Download CORBA introduction

CORBA, DCOM and Java
V. “Juggy” Jagannathan
CareFlow|Net, Inc.
CORBA
Distributed Object Infrastructure
References
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Robert Orfali, Dan Harkey, “Client/Server
Programming with Java and CORBA, Second
Edition.” John Wiley and Sons, 1998.
Andreas Vogel and Keith Duddy, “Java
Programming with CORBA, Second Edition,
Advanced Techniques for Building Distributed
Applications,” John Wiles and Sons, 1998.
John Siegal (written and edited by), “CORBA
Fundamentals and Programming,” John Wiley
and Sons, 1996.
Outline
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Overview
CORBA 1.1, 2.0, 3.0
IDL and Interface Repository
CORBA services
Background*
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Not-for-profit company based in United
States, with representation in Italy, United
Kingdom, Germany, Japan, Australia & India.
Founded April 1989.
Small staff (35 full time); no internal
development.
Almost all technical work done by engineers in
member companies
Almost 800 members (4/98)
* Viewfoil courtesy OMG: http://www.omg.org/library/diego.ppt
Worldwide Scope *
Andersen
Data Access
ICL
Nortel
APM
Eurocontrol
Informix
Novell
Alta
EDS
Intel
OHRA Insurances
AITECH
Ericsson
IBM
Oracle
Thomson CSF
AT&T
FinSiel
IONA Tech.
Open Group
Telecom Italia
BaaN
Fujitsu
Microsoft
POSC
Telefonica
Bull
Genesis
MITRE
Sodalia
Telia AB
Netscape
Siemens Nixdorf TRW
Unisys
Software AG
British Telecom HP
Bankers Trust
Harlequin
Sun
Microsystems
Sybase
Valtech…….
* Viewfoil courtesy OMG: http://www.omg.org/library/diego.ppt
Adoption Process *
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RFI (Request for Information) to establish range of
commercially available software.
RFP (Request for Proposals) to gather explicit
descriptions of available software; Architecture Board
approves.
Letters of Intent to establish corporate direction.
Task Force evaluation & recommendation; simultaneous
evaluation by Business Committee.
Architecture Board consideration for consistency.
Board decision based on recommendations from the
appropriate Technology Committee & Business
Committee.
Fast Track Process.
* Viewfoil courtesy OMG: http://www.omg.org/library/diego.ppt
Integration Technology - CORBA
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The Common Object Request Broker Architecture
is the standard adopted by OMG as a framework
to provide common ORB services and interfaces
to support portable clients and services.
The Object Request Broker (ORB) provides the
mechanisms by which clients transparently make
requests and receive responses.
CORBA (continued)
Advantages offered by CORBA:
architecture independence
 language independence
 location independence
The standard specifies the Interface Definition

Language (IDL) - language used to describe the
interfaces that client objects call and object
implementations provide.
De-facto Standards and products
available*
- Manufacturing:
- Electronic Commerce:
- Healthcare:
-Telecommunications:
- Finance
-Transportation
- …..
- Architecture guide and roadmap
- Working on Internet facilities.
CORBAdomains
CORBAfacilities
CORBA applications
CORBA 2 (IIOP)
- Naming
- Security (...ICL)
- Transactions (..Bull)
- Query Service
- Persistence
- Lifecycle
CORBAservices
* Viewfoil courtesy OMG: http://www.omg.org/library/diego.ppt
- ORBIX
- IONA
- Obj Broker BEA
- VisiBroker Borland
- Omni Orb Olivetti
- Component Broker IBM
The Evolution of CORBA*
CORBA 1.1 (1992)
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Basic ORB, IR, BOA, C Bindings, Naming, Events, Lifecycle etc.
CORBA 2.0 (1995)
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IIOP, Federated IR, C++ Bindings, Transactions, Concurrency,
Externalization, Relationships, Query, Licensing, Trader,
Security, Collections, Time etc.
CORBA 3.0 (1999)
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Messaging (MOM), Server Portability (POA), Business Objects
(JavaBeans), Java Bindings, Objects-by-value, CORBA/DCOM,
IIOP Firewall, Workflow, Domain-level Frameworks, Mobile
Agents etc.
* Adapted from Orafali and Harkey’s Book
ORB services
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Multiple language bindings - C, C++, Java,
Smalltalk, Ada, Cobol
Interface Repository stores the interface
specifications of all services
Transparency of object location
ORB services - Contd
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Object-oriented - provides for encapsulation,
polymorphism and inheritance
Can provide security for content and contextsensitive transactions
Common Object Service Specification 1(COSS1)
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Life cycle service - for instantiating, copying, moving and
deleting objects
Persistence service - interface for storing objects relational and OODBMS backends - Deprecated.
Naming service - to locate using OSF DCE, Sun NIS or
X.500 naming conventions. Can work with Internet
standards such as LDAP.
Event Service - dynamically register interest in events using a global object called event channel
COSS2
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Concurrecny control service - to provide for
locking
Transaction service - to provide for two-phase
commit coordination
Relationship service - to connect and group
multiple objects
Externalization service - to export and
interchange data
COSS3
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Security
Time management
CORBA Architecture
Servers
Server
Clients
C++ Java Ada
C
other
C++ Java Ada
Client Requests
IDL-based
Service
IDL-based
ORB
C
other
COSS4
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Query service - it is a superset of SQL based
on the Object Database Management Group
(ODMG)
Licensing service - to support metering and
support for payment for use of components
Properties service - to associate dynamic
properties with components
COSS5
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Trader
Collections
Change management
CORBA 2.0
Adds multivendor ORB interoperability to CORBA 1.1
specifications adopted in 1991
An ORB mediates requests for services from client applications
and service providers
2.0
Object
Implementations
Client
Interface
Repository
Dyamic Client
IDL
InvocStubs
ation
ORB
Interface
Static
Dynamic
Skeletons Skeletons
Object Adapter
Object Request Broker
Implementation
Repository
Client IDL Stubs
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Provides local proxies for remote
services
Provides for marshaling of
parameters into standard formats
Dynamic Invocation Interface
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Late binding of method calls
Run time discovery of service IDL interface
Interface Repository
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Runtime database of interfaces of all
registered objects
Supports the query of, registration of, and
update of interfaces of objects mediated by the
ORB
Repository IDs - uniquely and globally identify
interface repository across multivendor ORBs
ORB Interface
Provides for API that provide a number of utilities:
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Object-to-string : converts object reference to a string
String-to-object : converts a string reference to object
get-interface : to find the location of interface repository for a
given object
get-implementation: to find the reference to implementation
repository of an object
Server IDL Stubs
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Also called skeletons
provide static interfaces to each service
provided
both client side and server side stubs are
generated using IDL compilers
Dynamic Skeleton Interface (DSI)
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to implement dynamic services or dynamically
changing services
run-time binding of new services using
scripting languages and interpreters
to redirect messages to objects for which a
compiled idl-interface is not known
Object Adapter
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responsible for managing a collection of server
objects
instantiates server objects when needed and
provides them with object IDs called object
references
registers the classes and instances in the
Implementation Repository
All CORBA implementations must support a
Basic Object Adapter
Implementation Repository
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Keeps track of classes and run-time instances
of server objects supported by the ORB
Allows for mechanisms to support audit trails,
trace information and security related
infrastructure
Using the Static interface
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Definition of object services using IDL
Precompiler generates server and client stubs
Implementation code added to the server stubs
Compilation: client stubs, server stubs, code that implements
server classes and code to describe the classes to Interface
Repository
Register with Interface Repository
Instantiate server objects
Register run time instances with Implementation Repository
Using the Dynamic interface
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Lookup service name and get its interface specification from
Interface Repository
Put together a request made up of object reference, method
name and argument list
Invoke the method call on the desired object
Basic Object Adapter
Must provide for the following functionality:
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An implementation repository
Manage instantiation and destruction of objects
Manage object references and invoking methods
Mechanism to authenticate clients
BOA Object Activation Alternatives
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Shared Server: First time call to any object causes the server
process to startup. Future requests are handled by the server
directly. BOA notified with impl_is_ready and deactivate_impl.
Unshared Server: A new server process is started to support
each object requests.
Server-per-method: A new server process is started for each
request made (method call).
Persistent server: Servers are persistent and started
independently.
CORBA 2.0 Interoperability Standard
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CORBA 1.1 allowed for portability
CORBA 2.0 specifies the Internet Inter-ORB
Protocol (IIOP)
Public domain IIOPs are now available (from
Sun for instance)
Every CORBA-compliant ORB must
implement IIOP or provide a half bridge to it
General Inter-ORB Protocol (GIOP)
Common Data Representations (CDR) used in communications
CDR maps data types defined using IDL into a flat representation
for use in network layer
CDR takes care of inter-platform issues such as byte ordering
Specifies Interoperable Object References for use in multi-vendor
ORB environments
Internet Inter-ORB Protocol (IIOP)
Standardizes the way GIOP messages can be
exchanged using TCP/IP
Mandatory for CORBA 2.0 compliance
Environment-Specific Inter-ORB Protocols (ESIOPs)
Example is DCE/ESIOP
GIOP CDR-based messages defined using OMG IDL are
transported using DCE RPC using DCE native Network Data
Representation (NDR)
ORB-to-ORB Bridging
Server
Client
DSI
ORB From Vendor A
DII
ORB From Vendor B
IDL and Interface Repository
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Interface Definition Language (IDL):
fundamental mechanism supported by all
ORBs to separate interfaces and
implementations
Interface Repository is a queriable and
updateable runtime information store of
interface specification generated from IDL
IDL users
Client programmers who want to access the services provided by
an object - they need the IDL to determine how to call the
services
Server programmers who want to extend the service provided they need the IDL to subclass and provide additional capabilities
IDL Structure
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Module: groups a collection of class definitions
(interfaces)
Interface: defines a class; attributes and
exceptions
Operation: defines a method; the arguments;
argument types; in, out or in-out; exceptions;
optional context description
Data types: basic (short , float, char...) and
constructed
IDL Example : type codes
typedef string
typedef string
typedef string
typedef char
typedef char
typedef long
Date_T;
PersonID_T;
Address_T;
MaritalStatus_T;
Sex_T;
PolicyID_T;
IDL Example: Interfaces
interface FamilyMember;
interface Employer;
interface InsurancePolicy;
interface InsuranceCompany;
IDL Example: Interface definition
interface Demographics: PRObject
{
readonly attribute PersonID_T
personID;
attribute
attribute
string
string
lastName;
firstName;
attribute
attribute
long
Date_T
SSN;
DOB;
attribute
attribute
string
MaritalStatus_T
birthPlace;
maritalStatus;
readonly attribute sequence<InsurancePolicy> insurancePolicies;
readonly attribute sequence<FamilyMember> familyMembers;
readonly attribute sequence<Employer>
employers;
};
IDL Example: Interface definition
interface InsurancePolicy: PRObject
{
readonly attribute PolicyID_T
policyID;
attribute
attribute
string
string
insuranceType;
carrierProgram;
attribute
attribute
long
long
groupNo;
payorNo;
attribute
attribute
long
string
controlNo;
relationshipToPolicyHolder;
readonly attribute Demographics
policyHolder;
readonly attribute InsuranceCompany insuranceCompany;
};
Type Codes
represent IDL-defined data types
provide for self-describing data
Used by: DII, IIOP, Interface Repositories and any data type
Interface Repository - where used
Run-time type checking on method invocation parameters
Connecting multi-vendor ORBs
Support dynamic discovery of object interfaces and support for
DII
Provide for self-describing objects
CORBA 3.0
Trader Service
Java to IDL mapping
POA
Real-time CORBA
Workflow
Asynchronous Messaging
Business Objects Facility
Domain-specific frameworks
CORBA Component Model
Manufacturing, Transportation,
Finance, Healthcare, Telecom,
Electronic Commerce
and more!
CORBA Scripting Language
IIOP over firewall
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See: http://www.omg.org/news/pr98/9_9.htm for a press release
on CORBA 3.0 by major vendors for an early 1999 commercial availability.
Trader Service
Naming Service is like white pages
Trader is like yellow pages
Trader
ORB
Client
“Importer”
Import Service
Server
“Exporter”
Export Service
ORB
ORB
Invoke Service
Portable Object Adapter (POA)
To quote Orafali & Harkey: “The POA is simply the BOA done
right.”
Similar to BOA, POA supports management of:
 separate process for each method
 separate process for each object
 shared process for multiple objects
In addition, POA supports the notions of:
 servants and servant managers (instance managers)
 transient and persistent servants and instances
Essentially addresses scalability and flexibility in server
implementations
CORBA/Java Security
Reference: Andreas Vogel & Keith
Duddy, “Java Programming With
CORBA,” Second Edition, John Wiley
& Sons, 1998.
Applet Security Issues
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Java Virtual Machine for applet execution are
restricted in their ability to access local
resources - referred to as “applet sandboxing.”
Applets are only allowed to communicate with
the server they are downloaded from
CORBA clients can in principle communicate
with any object for which there is an IOR
reference.
Two approaches have been identified to
address applet security restrictions.
Working with Applet Security restrictions
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Establish an IIOP
gateway which
serves as a proxy to
the real server
Use signed applets
Browser
Client
Applet
Download
Applet
Server
Implementation
WWW Server
IIOP Gateway
IIOP
IIOP
Firewalls
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Firewalls restrict
and filter
network traffic
In a client-server
environment, in
the general
case, there may
be a client-side
and a serverside firewall
involved.
Types of firewalls
Origin/Destination Filtering - router-based
filtering based on IP address of target and
specific port numbers on that address
 filtering based on IP addresses of specific set
of source machines
Gateway firewall
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A machine with dual network cards - one
connects to Internet and the other to Intranet.
Only way for packet traffic to bridge the two
networks is a protocol-specific bridge that
forwards packets to the appropriate
applications
Client-side firewalls
Browser
Client
Applet
Proxy
Client-side
Firewall
Server
Implementation
Gatekeeper
HTTP
IIOP
HTTP Tunneling With Gatekeeper
Browser
Client
Applet
Proxy
Client-side
Firewall
HTTP
Server
Implementation
Port 80
Gatekeeper
IIOP
IIOP
Gatekeeper with dual functionality
IIOP over Secure Socket Layer (SSL)
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SSL is layered on top of TCP/IP
It is the basis for secure electronic commerce
over the Web - using Secure HTTP (SHTTP)
SSL authentication is based on public key
cryptography
Authentication is based on digitally signed
certificates.
Overview of CORBA Security
Service
Goals of a security service
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Confidentiality
Integrity
Accountability
Availability
Sample threats to security
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An authorized user accessing information they
should not
A user pretending to be someone else to
access information
Eavesdropping on others communication
Modifying, deleting or maliciously tampering
with information
Features of the Security Service
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Identification and authentication
Authorization and access control
Auditing
Communications security
Non-repudiation
Administration of security policy
Security Conformance Levels
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Security Level 1 - targeted at securing
security-unaware applications. Allows for client
authentication, secure communications and
method-level access control based on security
policies.
Security Level 2 - supports all of the above
and supports integrity, trust and auditing.
Targets security-aware application which may
enforce their own policies.
CORBA World Players
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Netscape (bundle CORBA-clients with their
web browser)
Oracle has adopted CORBA/Java as their
base platform
JavaSoft - is layering RMI on top of CORBA
IIOP. Sun has also adopted Visibroker as its
ORB technology.
IBM/Lotus: Adopted base infrastructure is
CORBA/Java.
CORBA World Players - Others
Iona
Symantec
Visigenic/Inprise
ParcPlace
HP
Penumra
Tandem
Inprise
Novell
Sybase
I-Kinetics
NetDynamics
Expersoft
Vignette
BEA
Allaire
and 800 more!
Distributed Component Object Model
(DCOM)*
Microsoft Version of Distributed Object Computing
* Material for this tutorial is based on Orafali and Harkey book on
Client Server Programming with Java and CORBA and Richard Grimes
book on DCOM Programming.
DCOM and CORBA
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Like CORBA, DCOM adheres to separation of
interface from implementation using Microsoft
Interface Definition Language (IDL) based on
DCE. Not compatible with CORBA IDL.
Also provides an additional IDL for OLE
automation called Object Definition Language
(ODL).
DCOM
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Unlike CORBA, DCOM does not support
multiple inheritance. It supports multiple
interfaces and compositional architectures.
Like CORBA, DCOM provides both static and
dynamic bindings for methods
Type Library in DCOM corresponds to
Interface Repository in CORBA.
DCOM registry corresponds to Implementation
Repository.
DCOM Interface
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DCOM Interface is collection of method calls.
DCOM interface is language neutral
Microsoft provides language bindings for
Visual C++, Visual Basic, and Visual J++.
Inprise ’s Delphi products provide bindings for
Pascal.
DCOM Interface
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To access DCOM interface, clients use pointer
to an array of function pointers - called virtual
table or vtable.
Implementation
of Member
Functions
Interface
Function
Table
Internal
Data
Object
User
Pointer
Pointers
to
Functions
DCOM Object
DCOM Conventions
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Plug-in-jack pictorial
representation of
components.
Interface by convention
is named with a letter
beginning with capital “I”
- e.g. IDispatch
At run time, interfaces
are known by their
Interface Identifier (IID)
IID is globally unique
(GIID) - 128-bit IDs.
DCOM Object
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Also known as
ActiveX object.
DCOM object
implements all the
interfaces the object
supports.
Identified by a unique
128-bit Class ID
(CLSID)
Implements
IUnknown interface
IUnknown
Interface A
Interface B
Interface C
DCOM
Object
DCOM Server
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DCOM servers is a
DLL or EXE or a
Java Class.
Server must
provide a class
factory for
instantiating new
Object
objects
Interfaces
IClassFactory2
interface needs to
be supported if
IClassFactory
object is
or
instantiated only if
IClassFactory2
a valid license file
is present
IUnknown
DCOM
Object
Class
Factory
DCOM Server
DCOM Local/Remote Transparency
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In-process servers: server DLLs that can be loaded in
the same process as the client.
Local servers: clients use DCOM’s Lightweight RPC
(LRPC) to communicate with server running on a
different process space in the same machine.
Remote servers: clients use DCE RPC-like mechanisms
to make method invocation.
DCOM IUnknown Interface
IUnknown Interface consists of:
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QueryInterface - for supporting interface
negotiations
AddRef - to support garbage collection
Release - to support life cycle management
Writing a Java Client/Server Application in DCOM
1. Create DCOM
IDL
2. Generate
GUIDs
3. MIDL
Class
Proxy
4. JavaTLB
Interface
Interface
8. Implement
Client
5. Implement
Server
9. jvc
6. jvc
10. JavaReg
7. JavaReg
11. jview
Client
Summary.txt
Server
DCOM Conclusions
DCOM is maturing rapidly
Java support is there, although it is for Microsoft version of
Java
Biggest differences with CORBA
 Support limited to Wintel platforms
 Range of services available limited (CORBA has from
Workflow to security to vertical domain standardization
efforts
Biggest drivers
 Freely available with Windows NT
 MTS and MSMQ and other internet Microsoft
technologies
Java Technology
Why Java
Java is a simple, efficient, secure, portable high-performance
programming language.
It is similar to C++, easy to learn.
It has no pointer arithmetic, no need to free memory which is a
serious problem in C/C++.
Very compact Runtime.
Some Java Modules
HTML
JavaBeans
Servlets
IIOP RMI and other Protocols
ENTERPRISE JAVABEANS COMPONENTS
J
Server
Platform
T
S
J
M
A
P
I
J
J
N
I
D
D
I
L
J
J
M
S
D
B
C
Other Java APIs
Java Telephony API
Java Wallet
Java Server API
Java Security API
JDBC
JDBC (Database Connectivity)
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JDBC is a “call level “ SQL interface from Java.
This facilitates the execution of SQL
statements and return the results.
JDBC is an interface level specification.
JDBC API is implemented on top of existing
SQL level API -- ODBC.
JDBC (Contd)
The most important JDBC interfaces are
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java.sql.Environment provides support for
creating new database connections
java.sql.Connection to connect to a particular
database
java.sql.Statement for executing a SQL
statement on a given Connection
java.sql.ResultSet to access the resultset for a
given SQL statement.
JDBC (Contd.)
Furthermore java.sql.Statement has two
additional interfaces
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java.sql.PreparedStatement for executing a
pre-compiled SQL statement
java.sql.CallableStatement for executing a
database stored procedure
JDBC (Contd)

A JDBC-ODBC Bridge driver currently sits
between the ODBC Drivers (provided by
database vendors) and the JDBC API.
JavaBeans
JavaBeans
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JavaBeans is a software Component model for
Java.
Facilitate third party ISVs to create and Ship Java
Components that can be composed together by
end -users (VB model)
JavaBean Component Model
BeanInfo
Methods
JavaBean
Component
Customizer
Events
Properties
* From Orafali’s book
JAR
CORBA Component Model*
Component Info/IDL
Methods
CORBA
Component
Customizer
Events
Properties
* From Orafali’s book based on the main submission team
CAR
JavaBeans
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JavaBeans provides a platform independent
component architecture.
Will bridge to ActiveX, OpenDoc and
LiveConnect.
Distributed Beans
JDBC
Database
Server
IIOP
CORBA
SERVER
RMI
JAVA
JAR
Java Beans are packaged as JAR (Java Archive). (Beans come
in a JAR :-)
Pragmatics
How does one create a Bean??
Creating Beans
Write the component in Java using the JDK 1.1 Event model
(Event Delegation model) and implement the appropriate
Listeners and add the listeners to the sources.
Creating a BeanInfo class is highly recommended.
Use Design Patterns for Creating the Java Beans
Design Patterns
In very simplistic terms, design patterns are naming conventions.
For example, if there is a property named Label, you would have


public String getLabel() { }
public void setLabel(String labelname) { }
Design Patterns
Boolean Properties

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public boolean isRed() { }
public void setRed( Color red) { }
Design Patterns
EventListeners

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public void add<ListenerType> (<ListenerType> listener)
public void remove<ListenerType> (<ListenerType> listener)
Example
addSriniEventListener(SriniEventListener srevt)
removeSriniEventListener(SriniEventListener srevt)
Bean Analysis
Introspection
Java beans
Bean
Boxes
Design
Patterns
Reflection
Introspection
Discovers Events, Properties and Methods when the Bean
Designer provides a BeanInfo Class
Reflection
Reads Events, Properties and Methods using Design Patterns
JAR Files
is similar to Zip Archives and may optionally have a MANIFEST
file with information describing the contents of the JAR file.
JAR files are used to collect class files, serialized objects, images
help files and similar resource files.
MANIFEST FILES
A JAR file that wants to advertise its contents should have a
MANIFESTS file. The MANIFEST file will indicate which of the
potential beans in the JAR archive are actual beans. If there is no
MANIFEST every potential class is regarded as a bean.
Authentication
JAR files can be signed to provide authentication.
Lotus InfoBus
Producer
Infobus
Consumers
Change Notification
Producers and Consumers notify changes performed by each
other to the shared data.
An Event based mechanism.
Supports security and access control
More Information
Can be found including specs and such at

http://java.sun.com/beans/infobus
Java Blend
What is Java Blend
Java Blend Integrates Objects in the Java Programming language
with Enterprise Objects.
Provides a single consistent, coherent object model for enterprise
based on the Java object.
Java Blend
Jointly developed by JavaSoft and Baan.
Currently the programmer must explicitly convert to and from
Java objects and database objects.
Current applications are split into Java part and SQL part and
both interacting through JDBC or some similar mechanism.
Object-Relational Mapping
Currently done by the programmer.
Decomposition of Objects into many tables or build objects from
many tables.
Mappings between foreign keys and Object references
Mappings between field types and SQL types
Object Relational Mapping
Map and or Infer Inheritance Relationships from Foreign keys
Perform Transactions and process the results of transactions.
Java Blend
Java Blend provides automatic mapping between Java Objects
and backend relational database tables.
The programmer only deals with Java Objects and is unaware of
the SQL.
SQL translations, Database representations (relational) are
transparent to the programmer.
Advantages
Easy access to enterprise relational data from Java Programming
Language.
No need to know, SQL, DDL and other relational database
specific ideas.
Significant reduction in application development time.
Java Blend Environment
development environment supports automatic object<-->
Relational Mapping.
Runtime environment supports



Transaction Management
Caching
Query Processing
Development Process
A mapping tool is used to map business objects written in Java to
database schema.
Queries can be written in OQL (ODMG compliant)
Enterprise Application
Persistent Objects
Java Blend Runtime
JDBC
JVM
Java Blend Runtime Architecture
Transaction
Manager
Run Time
Cache
Query
Engine
Other Advantages
Multiple threads for multiple user transactions
Multiple simultaneous transactions per application
Upon commit stores objects in the database
Rollback mechanisms
Performance
Increases performance by optimistic concurrency control(i.e.,
does not hold read and write locks on server)
Runtime caching
Support for stored procedures
EJB and Java Blend
Java Blend works with EJB APIs
Extremely scalable.
Enterprise Java Beans
What is EJB
It is a distributed Software Component Architecture.
It is a Framework for Creating Middleware.
EJB is part of the Enterprise Java API
EJB
It is a Server side Component model
It focuses on Business Logic.
Enterprise Java Bean Architecture*
*From EJB specification document
Enterprise Java Bean Containers*
OTS/JTS
Activation/
Deactivation
State
Management
Container
Metadata
Security
Factories
ORB
EJB1
EJB2
EJB3
* Orafali’s Book
EJB Architecture
EJB architecture consists of





EJB Servers
EJB Containers
EJB Clients
EJB
Auxiliary Systems such as JNDI, JTS and others.
The Various Players
EJB Providers
Provide Enterprise Java Beans (Components)
EJB Server Providers

Creates and sells EJB servers
EJB Container Providers



sell EJB containers
Usually the server provider will also provide containers
The Various Players
EJB Application Assemblers

Will enable application building using EJB components,
servers and Containers.
EJB Servers
EJB Servers are similar to CORBA ORBs
It Provides Naming(JNDI) and Transaction Services (JTS)
Makes containers visible to clients
EJB Containers
Encapsulate the Bean
Client accesses Beans’ methods through the container methods
Container methods map to Bean methods
Client only knows about container methods.
EJB Clients
First discover the services (ala CORBA ) using mechanisms such
as JNI
Request Bean Services through Container Services.
Types of Enterprise Java Beans
Usually categorized as Session Beans or Entity Beans
Session EJB
Is associated with a Single Client.
It is created and destroyed by a client
Entity EJB
Are Shared by multiple clients
Persistence is supported.
Structure of an EJB Client
First Locate a EJB container using a directory service
Allocate a bean
Use the bean services
Dispose the bean.
EJB Support
IBM
Oracle
Lotus
Novell
Iona
Netscape
Inprise
and many many others
More on EJB
can be found at www.java.sun.com/products/ejb/index.html
Some concluding thoughts....
CORBA/DCOM/JAVA Interoperability
Bridge
ActiveX
Clients
Proxy
CORBA
Orbix
Orb
Orbix
Srvc
Java Beans
Proxy
CORBA
Visi
Orb
Visi
Srvc
Java
Code
Java Beans
ActiveX
DCOM
COM
Srvc
Acknowledgement: Adapted from a view foil by Tad Davis, CareFlow|Net
ActiveX
Bottom Line for Distributed Object Computing
You can develop using CORBA
You can develop using DCOM
You can certainly use Java
They all in reality can inter-operate in fairly straight forward
fashion.