1. Chapter 5: Distributed objects and remote invocation
From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design
Edition 3, © Addison-Wesley 2001
Presentation based on slides by Coulouris et al;
modified by Jens B Jorgensen and Jonas Thomsen,
University of Aarhus

2. Distributed objects – middleware
Applications
Middleware
layers
Request reply protocol
External data representation
Operating System
RMI, RPC and events

3. Distributed objects – interfaces
Programs = communicating modules
Module interface specifies:
Available procedures (and variables).
Parameters (input, output, both).
Hiding of implementation details.
Allows implementation to be replaced without client knowledge
Ensures encapsulation (only referring to object data through methods)
Service interfaces (RPC), remote interfaces (RMI).
Interface Definition Language: A notation for defining interfaces.

4. Distributed objects – 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(); };

5. Distributed objects – basic model
invocation
remote
local A B C D E F
Local method invocation
Remote method invocation
Remote object
Remote object reference

6. Distributed objects – remote objects and remote interfaces
Data
implementation
object {
of methods

7. Remote method invocation – invocation semantics
Local invocations executed exactly once.
RMI invocation semantics:
Maybe (operations without answer).
At-least once (good for idempotent operations).
At-most once (used in Java RMI and CORBA).
RMI invocation semantics ensured by:
Maybe: No ensuring
At-least once: Retries
At-most once: Retries, history of replies

8. Remote method invocation – invocation semantics techniques
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

9. Remote method invocation – architectural model
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

10. Remote method invocation – communication module
Request
Server
Client
doOperation
(wait)
(continuation)
Reply
message
getRequest
execute
method
select object
sendReply
messageType
requestId
objectReference
methodId
arguments
int (0=Request, 1= Reply)
int
RemoteObjectRef
int or Method
array of bytes

11. Remote method invocation – remote reference module
Translates between local and remote object references.
Has a remote object table with entries for all remote objects held by the process, and a record for each local proxy.
Creates remote object references.

12. Remote method invocation – RMI software
Layer between application-level objects and the communication and remote reference modules.
Consists of:
Proxy.
Dispatcher.
Skeleton.
Classes for RMI software can be generated automatically by an interface compiler.

13. Remote method invocation – proxy
Makes remote method invocation transparent to clients.
Behaves like a local object to the invoker.
Implements the methods in the remote interface of the remote object it represents.
Does not execute invocation, but forwards request to remote object.
Marshals a reference to the target object, its own methodId and its arguments into a request message;
sends the request message to the target (via communication module);
awaits the reply message;
unmarshals the reply;
returns the result to the invoker;

14. Remote method invocation – dispatcher and skeleton
Receives request message from communication module;
Uses methodId to select the appropriate method in the skeleton;
Passes on the request;
Skeleton:
Unmarshals the arguments in the request message;
Invokes the corresponding method in the remote object;
Waits for result;
Marshals result;
Sends reply to sender’s proxy (via communication module);

15. Remote method invocation – location and activation
Binder
Maps textual names to remote object references.
Location service
Maps remote object references to remote objects
Active/passive objects
Active object are available in memory
Passive objects has been stored on disk and removed from memory
Clients only see remote object. Activation is transparent.

16. Remote method invocation – garbage collection
Local:
Objects are freed when no more references exist
Distributed:
Reference counting:
When a proxy is created, client sends addRef to server
When a proxy is freed, client sends removeRef to server
When remote objects holders is empty local garbage collection disposes it
If a client crashes, a removeRef is missing  remote object not being freed; solution…
Leases
Clients must renew remote object references periodically.
If not renewed, remote object might be freed.
Used in Java RMI.

17. Events and notifications – basics
One object can react to a change occurring in another object.
Publish-subscribe communication pattern.
Applicable in heterogeneous environments.
Asynchronous: Publishers and subscribers are decoupled.

18. Events and notifications – dealing room system
Dealer’s computer
Information
provider
Dealer
External
source
Notification

19. Events and notifications – delivery semantics
IP Multicast
suitable in applications where a missing update is acceptable; online games.
Reliable Multicast
All clients must receive all messages; dealing room example.
Real-time Reliable Multicast
Nuclear power plant events.

20. Events and notifications – observer roles
Forwarding
Object of interest sends all notifications to the forwarding observer, which handles communication with subscribers.
Filtering of notifications
To limit the number of event an subscriber receives, a filtering observer might filter events on behalf of the subscriber.
Patterns of events
Subscribers can specify patterns of interest. An observer does the pattern matching on behalf of the object of interest.
Notification mailboxes
When a subscriber is unable to receive notifications it can use a notification mailbox, which stores notification until it is ready for receiving them.

21. Remote method invocation (RMI). Events and notifications.
Summary
Distributed objects.
Remote method invocation (RMI).
Events and notifications.