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Defines h ow o bjects areactivated/deactivated Initializing server o bjects:

BOA.obj_is_ready( /* the object ref */ ); R egisters the o bject with the ORBBOA.impl_is_ready();

Tells the BOA /ORB to begin listening f or requests

Unders pecified Initially unclear which features w ould be

required on vari ous platf or ms

Basic O bject Ada pter ( BOA )

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Portable O bject Ada pter (P OA )

R e places BOA Mo st comm ercial i mp lementati ons still use

BOAEx panded sc ope of OA to include Activati on policies

Threading models O bject life cycle (transient/ persistent) Pre/ post inv ocati on ca pabilities

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(Portable) Object Adapter

(POA)Register class implementationsCreates and destroys objectsHandles method invocationHandles client authentication and accesscontrol

ORB

C++ O bject

Client Server

Java O bject

IIOPORB

Stub O bject Ada pter

Skelet on

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W hat is an ORB?

A distributed o bject bus

Hides trans port mechanis ms

Locati on Method inv ocati on Marshalling

OMG Interface Definiti on Language(IDL) pr ovides the languageinde pendent se mantics

ORB

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O bject R equest Br oker ( ORB )

Enables o bject t o trans parently make andreceive requests and res ponses

...like a telephone exchange, providing the b a s icmechani sm for making and receiving call s

ORB

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O bject R equest Br oker ( ORB )

A bstracts re mo te request and res ponsemechanis ms

Trans port f or di s

tribu

ting method inv ocati ons

ORB

O bject I mp lementati onClient

R equestR equest

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ORB Usage

ORB is a singlet onORB initializati on Single static call t o init() an ORB

After initializati on, y ou register o bjectswith the ORB using an O bject Ada pter

org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init();

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Pr oxy-based Inv ocati on

ORB

Skelet onStub

R equestR equest

Client O bject I mp lementati on

R equest R equest

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Object Request Broker ...Communication infrastructure sendingmessages bet ween objectsCommunication type:

GIOP (General Inter-ORB Protocol) IIOP (Internet Inter-ORB Protocol) (GIOP on

TCP/IP)

ORB

C++ O bject

Client Server

Java O bject

IIOPORB


Skelet on

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Internet Inter ORB Pr otocol (II OP)

Trans port pr otocol Defines inter ORB comm unicati on R uns on top of TCP/IP Defines CORBA messages

IIOP is a s pecificati on Vend ors must i mp lement t o be CORBA -

comp liant Allows f or multi-vend or inter operability

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CORBA Object

Server

Interface

IDL

C++ /Java

Implementati on

Inter operable O bject R eferenceCORBA

O bject

Servant

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Interoperable Object Reference(IOR)

Uniquely identifies an object (see objectreferences)

Server

InterfaceIDL

C++ /JavaImplementati on


O bject

Servant

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Interface Definition Language

(IDL)Describes interfaceLanguage independent

Client and server platform independentServer

InterfaceIDL

C++ /JavaImplementati on


O bject

Servant

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IDL (cont)

In RMI we had an RMI compiler run on the.class file we wanted to be remote. Itsmore complex under CORBA

The interface is written in InterfaceDefinition Language, IDL. IDL is language-neutral (by being a different language thananything else) though highly reminiscent of C++ (multiple inheritance, similar syntax,

etc.)

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Overall CORBA Architecture

ORB

Client Server

IIOPORB


Skelet on

C++ O bject IDL Java O bject

Implementationrepository

Interfacerepository

Implementation repositoryactivates registered servers on demand and locates running serversuses the object adapter name to register and activate servers

Interface repositoryinterface repository provides information about registered IDL interfaces to clientsand servers that require it.

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Devel opm ent Ste ps

1 write the IDL file2 compile with idltojava (stubs/skeleton generated automatically)3 write object implementation (servant)4 write client application

steps:

1

Stub Skeleton

Client Application O bject Implementation

ORB ORBrequest

response

IDL

idltojava

2

3

2

4

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Example of CORBA Services

Naming: Keeps track of associationbet ween object names and their reference. Allo ws ORB to locatereferenced objectsLife Cycle: Handles the creation,copying, moving, and deletion of objectsTrader: A yello w pages for objects.Lets you find them by the services theyprovideEvent: Facilitates asynchronouscommunications through events

Concurrency: Manages locks so objectscan share resources

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CORBAservices are the baseline servicesavailable to all objects sitting on the ORB

communication bus.1 . Naming Service2.Event

ManagementService

3. Life Cycle Service4. Persistent State

Service5. Transaction

Service6. Concurrency

Service

9. Query Service10 . Licensing Service11 . Property Service1 2. Time Service1 3. Security Service1 4. Notification

Service1 5. Trader Service1 6. Collections

Service

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Corba ServicesService Description

Collection Facilities for grouping objects into lists, queue, sets, etc.

QueryFacilities for querying collections of objects in a declarativemanner

Concurrency Facilities to allo w concurrent access to shared objects

Transaction Flat and nested transactions on method calls over multipleobjects

Event Facilities for asynchronous communication through events

Notification Advanced facilities for event-based asynchronouscommunication

Externalization Facilities for marshaling and unmarshaling of objects

Life cycleFacilities for creation, deletion, copying, and moving of objects

Licensing Facilities for attaching a license to an object

Naming Facilities for system wide name of objects

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RM I De ployment

O bject I mp lementati on

Stubs Skelet ons

InterfaceDefiniti on

Imp lementati onInstallati on

Client

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CORBA De ployment

O bject I mp lementati on

InterfaceR e pository

Imp lementati onR e pository

Stubs Skelet ons

IDL Definiti on Imp lementati onInstallati on

Client

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CORBA Features

Language inde pendenceLocati on trans parency

R euse of facilities & servicesOMG IDL defined interfacesStub & Skelet on generati on

Server activati on

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CORBA R esources

O bject Manage ment Gr ou p (OMG ) www. omg.org

www.c orba. orgDiscussion gr ou ps comp .o bject.c orba comp .lang.java.c orba

Imp lementers In prise: www.in prise.c om Iona: www.i ona.c om

Sun: java.sun.c om/ pr oducts/jdk/1.2/

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Creating a CORBA O bject

Initialize ORB and BOAInstantiate the o bjectEx port the o bject BOA .o bj_is_ready()

Op tionally register the o bject Na ming Context.rebind()

Tell the ORB to begin receiving requests BOA .imp l_is_ready()

bli i id i (S i [] ) {

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Activating the Qu oteServer public static void main(String[] args) {

try {// Initialize object request brokerORB orb = ORB.init(args, null);

// Initialize basic object adapterBOA boa = orb.BOA_init();

// Create a new QuoteServer ...QuoteServer quoteServer = new QuoteServer();

// ... and export the object boa.obj_is_ready(quoteServer);

// Object Request Broker Initialized System.out.println (QuoteServer ORB

initialized");

// Wait for incoming requests boa.impl_is_ready();

} catch (SystemExcpetion e) {

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Accessing a CORBA O bject

ORB uses Inter operable O bjectR eferences (I OR s)

O bject might be l ocal, client uses proxyClient must acquire first o bject reference Na ming/Trading service

Pr op rietary bind Pr op rietary U R L service Som e other pr op rietary means

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R egistering the Qu oteServer // Create a new QuoteServer ...

QuoteServer quoteServer = new QuoteServer();...

// Obtain reference for our nameserviceorg.omg.CORBA.Object object =

orb.resolve_initial_references("NameService");// Since we have only an object reference, we must// cast it to a NamingContext. We use a helper// class for this purpose

NamingContext namingContext = NamingContextHelper.narrow(object);

// Add a new naming component for our interface NameComponent name[] = { new NameComponent(QuoteServer", "") };

// Now notify naming service of our new interfacenamingContext.rebind(name, quoteServer);

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Using a CORBA O bject

Initialize the ORB on the clientGet a reference t o the re mo te o bject

IOR Actual reference (e.g. fr om bind())

Invoke methods

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Accessing the Qu oteServer // Create an object request brokerORB orb = ORB.init(args, null);// Obtain object reference for name service ...org.omg.CORBA.Object object =

orb.resolve_initial_references("NameService");// ... and narrow it to a NameContext

NamingContext namingContext = NamingContextHelper.narrow(object);// Create a name component array

NameComponent name[] = { new NameComponent(QuoteServer","") };// Get a QuoteServer object reference ...org.omg.CORBA.Object objectReference = namingContext.resolve(name);// ... and narrow it to get a QuoteServerQuoteServer quoteServer = QuoteServerHelper.narrow(objectReference);// invoke methods on referenceQuote noveraQuote = quoteServer.getQuote(MCTR);

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Technical/Architectural

Overvie wCORBA facilities come in t wo groups:1 . horizontal facilities

target client-side functionality

2. vertical facilities target domain-specific functionality

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Technical/Architectural

Overvie w (cont.)Horizontal facilities specifications:1 . Mobile Agents Facility2. Printing Facility3. Internationalization Facility

Vertical facilities domain-specific servicesare being developed for various businessareas:

1 . Common Enterprise Models2. Finance/Insurance

3. Electronic Commerce

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Technical/ArchitecturalOvervie w (cont.)

7. Transportation8. Life Science Research9. Utilities10 .C4I (Command, Control, Communications,

Computers, and Intelligence)11 .Space

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CORBA BasicsID L J a v a

m o d u l e p a c k a g e

in t e r f ace i n t e r f aces t ruc t c l a s sc o n s t p ub l i c s t a t i c f i na l

b o o l e a n b o o l eanc h a r c h a r w c h a r w c h a r o c t e t o c t e t

s t r i n g j ava . l ang .S t r i ngws t r i ng j ava . l ang .S t r i ng

sh o r t sh o r tu n s i g n e d s h o r t sh o r tl o n g i n tu n s i g n e d l o n g i n tl o n g l o n g l o n gu n s i g n e d l o n g l o n g l o n gf l o a t f l o a t

d o u b l e d o u b l ef i xed ( n o t s u p p o r t e d i n i d l j ) j a v a . m a t h . B i g D e c i m a l s e q u e n c e [ ] ( a r r a y ) [ ] ( a r r a y ) [ ] ( a r r a y )

F ig . 2 6 . 9 ID L k e y w o r d s, t y p e s a n d t h e i r m a p p in g s t o J a v a k e y w o rd s.

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CORBA Basics (cont.)

Method call types: synchronous

client makes call on remote blocking method standard method call on remote service

client makes call on remote non-blocking method remote method qualified with oneway modifier in IDL file remote method receives only in arguments

asynchronousclient makes synchronous call on remote method,remote service responds at a later time by callingmethod on client

must be qualified with oneway to prevent deadlock

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CORBA Basics (cont.)

Client Server

1 . registerClient( this )

2. clent.endMessage()Client and server are n o w dead loc ked.

Deadl ock caused by client calling a server that calls the client.

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Distributed Exceptions

CORBA specifies t wo exception types:1 . System Exceptions

defined for use by CORBA infrastructure

2. User Exceptions defined using IDL by developers

Standard CORBA exceptions map to

Java exceptions as final classes.

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Case Study: Chat

Chat is a basic net work application usinga central broadcasting point where acollection of clients pushes in messagesand the middle ware pushes outmessages.Following use cases:

1 . Connect: A client finds and connects to a chatserver.

2. Disconnect: A client completes a chat session bydisconnecting from the chat server.

3. Send message: A connected client creates a chat

a c nvoca on n er ace

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a c nvoca on n er ace ,Dynamic Invocation Interface (DII) and

Dynamic Skeleton Interface (DSI)

Tw o ways to invoke a request:1 . statically using S tatic Invocation Interface

( S II) relies on creating a request through invoking a

static method in a stub stub generated from compile-time definition of an object

type (IDL interface) and object operations (methodswithin IDL interface)

2. dynamically using Dynamic InvocationInterface (DII) programmatically creates and send invocation

request directly to ORB without assistance of stub

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Static Invocation Interface (SII), DynamicInvocation Interface (DII) and Dynamic

Skeleton Interface (DSI) (cont.)Interface Repository (IR) contains descriptive information about distributed

objects:

modules availableinterfaces definednames of operations defined within interfacesargument typesreturn types

exceptions raised

Steps needed to make DII call:1 . Obtain object reference to server object2. Look up desired method in Interface Repository

3. Build argument list using IRs OperationDef

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BOAs, POAs and TIEs (cont.)

POA connects an object reference todeveloper- written code using code foundin IDL generated skeleton. allo w fine-grained control

Distributed objects inherit from a POAbase definition generated by IDL compiler enables distributed object to be usable by a

POA, enables POA to control all access to servant

through policies

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BOAs, POAs and TIEs (cont.)

Another way for developer to us a POA isto wrap their servants in a TIE. enables interaction with a POA without having

servants object implementation inheritstructure from POAImpl .

servant can inherit from other base classes

freely

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CORBA services

CORBAservices define base servicesand a support structure useful to a widerange of applications.Five most commonly used services:

1 . Naming Service2. Security Service

3. Object Transaction Service4. Persistent State Service5. Event and Notification Services

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Naming Service

Associates name objects with arbitraryvalue (kno wn as name bindings ).A path to a name binding consists of zeroor more naming contexts (a collection of unique name bindings).Resolving a name binding returns objectassociated with name.Binding a name creates associationbet ween a name and an object.

Multi le name bindin s can oint to a

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Security Service

Consists of t wo levels Level 1

provides basic security for 1 . user authentication,2. invocation security, and3. availability of authentication principals to security-a ware

applications

allows applications to ignore system-securityrequirementsrequires support for no delegation and singledelegation models

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Security Service (cont.)

Level 2is everything level 1 provides in addition to:1 . more fine-grained user authentication

2. greater invocation security3. auditing4. finer control over secure invocations5. delegation6. administrators can set security policies

7. discovery of security policies by security-a wareapplications

8. discovery of security policies by ORBs and other services

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Object Transaction Service

Enables CORBA objects to execute asparts of distributed transactions .

a transaction describes a collection of interactions where multiple users may accessand/or modify data.

acronym ACID describes four standardrequirements for reliable transactions:

1 . Atomic completion of numerous steps must beconducted as one, other wise each step must beundone.

2. Consistent effects of transaction are repeatableand predictable.

3. Isolated transaction not interrupted from outside

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Object Transaction Service(cont.)

transactions complete in one of t wo ways:1 . committed

changes are made to persist

2. rolled backchanges made to data are discarded

POAs dictate transaction types supported byservants

shared transactionsunshared transactionsshared and unshared transactions

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Persistent State Service (cont.)

contains t wo definition types:abstract definition define portable definition of the persistable state of a

CORBA object

concrete definition

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Event and Notification Services

E vent S ervice defines mechanism thatdecouples delivery of events (messages)from source of events. no predefined event types in specification keeps track of action events and event

listeners

S upplier creates events that areprocessed by a consumer .In push model , supplier sends event

messages asynchronously to all

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Event and Notification Services(cont.)

Notification Service is a direct extension of the Event Service. objects can create and destroy event

channels arbitrarily, and can filter output usingF ilter Objects and Object Constraint Language

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Standard flo w of a StructuredEvent :1 . finding object reference to Notificaion Service

(instance of EventChannelFactory )

2.EventChannelFactory creates anEventChannel3. supplier asks EventChannel to return

SupplierAdmin object

4.SupplierAdmin returns a Consumer proxy(such as StructuredProxyPushConsumer ) toan event channel

5. access to distributed events (through push or pullmodels) is accessible through proxy

d f

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ProxySupplier

!

o ns " # er

EventChanne l

ProxyConsumer

S" $ $ lier

!

o ns " # er S" $ $ lier

!

o ns " # er

S" $ $ lier

EventChanne l

Even t

Eve nt

Even t

Even t

Event

Event

Even t

!

once$

t"

a ll%

, a S"

$ $

lie r send s a n Eve nt to a!

on s"

#

e r.

&

n EventChannel deco " $ les t'

e S " $ $ lier fro # t'

e!

on s " # er.

&

ddin ( a ProxyConsumer a nd ProxySupplier en a)

les f " rt'

er d eco " $ lin ( a nd # ake s it $ ossi

)

le to s " $ $ o rt)

o t'

t'

e $ " s'

an d $ " ll # od els.

Su pplier-t o-consu mer fl ow using the Event/N otificati on Service.

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EJBs and CORBA components

CORBA Component Model (CCM)Request for P roposal (R FP ) recommendsthe JavaBeans component model as basisof a server-side frame work.CORBAcomponents based on Component Implementation F ramework (CI F )

architecture.CIF defines superset of Persistent StateDefinition Language called Component

IDL (CIDL)

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EJBs and CORBAcomponents(cont.)

IDL ke y w o r d s t o su p p o r t t h e C O RBAC o m p o n e n t M o d e l ( C C M )

compo nent h om e pr ovides

consu m es i mpo rt set R aisesem its l ocal su ppo rt s

finder m ulti p le ty peIdget R aises pri m aryKey ty pePrefix

F ig. 27.4 IDL ke y w o rd s t o su p p o r t t h e C O RBA C o m p o n e n t M o d e l .

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Component Interface Definition Language(CIDL) is a superset of Persistent StateDefinition Language. defines

components in a way that enables automaticgeneration of components persistence code.component implementationstate management

compiled with a CIDL compiler

Developers organize components as anassembl with a descri tor .

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A Container class creates a containmenthierarchy grouping components and other containers together.Container programming model is a runtimeenvironment in which componentimplementations use their enclosing

containers to access various services thecontainer provides.

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Four areas make up CCM container programming model:

1 . External typesinterfaces seen by client wanting to communicate withcomponent

2. Container typesAPI component uses to communicate with runtimecontainer

3. Container Implementation typesdifferent containers have different relationships withsurrounding system. Three types: stateless ,conversational , and durable . Each type defines itssystem support

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Using a containers internal interfaces, acomponent has full access to services acontainer supports Container defines APIs for:

component securitypersistencetransactionseventslifecycle

Component implements call-back interfaces

to allo w container-to-component

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Components can be either transient, or

can only be created by factories

do not have primary keys

persistentcan be created by factories and located by findershave primary keyssupport t wo forms of persistence container managed component managed

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EJBs and CORBAcomponents(cont.)C o m p o n e n t Ty p e 0 e sc r ip t io n

S e r v i c e 1 o e s n o t m a i n t a i n s t a t e i n2

o r m a t i o n ( c o m p l e t e l y s t a t e l e s s )1 o e s n o t h a v e a u n i q u e i d ( p r i m a r y k e y )

Im p le m e n t s n e e d e d b e h a v i o r ( e . g . , c a l c u l a t e I n t e r e s t ,a d d I t e m T o S h o p p i n g C a r t , e t c . )

C a n u s e t r a n s a c ti o n s , i s n o t i n c lu d e d i n t h e c u r r e n tt r a n s a c t i o n

S e s s i o n M a i n t a in s i n t e r n a l -s t a t e i n 3 o r m a t i o n4 a s a u n i q u e i d t h a t i s u s a b l e o n l y b y i t s c o n t a i n e r

Im p le m e n t s n e e d e d b e h a v i o r C a n u s e t r a n sa c t i o n s , b u t i s n o t i n c lu d e d i n t h e c u r r e n t t r a n s a c t i o n M a p s t o S e s s i o n 5 J B s

6

n t i t y C o n t a i n e r - o r c o m p o n e n t - m a n a g e d p e r s i s t e n t s t a t e4 a s a u n i q u e i d ( p r i m a r y k e y )

Im p le m e n t s n e e d e d b e h a v i o r t h a t i s o p t i o n a l l y t r a n s a c t i o n a l M a p s t o 5 n t i t y 5 J B s

7

r o c e s s C o n t a i n e r - m a n a g e d o r c o m p o n e n t - m a n a g e d p e r s i s t e n t s t a te t h a t i s i n a c c e s s i b l e t o c l i e n t s

C o n t a i n e r - m a n a g e d o r c o m p o n e n t - m a n a g e d p e r s i s t e n c e o f t h e c o m p o n e n t s p r i m a r y k e y ( i d e n t i t y ) 8 i t h v i s i b i l i t y o f t h e p r i m a r y k e y t h r o u g h u s e r - d e f i n e dm e t h o d s

Im p le m e n t s n e e d e d b e h a v i o r a n d t h e b e h a v i o r is o p t i o n a l l y t r a n s a c t i o n a l

F i . . 5 C O R B A c 9 @ A 9 B C B t t y p e s D B E E e sc r i p t i 9 B s.

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Activation and passivation are actionsaround invocation boundaries of anobject operation.

activation lifecycle1 . request arrives to ORB to invoke operation on object2. ORB sends request to POA3. POA sends request to container managing component

4. container activates object

passivation policies defined by componentsdeployment information

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In distributed systems, clients dont createremote objects. Clients discover remoteobjects. discovery through file containing objects IOR discovery through Naming Service

Factories ( ComponentHome instances) of remote systems create objects for their corresponding systems. component definition must define component

factory

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CORBAcomponents use subset of CORBAservices for Component ImplementationFrame work.

Security Serviceevery component may have o wn security requirement

defined in components deployment descriptor

container must keep track of active policies and apply them

Object Transaction Service (lite version)container can control transaction boundariescomponent can control transaction boundaries

Persistent State Servicecontainer-managed persistence, PSS transparent to

component

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Notification Serviceaccessed indirectlycontainer mediates use of event service by

componentcontainer does not preclude direct use of Notification Service

component developers are encouraged to define eventsin IDL as way of keeping components functional

description in one locationkeywords:

publishes any number of consumers can subscribe to an event methods declared with publishes expected to be part

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CCM and EJB models are similar.CCM specification defines t wo componentlevels basic

mirrors EJB model almost exactlysingle threaded

uses security, transaction and persistence servicesonlyexposes only single interface

extended

can expose multiple interfaces

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CORBA vs. RMI

CORBA is comprehensive vie w of distributed systems architecture.RMI describes only communicationproxies and protocols bet ween clientobject and server object.

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When to Use RMI

RMI suitable for smaller distributedapplications in which scalability,architecture, heterogeneity, andextensibility are not major concerns.Mapping of RMI and RMI capabilities toOMA encompasses only Applications

Objects and the ORB.RMI does not define Quality of Service or asynchronous invocations.

RMI does not offer POA or range of

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When to Use CORBA

CORBA defines and providesimplementations to many commonrequirements for most complex distributedsystems: architecture Quality of Service (QoS)

Scalability Heterogeneity Extensibility

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RMI-IIOP

SUN and IBM implemented RMI-over-IIO P (RMI-IIOP ) to replace RMIs underlyingcommunication protocol ( J ava Remote Method P

rotocol or J

RM P ).

Inprise, Netscape, Oracle, Sun and IBMspecified reverse mapping of Java-to IDL. Mapping limitations:

Constants can be of primitive types or of classjava.lang.String onlyIDL normally does not support overloading of method names.A class cannot inherit a method with same signature fromtwo different interfaces.

Interfaces and value types must be public .

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RMI-IIOP (cont.)

runtime limitations:sending a tree graph from ORB to ORB may beproblematic if multiple nodes point to one object

CORBA does not define distributed garbagecollectionCasting stubs may not work properly, so using thestatic method narrow of classjava.rmi.PortableRemoteObject isencouraged.RMI downloads the stubs needed by client,CORBA does not.

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RMI-IIOP (cont.)

steps involved in writing distributedapplication using RMI-IIOP:

use javax.rmi.PortableRemoteObject

instead of usingjava.rmi.UnicastRemoteObjectuse JNDI ( J ava Naming and Directory Interface)instead of RMI Registrydo not do wncast remote objects to subclass types;use method narrow of classPortableRemoteObject to cast distributedobjects as subclass types

RMIMessenger Case Study

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RMIMessenger Case StudyPorted to RMI-IIOP

Porting RMI messenger example(Chapter 1 3) to RMI-IIOP is easier thanporting application to CORBA. remote interfaces remain same support classes remain same ne w implementations for

ChatServer remote interface(ChatServerImpl ),ChatServerAdministrator

MessageManager interface

(RMIIIOPMessageManager )

ChatServer RMI-IIOP

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ChatServer RMI-IIOPImplementation

ChatServerImpl implements ChatServer remote interface

subclass of class

javax.rmi.PortableRemoteObject does not implement method register handles registration with name services

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Future Directions

OMA guiding document for CORBAsystems falls short in average-size systems

CORBA Component Model abstracts system issues through programming

constructs:

containerssubsystems

CORBA working on architectural process

new corba

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