Corba introduction and simple example

CORBA Introduction
- Common Object Request Broker Architecture
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--by WangJianmin
http://weibo.com/lanxuezaipiao/
http://www.cnblogs.com/lanxuezaipiao

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Outline
 Distributed Computing
 CORBA Introduction
 Architecture
 Key components
 An example
 Advantages

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Distributed Computing
 Local vs. Remote objects
 An enterprise app is a collection of co-
operating objects located on different machines

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Existing Mechanisms
 Sockets (TCP, UDP) not OO
 RPC (not OO)
 CORBA
 RMI
 EJB
 JMS
 DCOM
 WebService
 ……

CORBA vs. WebService
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 One important observation concerning CORBA and Web services
is that whatever can be accomplished by CORBA can be
accomplished using Web service technologies and vice versa,
although the amount of effort required would be noticeably
different. In particular, one can implement CORBA on top of
SOAP, or SOAP on top of CORBA.

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Aspect CORBA Web services
Data model Object model SOAP message exchange
model
Client-Server coupling Tight Loose
Location transparency Object references URL
Type system IDL XML schemas
static + runtime checks runtime checks only
Error handling IDL exception SOAP fault messages
Serialization built into the ORB can be chosen by the user
Parameter passing by reference by value (no notion of
objects)
by value (valuetype)
Transfer syntax CDR used on the wire XML used on the wire
binary format Unicode
State stateful stateless
Request semantics at-most-once defined by SOAP
Runtime composition DII UDDI/WSDL
Registry Interface Repository UDDI/WSDL
Implementation repository
Service discovery CORBA naming/trading
service
UDDI
RMI registry
Language support any language with an IDL
binding
any language
Security CORBA security service HTTP/SSL, XML signature
Firewall Traversal work in progress uses HTTP port 80
Events CORBA event service N/A

CORBA vs. RMI
 CORBA interfaces are defined in IDL, RMI interfaces are defined
in Java
 CORBA is language-independent, RMI is not
 CORBA objects are not garbage collected, RMI objects are
garbage collected automatically.
 RMI does not support “out” and “inout” operations since local
objects are copied, and remote objects passed by reference to
stub
 RMI-IIOP(RMI and CORBA): less resources and more robust
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CORBA vs. EJB
 These have many common objectives
– definition, packaging and deployment of components
 CORBA has always been component oriented
– EJB ideas are being extended and incorporated into CORBA 3
– EJB is like CORBA without language independence
 A Java-based CORBA will then be the best EJB
– a EJB flavor on the richness of CORBA
 EJB mandates CORBA interoperability
– and many EJB services are very close derivatives of their
CORBA forerunners.
 Why is this so important?
– Because more component frameworks will emerge over time!

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CORBA Introduction
 A standard controlled by the Object
Management Group (OMG)
 A spec for creating distributed objects
 Its architecture is based on the object model
 Promotes design of applications as a set of
cooperating objects
 based on client/server concepts

CORBA Objects
 It is important to note that CORBA objects differ
from typical programming objects in three
ways:
– CORBA objects can run on any platform.
– CORBA objects can be located anywhere on the
network.
– CORBA objects can be written in any language that
has IDL mapping.
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CORBA Architecture
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ORB core
Dynamic
Invocation
IDL
Stubs
ORB
Interface
Object
Adapter
Static IDL
Skeleton
Dynamic
Skeleton
Client Object Implementation
Standard Interface Per-Object Type
Generated Interface
ORB Dependent
Interface
Interface
Repository
Implementation
Repository

First key - ORB
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Client
Client
Client
Server
Server
Server
ORB

First key - ORB
 Provides a communications hub for all objects
– analogous to a hardware bus
 Uses a broker to handle messages requests
between clients and servers
– broker can choose server that best fits needs of
client
– allows separation of interface and implementation
– allows building block approach to development and
evolution
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First key - ORB
 Object bus that provides object location
transparency
 Responsible for mechanisms to:
– Find the object implementation of the request
– Prepare object implementation to receive the
request
– Communicate the data making up the request

Object adapter
 An object adapter is the primary means for an object
implementation to access ORB services such as object
reference generation.
 an object adapter bridges the gap between
– CORBA objects with IDL interfaces and
– the programming language interfaces of the corresponding servant
(classes)
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Object adapter
 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 object
name.
– 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 active CORBA object is registered with its object adapter,
 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.
 OA helps the ORB to operate with different type of objects.
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Second key - IDL
Java
C++
C
VB
Ada
Implementation is
Hidden behind
interface
Service or Contract-
oriented View

Why IDL?
 IDL provides facilities for defining modules, interfaces,
types, attributes and method signatures.
 IDL has the same lexical rules as C++ but has
additional keywords to support distribution,
– e.g. interface, any, attribute, in, out, inout, readonly, raises.
 It allows standard C++ pre-processing facilities.
– e.g. typedef for All.
 The grammar of IDL is a subset of ANSI C++ with
additional constructs to support method signatures.
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IDL Structure
module <identifier> {
<type declarations>;
<constant declarations>;
<exception declarations;
<interface definition>;
<interface definition>;
};
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CORBA Software Bus
C++ COBOL Smalltalk
Interface
Definition
Language
Protocol for
communication:
IIOP
Java VB

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Third key - IIOP
 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 an
object, for cancelling requests and for reporting errors.
 The Internet Inter-ORB protocol (IIOP) defines
a standard form for remote object references.
– IIOP is a specialized form of GIOP for TCP/IP
networks.

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Third key - IIOP
 Don't be put off by GIOP and IIOP, they are just
names for familiar things
– GIOP is just about external data representation and
a Request-reply protocol allowing for objects to be
activated
– IIOP is just about remote object references

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Corba Services
 Naming Service
 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
 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

Steps of a CORBA-based App
 The steps involved:
– Define an interface(IDL file)
– Map IDL to Java (idlj compiler)
– Implement the interface
– Write a Server
– Write a Client
– Run the application
 Example: a step-by-step Hello example
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Step 1: define the interface
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 Hello.idl
module HelloApp {
interface Hello {
string sayHello();
};
};

Step 2: map Hello.idl to Java
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 Use the idlj compiler
idlj -fall Hello.idl (Inheritance model)
idlj -fallTie -oldImplBase Hello.idl (Tie model)
 This will generate:
_HelloImplBase.java (server skeleton)
HelloStub.java (client stub, or proxy)
Hello_Tie.java (only Tie model)
Hello.java
HelloHelper.java
HelloHolder.java
HelloOperations.java

Step 3: implement the interface
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 Implement the servant
public class HelloImpl extends _HelloImplBase {
public String sayHello() {
return "nHello world !!n";
}
}

Step 4: implement the server
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 Creates and initializes an ORB instance
– ORB orb = ORB.init(args, null);
 Creates a servant instance (the implementation of one CORBA
Hello object)
– HelloImpl helloImpl = new HelloImpl();
 Obtain a reference for the desired object and register it with ORB
– By creating a tie with the servant being the delegate(Tie model)
– By a naming context (Inheritance model)
 "Stringify" the reference
– String ior = orb.object_to_string(hello);
 Waits for invocations of the new object from the client
– orb.run();
– Thread.currentThread().join();

Step 4: implement the server
 Note: to use a CORBA object, you must posses a valid reference
to it. An object reference may be obtained in one of three ways:
– CORBA::ORB::resolve_initial_references( <class-name> )
returns a reference to the specified class if the ORB can find it
– Invoking a "factory" method on an object whose reference you
already have (a "factory" creates other objects)
– Converting a "stringified" reference to a real reference
 the preferred way to obtain an object reference is the latter
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Step 5: write a client
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 Creates and initializes an ORB instance
– ORB orb = ORB.init(args, null);
 Obtain a reference for the desired object from IOR.
– read stringified object from IOR
– resolve the Object Reference(by HelloHelper.narrow() method)
 "Invokes the object's sayHello() operations and prints the result

Step 6: run the application
 Compile the .java files, including the stubs and skeletons
– javac *.java HelloApp/*.java
 Start orbd
– orbd -ORBInitialPort 1050 -ORBInitialHost localhost&
 Start the Hello server
– java HelloServer -ORBInitialPort 1050 -ORBInitialHost localhost&
 Run the client application
– java HelloClient -ORBInitialPort 1050 -ORBInitialHost localhost
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Advantages of CORBA
 Object Location Transparency
 Server Transparency
 Language Transparency
 Implementation Transparency
 Architecture Transparency
 Operating System Transparency
 Protocol Transparency

References
 Java IDL: The "Hello World" Example With The ImplBase Server-Side
Model (old version)
 Java IDL: The "Hello World" Example
 Java IDL: The "Hello World" Example With The ImplBase Server-Side
Model
 CORBA Tutorial
 Locating CORBA objects using Java IDL
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Corba introduction and simple example

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