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Chapter 17

CORBA case study

17.1 Introduction

17.2 CORBA RMI Client and server example

Architecture

CORBA IDL

CORBA object references

17.3 CORBA Services

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Introduction to CORBA

The Object Management Group (OMG) was formedin 1989. Its aims were: to make better use of distributed systems

to use object-oriented programming

to allow objects in different programming languages to communicate

with one another

The object request broker (ORB) enables clients to

invoke methods in a remote object

CORBA is a specification of an architecture

supporting this. CORBA 1 in 1990 and CORBA 2 in 1996.

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Introduction to CORBA

The main components of CORBAs RMI framework are:1. An interface definition language known as IDL.

2. An architecture.

3. The General Inter-ORB protocol (GIOP) defines an external data representation, called CDR

specifies formats for the messages in a request-reply protocol.

including messages for enquiring about the location of anobject, for cancelling requests and for reporting errors.

1. The Internet Inter-ORB protocol (IIOP) defines a standardform for remote object references.

IIOP is GIOP implemented in TCP/IP

CORBA services - generic services useful in distributedapplications e.g. Naming Service, Event Service.

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CORBA RMI

CORBA RMI is a multi-language RMI system.

The programmer needs to learn the following new concepts: the object model offered by CORBA;

the interface definition language and its mapping onto the implementationlanguage.

CORBA's object model similar to the remote object model in Chapter 5 clients are not necessarily objects a client can be any program that sends

request messages to remote objects and receives replies.

The term CORBA objectis used to refer to remote objects. a CORBA objectimplements an IDL interface, has a remote object reference

and its methods can be invoked remotely. A CORBA object can be implemented by a language without classes.

the class concept does not exist in CORBA.

therefore classes cannot be defined in CORBA IDL, which means thatinstances of classes cannot be passed as arguments.

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CORBA IDL interfaces Shape and ShapeList

struct Rectangle{

long width;long height;

long x;

long y;

} ;

struct GraphicalObject {

string type;Rectangle enclosing;

boolean isFilled;

};

interface Shape {

long getVersion() ;GraphicalObject getAllState() ; // returns state of the GraphicalObject

};

typedef sequence All;

interface ShapeList {

exception FullException{ };

Shape newShape(in GraphicalObject g) raises (FullException);All allShapes(); // returns sequence of remote object references

long getVersion() ;

};

Figure 17.1

an interface specifies a name and a set of methods

interface ShapeList

the parameter ofnewShape is an in parameterand

of type Graphical Object The return value is an

extra outparameter of type Shape.

sequences and arrays in typedefs

Exceptions defined by raises and set

by throw. They can have arguments.

this structis used as a parameter or result

type in methods in the remote interfaces.

this structis used in

defining anotherstruct.

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Parameters in CORBA IDL

Passing CORBA objects: Any parameter or return value whose type is specified by the name of a IDL

interface, e.g. Shape, is a reference to a CORBA object (see newShape)

and the value of a remote object reference is passed.

Passing CORBA primitive and constructed types: Arguments of primitive and constructed types are copied and passed by value.

On arrival, a new value is created in the recipients process. E.g., the structGraphicalObject(argument ofnewShape and result ofgetAllState)

Note: the method allShapes returns an array of remote objectreferences as follows:


All allShapes();

Type Object- is a supertype of all IDL interfaces (its values areobject references).

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CORBA Naming Service

It is a binder that provides methods including rebindfor servers to register the remote object references of CORBA objects

by name (e.g. rebind(path, Object)

resolve for clients to look them up by name.(e.g.Object= resolve(path))

The names are structured in a hierarchy,

a path is an array ofNameComponent(a structwith a name in it) the path starts from an initial context provided by CORBA

This makes access in a simple example seem rather complex!

The name service is present in all CORBA installations. (Its

role is like the Java RMI registry)

Its use will be shown in program examples

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CORBA pseudo objects

Implementation of CORBA provides some interfaces to thefunctionality of the ORB that programmers need to use. They

are called pseudo objects.

Because they can not be used like CORBA objects.

ORB is the name of an interface. It includes:

The method init, which must be called to initialize the ORB

The method connect, which is used to register CORBA objects with

the ORB

Other methods which enable conversions between remote object

references and strings.

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CORBA client and server example

Illustration of CORBA with a Java client and server The interface compiler is called idltojava,when given an IDL

interface, it produces: the equivalent Java interfaces (e.g. ShapeListbelow)

server skeletons for each idl interface (e.g. _ShapeListImplBase)

proxy classes or client stubs (e.g. _ShapeListStub) a Java class for each structe.g. Rectangle, GraphicalObject

helper classes (one for each of the types defined in the IDL interface)

and holder classes (forout and inoutarguments which can not be

directly map on to java)

public interface ShapeList extends org.omg.CORBA.Object {Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException;

Shape[] allShapes();

int getVersion();

}

Figure

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The ShapeListServantclass of the Java server program for the

CORBA interface ShapeList.

import org.omg.CORBA.*;

class ShapeListServant extends _ShapeListImplBase {

ORB theOrb;

private Shape theList[];

private int version;

private static int n=0;

public ShapeListServant(ORB orb){

theOrb = orb;

// initialize the other instance variables

}

public Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException {

version++;

Shape s = new ShapeServant( g, version);

if(n >=100) throw new ShapeListPackage.FullException();

theList[n++] = s;

theOrb.connect(s);return s;

}

public Shape[] allShapes(){ ... }

public int getVersion() { ... }

}

A Java server has classes for its

IDL interfaces (e.g. Shape andShapeList). Here is the classShapeListServant

a servant class extends the corresponding

skeleton class (e.g. ShapeListImplBase)

a servant class implements the methods in theinterface (ShapeList).

newShape is a factory method. It creates new

CORBA objects. It uses the connectmethod to

register it with the ORB. (it has a remote object

reference)

CORBA objects are instances of servant classes.In non-OO languages implementations of CORBA

objects cant be classes.

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Java class ShapeListServer(the server class)

import org.omg.CosNaming.*;

import org.omg.CosNaming.NamingContextPackage.*;import org.omg.CORBA.*;

public class ShapeListServer {

public static void main(String args[]) {

try{

ORB orb = ORB.init(args, null);

ShapeListServant shapeRef = new ShapeListServant(orb);

orb.connect(shapeRef);

org.omg.CORBA.Object objRef =

orb.resolve_initial_references("NameService");

NamingContext ncRef = NamingContextHelper.narrow(objRef);

NameComponent nc = new NameComponent("ShapeList", "");

NameComponent path[] = {nc};

ncRef.rebind(path, shapeRef);java.lang.Object sync = new java.lang.Object();

synchronized (sync) { sync.wait();}

} catch (Exception e) { ... }

}

}

Figure

The server class contains the main method

it creates and initialises the ORB

it creates an instance ofShapeListServantclass - a Java

object - which is made a CORBA object

by using the connectmethod to register it with the ORB

it waits for client requests

1. it gets a reference to the Naming Service, it

first get a root naming context2. narrows it to NamingContext- from Object

3. makes a NameComponentcontaining the

name ShapeList

4. makes a path

5. uses rebindto register the name and object

reference

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Java client program for CORBA interfaces Shape andShapeList

import org.omg.CosNaming.*;

import org.omg.CosNaming.NamingContextPackage.*;

import org.omg.CORBA.*;

public class ShapeListClient{

public static void main(String args[]) {

try{

ORB orb = ORB.init(args, null);

org.omg.CORBA.Object objRef =orb.resolve_initial_references("NameService");

NamingContext ncRef = NamingContextHelper.narrow(objRef);

NameComponent nc = new NameComponent("ShapeList", "");

NameComponent path [] = { nc };

ShapeList shapeListRef =

ShapeListHelper.narrow(ncRef.resolve(path));Shape[] sList = shapeListRef.allShapes();

GraphicalObject g = sList[0].getAllState();

} catch(org.omg.CORBA.SystemException e) {...}

} Figure

it creates and initialises an ORB

1. it contacts the NamingService for initial context2. Narrows it to NamingContext

3. It makes a name component

4. It makes a path

5. It gets a reference to the CORBA object called

ShapeList, using resolve and narrows it

it invokes the allShapes method in the CORBA object to get an array

containing remote references to all of the GraphicalObjects currently

stored by the server

it uses one of the remote references in the array to

invoke the getAllState method in the corresponding

CORBA object whose type is Shape

the value returned is of type GraphicalObject

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The main components of the CORBA architecture

clientserver

proxy

or dynamic invocation

implementation

repository objectadapter

ORBORB

skeleton

or dynamic skeleton

clientprogram

interface

repository

Request

Replycorecorefor A

ServantA

The CORBA architecture is designed to allow clientsto invoke methods in CORBA objects clients and objects can be implemented in a variety of programming

languages

it has the following additional components compared to Figure 5.6 object adapter, implementation repository and interface repository

Figure

ORB coreThe role of the ORB core is similar to that of the communication module of Figure 5.6.

In addition, an ORB core provides an interface that includes the following:

- operations enabling it to be started and stopped;

- operations to convert between remote object references and strings;

- operations to provide argument lists for requests using dynamic invocation.

Dynamic invocation interfaceIn some applications (e.g. browsers), a client without the appropriate

proxy class may need to invoke a method in a remote object.

CORBA does not allow classes for proxies to be downloaded at run time

as in Java RMI.

The dynamic invocation interface is CORBAs alternative. (we will

discuss it later with the Interface Repository)

Object adapteran object adapter bridges the gap between

CORBA objects with IDL interfaces and

the programming language interfaces of the corresponding servant classes.

it does the work of the remote reference and despatcher modules in Fig. 5.6

more about the object adapter later.

Skeletons

skeleton classes (for OO languages) are generated in thelanguage of the server by an IDL compiler.

remote method invocations are dispatched via the appropriateskeleton to a particular servant,

the skeleton unmarshals the arguments in request messagesand marshals exceptions and results in reply messages.

Client stubs/proxies

these are in the client language. an IDL compiler for the client language uses an IDL interface togenerate one of the following:

for object-oriented languages the class of a proxy

for procedural languages a set of stub procedures.

as before, the client stubs/proxies marshal the arguments ininvocation requests and unmarshal exceptions and results in replies.

Implementation repository

activates registered servers on demand and locates running servers

uses the object adapter name to register and activate servers.

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

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Object adapter

an object adapter bridges the gap between

CORBA objects with IDL interfaces and the programming language interfaces of the corresponding servant (classes).

it does the work of the remote reference and despatcher modules in RPC.

An object adapter has the following tasks: it creates remote object references for CORBA objects;

it dispatches each RMI via a skeleton to the appropriate servant;

it activates objects.

An object adapter gives each CORBA object a unique objectname. the same name is used each time an object is activated.

it is specified by the application program or generated by the object adapter.

Each object adapter has its own name - specified by theapplication program or generated automatically.

CORBA 2.2 standard for object adapters is called POA.

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Implementation repository

Implementation repository it activates registered servers on demand and locates running servers

it uses the object adapter name to register and activate servers.

it stores a mapping from the names of object adapters to the

pathnames of files containing object implementations.

when a server program is installed it can be registered with the

implementation repository.

when an object implementation is activated in a server, the hostname and

port number of the server are added to the mapping.

Implementation repository entry:

object adapter

name

pathname of object

implementation

hostname and port number

of server

- not all CORBA objects (e.g. call backs) need be activated on demand

- access control information can be stored in an implementation repository

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Interface repository

it provides information about registered IDL interfaces for an interface of a given type it can supply the names of the methods and foreach method, the names and types of the arguments and exceptions.

a facility for reflection in CORBA.

if a client has a remote reference to a CORBA object, it can ask the interfacerepository about its methods and their parameter types

the client can use the dynamic invocation interface to construct an invocationwith suitable arguments and send it to the server.

the IDL compiler gives a type identifier to each IDL type

a type identifier is included in remote object references

this type identifier is called the repository ID

because the interface repository stoes interfaces against their IDs applications that use static invocation with client proxies and

IDL skeletons do not require an interface repository. Not all ORBs provide an interface repository.

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CORBA IDL

IDL provides facilities for defining modules,interfaces, types, attributes and method signatures. examples of all of the above, except modules, is shown in Figure IDL

interfaces shape and shape and shapelist.

IDL has the same lexical rules as C++ but has

additional keywords to support distribution, for example interface, any, attribute, in, out, inout, readonly, raises.

It allows standard C++ pre-processing facilities. e.g.

typedefforAll.

The grammar of IDL is a subset of ANSI C++ with

additional constructs to support method signatures.

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IDL module Whiteboard

Modules allow interfaces and IDL type definitions tobe grouped.

A module defines a naming scope, which prevents

names define within a module clashing with names

defined outside it.module Whiteboard {

struct Rectangle{

...} ;

struct GraphicalObject {

...};

interface Shape {...};


interface ShapeList {

...};

};

Figure

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IDL method signatures

[oneway] (parameter1,..., parameterL)[raises (except1,..., exceptN)]

[context (name1,..., nameM)]

each parameter is labelled as in, outorinout, e.g. void getPerson(in string name, out Person p);

onewaye.g. oneway void callback(in int version) the client will not be blocked and maybe semantics is used at-most-once call semantics is the default

The optional raises expression indicated user defineexceptions

The optional context expression is used to supplies mapping

form string names to string values

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Inheritance

IDL interfaces may extend one or more interfaces an extended interface may add new methods, types, constants

and exceptions It may redefine types, constants and exceptions but not

methods

Interface A{};

Interface B: A{};

Interface C{};

Interface Z: B and C{};

Figure

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FigureIDL constructed types 1

Type Examples Use

sequence typedef sequence All;typedef sequence Allbounded and unbounded sequencesof Shapes

Defines a type for a variable-lengthsequence of elements of a specifiedIDL type. An upper bound on thelength may be specified.

string String name;

typedef string SmallString;unbounded and boundedsequences of characters

Defines a sequences of characters,

terminated by the null character. Anupper bound on the length may bespecified.

array typedef octet uniqueId[12];

typedef GraphicalObject GO[10][8]

Defines a type for a multi-dimensionalfixed-length sequence of elements of aspecified IDL type.

Figure

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FigureIDL constructed types 2

Type Examples Userecord struct GraphicalObject {

string type;Rectangle enclosing;boolean isFilled;

};

Defines a type for a record containing agroup of related entities. Structs are

passed by value in arguments andresults.

enumerated enum Rand(Exp, Number, Name);

The enumerated type in IDL maps a

type name onto a small set of integervalues.

union union Exp switch (Rand) {case Exp: string vote;

case Number: long n;case Name: string s;

The IDL discriminated union allowsone of a given set of types to be passedas an argument. The header is

parameterized by an enum, whichspecifies which member is in use.

};

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CORBA remote object references

CORBA 2.0 specifies a format for remote object references that issuitable for use,whether or not the object is activatable. Referencesusing this format are called 'interoperable object references'(IORs).

First field species the type name of the IDL interface of the CORBA object

Second field specifies the transport protocol Third field is used by the ORB to identify a CORBA object

IOR format

IDL interface type name Protocol and address details Object key

interface repositoryidentifier

IIOP host domainname

port number adapter name object name

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CORBA remote object references cont...

Transient IORs are for objects that last as long as the host process

they contain the address of the server hosting the CORBA object The server ORB core receives the request message containing the

object adapter name and object name of the target. It uses the object adapter name to locate the object adapter, which uses the object name to locate the servant.

Persistent IORs last between activations they contain the address of the implementation repository

the implementation repository receives the request and uses theobject adapter name to activate the object, then gives the serveraddress to the client

the client sends subsequent invocations to the server

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CORBA language mappings

Primitive types in IDL are mapped to the correspondingprimitive types in Java

Sequences and arrays in IDL are mapped to arrays in Java

Structs, enums and unions are mapped to Java classes

An IDL expression is mapped to a Java class

IDL allows methods to return several separate values visoutput parameters, where as Java can have only single result.The holder classes are provided to overcome this difficulty

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17.3 CORBA services

Naming Service Event Service and Notification Service Security service Trading service Transaction service and concurrency control service

Persistent object service

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CORBA services cont...

Naming Service It allows names to be bound to the remote object references of

CORBA objects within naming context

In fig CORBA objects are shown in the normal way, but namingcontexts are shown as plain ovals

Initial naming contexts provides a root for a set of bindings

Each name is consist of two strings, one for the name other for thekind of the object

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Naming Service cont...


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CORBA Event service: Defines interfaces allowing objects of interest, calledsuppliers to communicate notification to subscribers, calledconsumers

Notification are communicated as arguments or results

Notification may be propagated either by being pushed bysupplier to the customer or pulled by the consumer bysuppliers.

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CORBA Notification service: Extent the CORBA Event Service retaining all of it's features It includes a filtering events that specify exactly which

events they are interested

Notification may be define as data structure

Event subscriber are provided with a mean of discoveringthe events the consumers are interested in

Event consumers can discovered the event type offered bythe suppliers by the channel

Possible to configure the properties of a channel, a proxy or

a particular eventAn event type repository is an optional extra

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CORBA Security service: Authentication of principals (users and servers);

generating credential for principals (i.e. certificatestating their rights)

Access control can be applied to CORBA objectswhen they receive remote method invocations.

Auditing by servers of remote method invocations

Facilities for non-repudiation.

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Trading service: allows CORBA objects to be located by attribute Attribute is a name value pair

The service type is a name

It's database contains a mapping from service types and theirassociated attributes onto remote object references of CORBA objects

Trading servers can perform quires on behalf of one another's clients.

Transaction service and concurrency control service TS provides flat or nested transactions

Client specifies a transaction as a sequence of RMI calls

Which are introduce by begin and terminated by commit or rollback

(abort). ORB attaches a transaction identifier to each remote invocation

CCS provides locking of CORBA objects

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Persistent object service: for storing the state of CORBA objects in a passive form andretrieving it

The CORBA POS is intended to be suitable for use as an persistentobject store as an CORBA object.

Each persistent object has an identifier which includes the identity of

its data store and object number of that store. The implementation of persistent CORBA object must choose a

protocol for communicating with data store

E. g. The direct access protocol allows CORBA objects to access theattributes of persistent objects in the data store.

ch 17. corba

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