CORBA Framework Eelements 1 CORBA Framework Elements Object Request Broker (ORB) This is the...

CORBA Framework Eelements 1

CORBA Framework Elements

Object Request Broker (ORB) This is the object manager in CORBA

Mechanisms for specifying interfaces Interface Definition Language (IDL) - for static interface definitions Dynamic Invocation Interface (DII) - lets clients access interfaces as first-class

objects at run-time from an Interface Repository.

Internet Inter-Orb Protocol (IIOP) A binary protocol for communication between ORBs. Was added in CORBA 2.0


Object Request Broker (ORB)

One standardised interface

One interface per object operation

ORB-dependent interfaceOne interface per object adapter

DynamicInvocation DynamicInvocation

ClientStubsClientStubs

ORBInterface ORBInterface

Implementation SkeletonsImplementation Skeletons

ClientClient Object ImplementationObject Implementation

ORB CoreORB Core

ObjectAdapter ObjectAdapter

There can be more than one object adapters


Object Request Broker (ORB) The Object Manager in CORBA Both on the client side and the server side (allows agents to act

as both clients and servers of remote objects) 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


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.


Internet Inter-Orb Protocol (IIOP)

ORBORB

OLE

RPC

BridgeBridge

ORB

ComponentComponentComponentComponent

ComponentComponent

ComponentComponent

ComponentComponent

ComponentComponent

ComponentComponent


Passing Objects by Reference In a distributed application, there are two possible methods for

an application component to obtain access to an object in another process: When an object is passed by reference, the object itself remains "in place"

while an object reference for that object is passed. Operations on the object through the object reference are actually processed by the object itself.

When an object is passed by value, the object's state is copied and passed to its destination (via object serialization), where a new copy of the object is instantiated. Operations on that object's copy are processed by the copy, not by the original object.

Note: in CORBA, objects are only passed by reference (however, the new CORBA specifications include facilities for passing objects by value).


Object References An Object Reference is the information needed to specify an object

within an ORB. The representation of an object reference handed to a client is only valid for the lifetime

of that client. The language mapping also provides additional ways to access object references in a

typed way for the convenience of the programmer. There is a distinguished object reference, the null reference, guaranteed to be different

from all object references, that denotes no object. In Java, this is a Java null.

To invoke a CORBA object, you need a reference for the object. There are two ways to get a reference for a CORBA object: from another object, such as a factory or a name service

from a string that was specially created from an object reference

Interoperable Object References CORBA uses IOR as a “pointer” to a specific instance of a class in a distributed

environment encodes host, port, object identity may be externalized (using object_to_string)


Object References Lifecycle and Longevity of Object Reference

Object Reference can be created without instantiating any servant object Object Reference outlives the CORBA object to which it refers.

(CORBA::OBJECT_NOT_EXIST meaning the object has been permanently deleted.)

Factory Objects Create objects on remote servers (Example: a customer at a bank needs to create an

Account object when opening a new account.) ‘remote constructor’ Factory design pattern IDL definition:

…Interface Account{

…};Interface AccountFactory {

Account create (in string name, in long initialBalance);Account find (in string name);

};…


Object References Interoperable Object Reference

Structure of an IOR

Repository id identifies the type of object Number indicate the version of the ID interface, and usually is 1.0 Each profile is specific to a particular transport protocol and contains

complete details about the location of an object and how to open a connection to the object

– Make the same object accessible via different protocol

– Multiple profiles can be used as a way of implementing fault tolerance.

“ID:::Foo:1.0” n profile1 Profile n…

RepositoryId Profiles


Object References IIOP

0 1.2 host Optional components

Tag IIOP Version

port object_key


CORBA Components Client stub

Each stub represents (it is a proxy) an object operation (a possible request) which a client invokes in a language-dependent manner (e.g., by calling a subroutine which represents the operation).

The stubs make calls on the rest of the ORB using interfaces that are private to JavaIDL. Alternatively, a client may dynamically construct and invoke request objects which can

represent any object operation.

Implementation Skeleton Each skeleton provides the interface through which a method receives a request (dynamic

and static skeletons)

Object Adapter Purpose is to interface an object's implementation with its ORB 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.

ORB Interface The interface to the small set of ORB operations common to all objects, e.g., the

operation which returns an object's interface type.


ORB and Object Interface ORB Interface

module CORBA {interface ORB {// PIDL string object_to_string(in Object obj); Object string_to_object(in string obj); Object resolve_initial_references(in ObjectId identifier); … ….};interface Object { interfaceDef get_interface();

boolean is_nil(); Object duplicate(); void release(); boolean is_a (in string logical_type_id); boolean non_existent(); boolean is_equivalent(in Object other_object)

…};ORB ORB_init();

};


Duplicate() and release()Text book Chapter 2: figure 2.8 – 2.11


The Portable Object Adapter (POA) The POA defines standard interfaces to do the

following: Map an obj ref to a servant that implements that object Allow transparent activation of objects Associate policy information with objects Make a CORBA object persistent over several server process lifetimes POA interface is locality constrained interface (i.e., references to the

POA cannot be passed outside of a server’s address space). Main functionality: dispatch incoming invocation requests to the

correct servant There can be multiple POAs active in a particular server There is always a root POA from which all of the other POAs are

created Relative name to the parent POA


The Portable Object Adapter (POA) Related concept

Servant Object ID Active Object Map Incarnate Etherealize Default Servant

Server details

Host, port (for client toLocate the server process)

Object_Key

Used by server

POA name ObjectID

POA instance Servant Objectmanages

Not necessarily 1-1

IOR Location Details


POA ArchitectureText book: figure 2.12


Object and Servant Lifetimes Servant object is associated with server process Incarnation: binding the servant object to CORBA

object Etherealization: break the binding

Text book: figure 2.13


ObjectAdapter

ImplementationSkeletons

ORB Core

Client Object Implementation

ClientStubs

ORBInterface

DynamicInvocation

standard interface

One interface per object operation

One interface per object adaptor

Proprietary ORB interface

Normal call interfaceUp call interface

CORBA Components


Client

ClientStubs

ORBInterface

DynamicInvocation

Clients perform requests using object references.

Clients may issue requests through object interface stubs (static) or dynamic invocation interface.

Clients may access general ORB services:

• Interface Repository.• Context Management.• List Management.• Request Management.

Client Side


Object Implementation

ORBInterface

ObjectAdapter

Implementations receive requests through skeletons (without knowledge of invocation approach).

The Object Adapter provides for:• management of references;• method invocation;• authentication;• implementation registration;• activation/deactivation.

ImplementationSkeletons

Implementation Side


Static v. 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.


CORBA Interface Repository The Interface Repository is a service that provides persistent

objects that represent the IDL information in a form available at runtime. Note: The JavaIDL runtime does not include an implementation of an Interface

Repository and one is not generally required by clients at runtime. Using the IR, it is possible for a program to encounter an object whose

interface was not known at compile time, yet be able to determine what operations are valid on the object and make invocation on it.

Interface Repository provides: Dynamic client access to interface definitions to construct a request. Dynamic type-checking of request signatures. Traversal of inheritance graphs. ORB-to-ORB interoperability.


CORBA Implementation Repository The Implementation Repository contains information that allows the

ORB to locate and activate implementations of objects. Ordinarily, installation of implementations and control of policies

related to the activation and execution of object implementations is done through operations on the Implementation Repository.

In addition to its role in the functioning of the ORB, the Implementation Repository is a common place to store additional information associated with implementations of ORB objects. (e.g., debugging information, administrative control, resource allocation, security, etc)

The Implementation Repository supports the implementation of object servers. It is not needed by clients in order to access servers.


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.


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 normally 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.

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