CS 843 - Distributed Computing Systems Chapter 17: CORBA Chin-Chih Chang, [email protected] From...

CS 843 - Distributed Computing Systems

Chapter 17: CORBAChin-Chih Chang, [email protected]

From Coulouris, Dollimore and Kindberg

Distributed Systems: Concepts and Design

Edition 3, © Addison-Wesley 2001

Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3

© Addison-Wesley Publishers 2000

Introduction to CORBA

• The Object Management Group (OMG) was formed in 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 (Common Object Request Broker Architecture) is a specification of an architecture supporting this. • CORBA 1 in 1990 and CORBA 2 in 1996.

http://www.omg.org/

http://www.corba.org/



CORBA RMI

• The main components of CORBA’s RMI framework are:1. An interface definition language known as IDL.2. An architecture.3. The General Inter-ORB protocol (GIOP) defines

o an external data representation, called CDRo specifies formats for the messages in a request-reply

protocol including messages for enquiring about the location of an object, for cancelling requests, and for reporting errors.

4. The Internet Inter-ORB protocol (IIOP) defines a standard form for remote object references. • IIOP is GIOP implemented in TCP/IP

•



CORBA RMI

• CORBA services are generic services useful in distributed applications e.g. Naming Service, Event Service.

• 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

implementation language. (e.g. a struct in IDL is mapped onto what in Java?)

•



CORBA Object Model

• The CORBA object has a 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 object is used to refer to remote objects. A CORBA object implements an IDL interface. It has a remote object reference.

Its methods can be invoked remotely.

•



CORBA Object Model

• 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 that instances of classes cannot be passed as arguments.

•



CORBA IDL

• A CORBA interface specifies a name and a set of methods that clients can request (Figure 17.1). Interfaces are used to specify the methods that clients

can request. Strucs are used to define the data structure used as

parameter types in defining the methods.

•



Figure 17.1 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 <Shape, 100> All; interface ShapeList {

exception FullException{ }; Shape newShape(in GraphicalObject g) raises (FullException);All allShapes(); // returns sequence of remote object referenceslong getVersion() ;

};

Figure 17.1

an interface specifies a name and a set of methods

interface ShapeList

the parameter of newShape is an in parameter and of type Graphical Object The return value is an extra out parameter of type Shape. No classes can be passed as arguments or results

sequences and arrays in typedefs

Exceptions defined by raises and set by throw. They can have arguments.

•

this struct is used as a parameter or result type in methods in the remote interfaces.

this struct is used in defining another struct.

Differences from a Java remote inteface?

why are argument types defined in the IDL?(not necerssary in Java remote interfaces)



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)

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 recipient’s process. E.g., the struct GraphicalObject (argument of newShape and result of getAllState)

•




• Note: the method allShapes returns an array of remote object references as follows:

typedef sequence <Shape, 100> All;

All allShapes();

• Type Object is a supertype of all IDL interfaces (its values are object references). When would it be useful? Hint: Think about the name server

• CORBA IDL allows exceptions to be defined in interfaces and thrown by their methods.

•




• Implementations of CORBA provide some interfaces to the functionality of the ORB called pseudo-objects.

• ORB is the name of an interface that represents the functionality of the ORB that programmers need to access: 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.

•




• Remote invocation in CORBA: At-most-once call semantics is the default. Maybe semantics by using the oneway keyword. This is

only used for methods without results.

• The CORBA Naming Service is a binder that provides methods including rebind for servers to register the remote object references

of CORBA objects by name (e.g. rebind (path, Object) e.g of 2nd argument?

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

resolve(path))

•



CORBA Pseudo Objects

• ORB is the name of an interface that represents the functionality of the ORB that programmers need to use. It includes: The method init, which must be called to initialise 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.

•



CORBA Client and Server Example

• The Hello Client-Server Example.

http://java.sun.com/j2se/1.3/docs/guide/idl/GShome.html



Figure 17.2Java interface ShapeList generated by idltojava from CORBA interface ShapeList

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

}



CORBA Naming Service

• It is a binder that provides methods including rebind for servers to register the remote object references of CORBA

objects by name (e.g. rebind (path, Object) e.g of 2nd argument? resolve for clients to look them up by name.(e.g.Object = resolve(path)) these methods belong to an interface called NamingContext (Fig

17.10)

• The names are structured in a hierarchy, a path is an array of NameComponent (a struct with 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. (It’s role is like the Java RMI registry)

• Its use will be shown in program examples

•



Illustration of programming CORBA

• We illustrate CORBA with a Java client and server• The interface compiler is called idltojava

when given an IDL interface, it produceso server skeletons for each class (e.g. _ShapeListImplBase)

o proxy classes (e.g. _ShapeListStub)

o a Java class for each struct e.g. Rectangle, GraphicalObject

o helper classes (narrow method) and holder classes (for out arguments)

o the equivalent Java interfaces (e.g. ShapeList below)

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

}Figure 17.2

•What is the problem with out arguments in Java?e.g. void getPerson(in string name, out Person p);

Consider resolve in the naming service, we ask it to look upa reference to ShapeList, it returns an Object. What is the problem?



The ShapeListServant class of the Java server program for the CORBA interface ShapeList. Figure 17.3

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 and ShapeList). Here is the class ShapeListServant

a servant class extends the corresponding skeleton class (e.g. ShapeListImplBase)

a servant class implements the methods in the interface (ShapeList). newShape is a factory method. It creates new CORBA objects. It uses the connect method to inform the ORB about the new CORBA object. (it has a remote reference module)

CORBA objects are instances of servant classes.In non-OO languages implementations of CORBA objects can’t be classes. What might they be in C?

•

This class has to create CORBA objects of type Shape. How does it do that?



Figure 17.4 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 17.4

The server class contains the main method

it creates and initialises the ORB

it creates an instance of ShapeListServant class - a Java object - which is made a CORBA object by using the connect method to register it with the ORB

it waits for client requests

•

1. it gets a reference to the Naming Service 2. narrows it to NamingContext- from Object 3. makes a NameComponent containing the

name “ShapeList” 4. makes a path

5. uses rebind to register the name and object reference



Figure 17.5 Java client program for CORBA interfaces Shape and ShapeList

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 17.5

it creates and initialises an ORB

1. it contacts the NamingService for initial context2. Narrows it to NamingContext3. It makes a name component4. It makes a path5. 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 Shapethe value returned is of type GraphicalObject

•



CORBA ORB Architecture (17.2.2)



CORBA ORB Architecture

• Object - This is a CORBA programming entity that consists of an identity, an interface, and an implementation, which is known as a Servant.

• Servant - This is an implementation programming language entity that defines the operations that support a CORBA IDL interface. Servants can be written in a variety of languages, including C, C++, Java, Smalltalk, and Ada.

•




• Client This is the program entity that invokes an operation on an

object implementation. Accessing the services of a remote object should be

transparent to the caller. Ideally, it should be as simple as calling a method on an object, i.e., obj->op(args). The remaining components in the Figure help to support this level of transparency.

•




• Object Request Broker (ORB): The ORB provides a mechanism for transparently

communicating client requests to target object implementations.

The ORB simplifies distributed programming by decoupling the client from the details of the method invocations.

This makes client requests appear to be local procedure calls. When a client invokes an operation, the ORB is responsible for finding the object implementation, transparently activating it if necessary, delivering the request to the object, and returning any response to the caller.

•




• ORB Interface An ORB is a logical entity that may be implemented in

various ways (such as one or more processes or a set of libraries).

To decouple applications from implementation details, the CORBA specification defines an abstract interface for an ORB.

This interface provides various helper functions such as converting object references to strings and vice versa, and creating argument lists for requests made through the dynamic invocation interface described below.

•




• Dynamic Invocation Interface (DII) This interface allows a client to directly access the

underlying request mechanisms provided by an ORB. Applications use the DII to dynamically issue requests to

objects without requiring IDL interface-specific stubs to be linked in.

Unlike IDL stubs (which only allow RPC-style requests), the DII also allows clients to make non-blocking deferred synchronous (separate send and receive operations) and oneway (send-only) calls.

•




• Dynamic Skeleton Interface (DSI) This is the server side's analogue to the client side's DII. The DSI allows an ORB to deliver requests to an object

implementation that does not have compile-time knowledge of the type of the object it is implementing.

The client making the request has no idea whether the implementation is using the type-specific IDL skeletons or is using the dynamic skeletons.

•




• Object Adapter This assists the ORB with delivering requests to the object

and with activating the object. More importantly, an object adapter associates object implementations with the ORB.

Object adapters can be specialized to provide support for certain object implementation styles (such as OODB object adapters for persistence and library object adapters for non-remote objects).

•



The Architecture of CORBA (Textbook)

• CORBA makes the distinction between static and dynamic invocations. Static invocations are used when the remote interface of

the CORBA object is known at compile time, enabling client stubs and server skeletons to be used.

If the remote interface is not known at compile time, dynamic invocation must be used.

Most programmers prefer to use static invocation because it provides a more natural programming model.



The Architecture of CORBA

• CORBA Primary components in CORBA: ORB core Object adapter Skeletons Client stubs/proxies

• ORB core is the communication module for the CORBA object and provides an interface including: 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.



Object Request Broker (ORB)

• On client side the ORB is responsible for accepting requests for a remote object finding implementation of the object accepting client-side reference to the remote object(converted to a

language specific form, e.g., a Java stub object) routing client method calls through the object reference to the object

implementation

• On server side the ORB lets object servers register new objects receives requests from the client ORB uses object’s skeleton interface to invoke object’s activation method creates reference for new object and sends it back to client



Figure 17.6The main components of the CORBA architecture

client server

proxy

or dynamic invocation

implementation repository object

adapter

ORBORB

skeleton

or dynamic skeleton

client program

interface repository

Request

Replycorecore for A

Servant A



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 dispatcher modules in

Fig. 5.6.

• An object adapter has the following tasks: it creates remote object references for CORBA objects; Each object adapter provides access to those services of an ORB

(such as activation, deactivation, object creation, object reference management) used by a particular type of object implementation.

o it dispatches each RMI via a skeleton to the appropriate servant;o it activates objects.

•



Object Adapter

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

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

Each active CORBA object is registered with its object adapter,

o which keeps a remote object table to maps names of CORBA objects to servants.

• Each object adapter has its own name - specified by the application program or generated automatically.

•



The Architecture of CORBA (Textbook)

• Skeleton classes are generated in the language of the server by an IDL compiler. Remote invocations are dispatched via the appropriate skeleton to a particular servant.

• The class of a proxy or a set of stub procedures is generated from an IDL interface by an IDL compiler for the client language.

•



Static vs. Dynamic Invocation

• Static Invocation Static interfaces are generated in form of client stubs by the IDL (pre-)

compiler. This means that the structure of the object has to be known before

hand (at compile time). Allows for better type checking; less runtime overhead; self-

documentation.

• Dynamic Invocation Dynamic Invocation Interface (DII) allows clients to invoke operations

on remote objects without having access to object stubs (another way to do this without dynamic invocation is to download static client stubs via a Java applet).

Clients must discover interface-related information at runtime (e.g., using the interface repository)

Servers can offer new services anytime without the need for recompilation on the client side.

•



Dynamic Requests

• The Dynamic Invocation Interface (DII) allows clients to dynamically: discover objects; discover objects’ interfaces; create requests; invoke requests; receive responses.

• Major features of Dynamic Invocation Interface: requests appear as objects themselves; requests are reusable; invocation may be synchronous or asynchronous; requests may be generated dynamically, statically or in combination

approach.

•



Implementation repository

• Implementation repository (Server) 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. o when a server program is installed it can be registered

with the implementation repository.o when an object implementation is activated in a server,

the hostname and port number of the server are added to the mapping.

•



Implementation repository

• Implementation repository Implementation repository entry: Not all CORBA objects (e.g. call backs) need be activated

on demand Access control information can be stored in an

implementation repository

object adapter name

pathname of object implementation

hostname and port number of server

•



Interface Repository (Client)

• It provides information about registered IDL interfaces For an interface of a given type it can supply the names of

the methods and for each 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 interface repository about its methods and their parameter types

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

•



Interface Repository

• 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.

•



IDL InterfaceDefinitions

ImplementationInstallation

ClientStubs

InterfaceRepository

ImplementationRepositoryImplementation

Skeletons

Client Object Implementation

Accesses Includes DescribesIncludes

Summary of CORBA Interfaces

• All objects are defined in IDL by specifying their interfaces.• Object definitions (interfaces) are manifested as objects in the

Interface Repository, as client stubs, and as implementation skeletons.

• Descriptions of object implementations are maintained as objects in the Implementation Repository



OMG IDL

• OMG Interface Definition Language (IDL): 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

o a namespace Interface

o abstract typeo multiple inheritance

Structo structured data

Module auction { exception NotAllowed {};

struct Sale { int price; string item; }

interface Auction { void bid (in long price) raises NotAllowed; }}

Module auction { exception NotAllowed {};

struct Sale { int price; string item; }

interface Auction { void bid (in long price) raises NotAllowed; }}



CORBA IDL (17.2.3)

• IDL provides facilities for defining modules, interfaces, types, attributes and method signatures. Examples of all of the above, except modules, in Figures

5.2 and 17.1.

• 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. typedef for All in Figure 17.7.

• The grammar of IDL is a subset of ANSI C++ with additional constructs to support method signatures.

•



IDL module Whiteboard

• Modules allow interfaces and associated definitions to be grouped.

• A module defines a naming scope.

module Whiteboard {struct Rectangle{...} ;struct GraphicalObject {...};interface Shape {...};typedef sequence <Shape, 100> All;interface ShapeList {...};

};

Figure 17.7

•



Figure 17.7IDL module Whiteboard

module Whiteboard {struct Rectangle{...} ;struct GraphicalObject {...};interface Shape {...};typedef sequence <Shape, 100> All;interface ShapeList {...};

};



IDL method signatures

[oneway] <return_type> <method_name> (parameter1,..., parameterL) [raises (except1,..., exceptN)]

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

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

• oneway e.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

•

we saw raises in the newShape method of ShapeList



IDL method signatures

• Inheritance - IDL interfaces may extend one or more interfaces all IDL interfaces are compatible with Object

o ee can use type Object for parameters that may be of any type e.g. bind and resolve in the Naming Service

an extended interface may add new methods, types, constants and exceptions

It may redefine types, constants and exceptions but not methods

•

we saw raises in the newShape method of ShapeList



Figure 17.8IDL constructed types – 1

Type Examples Use

sequence typedef sequence <Shape, 100> All;typedef sequence <Shape> Allbounded and unbounded sequencesof Shapes

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

string String name; typedef string<8> 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.

this figure continues on the next slide

•See Fig 5.1 for an example of string



Figure 17.8 IDL constructed types – 2

Type Examples Use

record struct GraphicalObject {

string type; Rectangle enclosing; boolean isFilled;

};

Defines a type for a record containing agroup of related entities. Structs arepassed by value in arguments andresults.

enumerated enum Rand (Exp, Number, Name);

The enumerated type in IDL maps atype 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 isparameterized by an enum, which specifies which member is in use. };

•



Internet Inter-Orb Protocol (IIOP)

• CORBA specification is neutral with respect to network protocols the CORBA standard specifies what is known as the General Inter-ORB Protocol

(GIOP) GIOP is a high-level standard protocol for communication between ORBs not used directly; instead, it is specialized by a particular protocol that would

then be used directly

• Internet Inter-ORB Protocol (IIOP) IIOP is the GIOP-based protocol for TCP/IP networks As of the 2.0 version of the CORBA specification, vendors are required to

implement the IIOP protocol

• CORBA Networking Model CORBA applications are built on top of GIOP-derived protocols such as IIOP these protocols, in turn, rest on top of TCP/IP, DCE, or other underlying

transport protocol the network uses an application architecture can be designed to use a bridge that would

interconnect, for instance, DCE-based application components with IIOP-based ones.



CORBA remote object references

• Interoperable Object References (IOR) An IOR is an object reference that is understood by ORBs

that can interoperate using the OMG-defined protocols General Inter-ORB Protocol (GIOP) and Internet Inter-ORB Protocol (IIOP).

A client can obtain an object reference using orb.object_to_string(objRef), as shown in the Browsing the Namespace example, or as a result of an invocation on another object reference.

•

http://java.sun.com/j2se/1.4/docs/guide/idl/jidlNaming.html#example3



17.2.4 CORBA remote object references

• 'interoperable object references' (IORs) – CORBA 2.0 suitable whether or not the object is activatable.

• Transient IORs are for objects that last as long as the host process they contain the address of the server hosting the CORBA

objecto 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.

•



17.2.4 CORBA remote object references

• Persistent IORs last between activations they contain the address of the implementation repository the implementation repository receives the request and

uses the object adapter name to activate the object, then gives the server address to the client

the client sends subsequent invocations to the server

•

IOR format

IDL interface type name Protocol and address details Object key

interface repositoryidentifier

IIOP host domainname

port number adapter name object name

Page 684



OMG Reference Model Architecture




• Object Services These are domain-independent interfaces that are used by

many distributed object programs. For example, a service providing for the discovery of other available services is almost always necessary regardless of the application domain.

Two examples of Object Services that fulfill this role are: object

o The Naming Service -- which allows clients to find objects based on names;

o The Trading Service -- which allows clients to find objects based on their properties.

•




• Common Facilities Like Object Service interfaces, these interfaces are also

horizontally-oriented, but unlike Object Services they are oriented towards end-user applications.

An example of such a facility is the Distributed Document Component Facility (DDCF), a compound document Common Facility based on OpenDoc. DDCF allows for the presentation and interchange of objects based on a document model, for example, facilitating the linking of a spreadsheet object into a report document.

•




• Domain Interfaces These interfaces fill roles similar to Object Services and

Common Facilities but are oriented towards specific application domains.

For example, one of the first OMG RFPs issued for Domain Interfaces is for Product Data Management (PDM) Enablers for the manufacturing domain. Other OMG RFPs will soon be issued in the telecommunications, medical, and financial domains.

•




• Application Interfaces These are interfaces developed specifically for a given

application. Because they are application-specific, and because the OMG does not develop applications (only specifications), these interfaces are not standardized.

However, if over time it appears that certain broadly useful services emerge out of a particular application domain, they might become candidates for future OMG standardization.

•



CORBA services include the following

• Trading service: allows CORBA objects to be located by attribute

• Transaction service and concurrency control service TS provides flat or nested transactions CCS provides locking of CORBA objects

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

and retrieving it

•



CORBA services include the following

• Naming Service (it would be a good idea to study it!)• Event Service and Notification Service:

in ES suppliers and consumers communicate via an event channel

NS extends this to allow filtering and typed events

• Security service: authentication of principals and access control of CORBA

objects with policies auditing by servers, facilities for non-repudiation

•



Figure 17.9Naming graph in CORBA Naming Service

initial naming context

ShapeList

CD E

B


P

R S T

V

Q U


XX



Figure 17.10Part of the CORBA Naming Service NamingContext interface in IDL

struct NameComponent { string id; string kind; };

typedef sequence <NameComponent> Name;

interface NamingContext {void bind (in Name n, in Object obj);

binds the given name and remote object reference in my context.void unbind (in Name n);

removes an existing binding with the given name.void bind_new_context(in Name n);

creates a new naming context and binds it to a given name in my context.Object resolve (in Name n);

looks up the name in my context and returns its remote object reference. void list (in unsigned long how_many, out BindingList bl, out BindingIterator bi);

returns the names in the bindings in my context.};



Figure 17.11CORBA event channels

consumersupplier

proxy consumer

notification

proxy supplier

event channel

notificationnotification



Flat or Nested Transactions

• A flat transaction is a transaction which conform to atomicity and isolation. Atomicity means that a transaction cannot be half-done. Isolation means that each transaction must remain unique.

• A nested transaction is a transaction created within the scope of another transaction. A nested transaction commits with respect to its parent transaction. If

a nested transaction commits, the effects of the commit are not permanent until the parent transaction commits. If the parent transaction aborts, the nested transaction is also aborted.

Changes made within the nested transaction are invisible to the top-level transaction until the nested transaction is committed.

•



Summary

• The main component of CORBA is the Object Request Broker or ORB, which allows clients written in one language to invoke operations in remote objects (called CORBA objects) written in another language.

•



Summary

• CORBA addresses other aspects of heterogeneity as follows: The CORBA General Inter-ORB protocol (GIOP) includes

an external data representation called CDR. o It makes it possible for clients and servers to communicate

irrespective of their hardware.o It also specifies a standard form for remote object references.

GIOP also includes a specification for the operations of a request-reply protocol that can be used irrespective of the underlying operating system.

The Internet Inter-ORB Protocol (IIOP) implements the request-reply protocol over TCP/IP. IIOP remote object references include the domain name and port number of a server.

•



Summary

• A CORBA object implements the operations in an IDL interface. All that clients need to know to access a CORBA object is

the operations available in its interface. The client program accesses CORBA objects via proxies

or stubs, which are generated automatically from their IDL interfaces in the language of the client.

Server skeletons for CORBA objects are generated automatically from their IDL interfaces in the language of the client.

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Summary

• The object adapter is an important component of CORBA servers. Its role is to create remote object references and to relate

remote object references in request messages to the implementations of CORBA objects.

• The CORBA architecture allows CORBA objects to be activated on demand. This is achieved by a component called the

implementation repository (server object database), which keeps a database of implementations indexed by their object adapter names.

When a client invokes a CORBA object, it can be activated if necessary in order to carry out the invocation.

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Summary

• An interface repository is a database of IDL interface definitions indexed by a repository ID, which is included in IORs. An interface repository (client interface database) can be

used to get information about the methods in the interface of a CORBA object to allow dynamic method invocations.

• CORBA services provide functionality above RMI, which may be required by distributed applications, allowing them to use additional services such as naming and directory services, event notifications, transactions or security as required.

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Summary: CORBA Remote Method Invocation

• Clients use “object interfaces” through language mapping Java clients should work on any ORB that supports the Java language

bindings. Clients can call any object instance remotely, so long as the object

instance implements the interface.

• Clients can call remote objects statically or dynamically The server cannot tell whether the client is using static or dynamic

invocation.

• Objects are identified using a unique id: Interoperable Object Reference (IOR) CORBA passes objects by reference IOR was Introduced in CORBA 2.0 Object references can be converted to strings and back to “live”

objects via ORB interface functions.



Steps in Developing CORBA Applications

• Write specification for each object using IDL• Use IDL Compiler (e.g., idlj, idl2java) to generate:

Client Stub code Server Skeleton code

• Write the client (in Java, can be applications or applets)• Write the server object implementation code (the “servant”)• Compile the client and server code• Start the server• Run the client application



Data Types in CORBA IDL

• Basic Types• short, long, unsigned long, unsigned short, float, double, long double, char,

wchar, boolean, string, octet, etc.

• Constructed Types• struct and union (similar to C++; can be used in conjunction with a

typedef)• sequence (variable sized arrays of objects)• any (generic type which represents any possible IDL types; similar to the

Java Object type)• enum (enumerated type with named integer values)• arrays• valuetypes (similar to interfaces; preceded with keyword valuetype to

provide pass-by-value semantics)

• Each CORBA IDL data type gets mapped to a native data type via the appropriate language binding (e.g, IDL-to-Java mapping).




module HelloApp { interface Hello { string sayHello(); }; };



What Does the IDL Compiler Generate?

idlj

Hello.idl

_HelloStub.java

HelloHelper.java HelloHolder.java

_HelloImplBase.java

Hello.java _HelloOperations.java

Client-Side Server-Side

Client Stub Implementation Skeleton

Note: these source files will be a part of the Java package Hello and will be placed ina directory with the same name.




• _HelloImplBase.java - This abstract class is the server skeleton, providing basic CORBA functionality for the server. It implements the Hello.java interface. The server class HelloServant extends _HelloImplBase.

• _HelloStub.java - This class is the client stub, providing CORBA functionality for the client. It implements the Hello.java interface.




• _Hello.java - This signature interface contains the Java version of our IDL interface. The Hello.java interface extends org.omg.CORBA.Object, providing standard CORBA object functionality. It also extends IDLEntity, and is used as the signature type in method declarations when interfaces of the specified type are used in other interfaces.

• HelloHelper.java - This final class provides auxiliary functionality, notably the narrow() method required to cast CORBA object references to their proper types.




• HelloHolder.java - This final class holds a public instance member of type Hello. It provides operations for out and inout arguments, which CORBA allows, but which do not map easily to Java's semantics.

• HelloOperations.java - This operations interface contains the single method sayHello(). The IDL-to-Java mapping puts all of the operations defined on the IDL interface into this file. The operations interface is used in the server-side mapping and as a mechanism for providing optimized calls for co-located clients and servers.

CS 843 - Distributed Computing Systems Chapter 17: CORBA Chin-Chih Chang, [email protected] From...

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Transcript of CS 843 - Distributed Computing Systems Chapter 17: CORBA Chin-Chih Chang, [email protected] From...