely. – local method invocations provide exactly-once semantics; the best RMI can guarantee is at-most-once – Middleware components (proxies, skeletons and dispatchers) hide details of marshalling, message passing and object location from programmers. • 16