1. II. Middleware for Distributed Systems
Outline
Principles of Object-Oriented Middleware
CORBA, COM and Java/RMI
Resolving Heterogeneity
Dynamic Object Requests
CORBA, COM & Java/RMI

2. CORBA, COM and Java/RMI Outline CORBA COM Java/RMI

3. Object Management Architecture
Vertical domain-specific interfaces
Non-standardized application-specific interfaces
Horizontal facility interfaces
Domain Interfaces
Application Interfaces
CORBA facilities
Object Request Broker
CORBA services
General service interfaces
CORBA, COM & Java/RMI

4. Object Management Architecture
CORBA Services
foundational services for use by developers of implementation objects
some published services:
Naming
Events
Transactions
Security
Trading
Common Facilities
object services provide functionality for use by objects, CORBA facilities provide standards for services used by applications
generic functionality needed by many applications (e.g., printing, document management, , etc.)
Domain Interfaces
provide domain-specific objects for vertical application domains
Examples: Finance, Healthcare, Manufacturing, Telecom, Electronic Commerce, Transportation
CORBA, COM & Java/RMI

5. Overview of CORBA Objects
CORBA objects differ from typical programming language objects:
CORBA objects can be located anywhere on a network.
CORBA objects (like Java objects) can run on any platform.
CORBA objects can be written in any of several languages.
CORBA object references are persistent.
CORBA object developers need know nothing of where their clients will be, what hardware or OS they will run on, or what language they will be written in.
CORBA objects approach universal accessibility.
CORBA, COM & Java/RMI

6. Overview of CORBA Objects
A client of an object has access to an object reference for the object, and invokes operations on the object.
A client knows only the logical structure of the object according to its interface and experiences the behavior of the object through invocations.
Client code has no knowledge of the implementation of the object or which ORB is used to access the implementation.
An object implementation provides the semantics of the object, usually by defining data for the object instance and code for the object's methods.
CORBA, COM & Java/RMI

7. Overview of CORBA Object Types
CORBA object model is statically typed
Object types have a unique name
Modules are used to provide scope
Nested modules
Object types are defined in IDL by interface
Interface contains operation, attribute, type and exception
Atomic type: boolean, char, short, long, float, string
Complex types can be constructed from objects and atomic types
Instance of complex types are complex values that do not have identity and cannot be referenced
CORBA, COM & Java/RMI

8. OMG IDL OMG Interface Definition Language (IDL): IDL Structure
mappings for many languages/compilers;
independent of any particular language/compiler;
multiple-inheritance, public interface-structured specification language;
not for implementation.
primary support for interoperability between static and dynamic requests mechanisms.
IDL Structure
Module
a namespace
Interface
abstract type
multiple inheritance
Struct
structured data
Module auction {
exception NotAllowed {};
struct Sale {
int price;
string item; };
interface Auction {
void bid (in long price)
raises NotAllowed; }
CORBA, COM & Java/RMI

9. Object Implementation Side Internet InterORB Protocol (IIOP)
Role of OMG IDL
Client Side
Object Implementation Side C
COBOL
C++ C
I D L
I D L
I D L
I D L
COBOL
Ada
I D L
ORB
I D L
ORB
I D L
Internet InterORB Protocol (IIOP)
I D L
Ada
Small
talk
I D L
I D L
I D L
I D L
Small
talk
C++
C++
JAVA
JAVA
CORBA, COM & Java/RMI

10. Overview of CORBA Object Attributes
Attributes make state information of server objects accessible to client objects
Attributes are treated as a pair of operations (get and set)
All attributes that are declared in the interface are accessible to clients
Read-only attributes
Constants
Read-only for client objects
Read-only for server objects
Scoping rules
Define, then use
Forward declarations
interface Player;
typedef sequence<Player> PlayerList
interface Trainer;
typedef sequence<Trainer> TrainerList;
interface Team {
readonly attribute string name;
attribute TrainerList coached_by;
attribute Club belongs_to;
attribute PlayerList players; … };
(**** there is a mistake here ****)
CORBA, COM & Java/RMI

11. Overview of CORBA Object Operations
CORBA operations specify the services that clients can request from server objects
Operation name
Return type
List of parameters In
Out
Inout
List of exceptions
Accessible to client objects (no private, or protected operations)
No overloading
interface Team { …
string print();
void bookGoalies(in Date d)…; };
CORBA, COM & Java/RMI

12. CORBA
CORBA, COM & Java/RMI

13. Object Implementation
A Request
A request consists of:
Target object (target object identified by a unique object reference)
Operation
Parameters (the input, output and in-out parameters defined for the operation; may be specified individually or as a list)
Optional request context
Results (the result values returned by the operation)
Client
Object Implementation
Client Proxy
(stub code)
Skeleton
code
Request
ORB
CORBA, COM & Java/RMI

14. Failure Handling
Exceptions are used to inform client object about their request failures
About 25 system failures
User defined exceptions
exception PlayerBooked{};
interface Team { …
void bookGoalies(in Date d) raises (PlayerBooked); };
CORBA, COM & Java/RMI

15. Subtypes and Multiple Inheritance
CORBA object model support (multiple) inheritance
Public inheritance (inherit all)
Virtual inheritance (operations are dynamically bound)
Object is the root object
Conflict resolved at IDL compile time
interface Organization{
readonly attribute string name; }
interface Club: Organization {
exception NotInClub();
attribute TeamList teams;
readonly attribute short noOfMembers;
void transfer (in Player p) raises (NotInClub); };
CORBA, COM & Java/RMI

16. Polymorphism
CORBA object model support restricted form of polymorphism
Inheritance relationship
Redefinition only on operations, not on attributes
CORBA, COM & Java/RMI