Copyright © George Coulouris, Jean Dollimore, Tim Kindberg 2001 email: authors@cdk2.netThis material is made available for private study and for direct use by individual teachers.It may not be included in any product or employed in any service without the written permission of the authors.

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Teaching material based on Distributed Systems: Concepts and Design, Edition 3, Addison-Wesley 2001.

Distributed Systems Course

CORBA case study

17.1 Introduction

17.2 CORBA RMI

17.2.1 Client and server example

17.2.2 Architecture

17.2.3 CORBA IDL

17.2.4 CORBA object references

17.3 Services (just a summary)

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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 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 CORBA’s 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 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 services - generic services useful in distributed applications e.g. Naming Service, Event Service.

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Don't be put off by GIOP and IIOPThey are just names for familiar things

GIOP is just about external data representationand a Request-reply protocol allowing for objects to be activated

The architecture allows for mixed languagesand object activation (added to Figure 5.6)

IIOP is just about remote object references

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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 implementation

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

CORBA's object model– similar to the remote object model in Chapter 5 (what are the main features?) – clients are not necessarily objects (why not?)– 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, 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 that instances

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 <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.

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

this struct is used in defining another struct.

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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 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

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CORBA Naming Service (see Section17.3.1)

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

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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 produces server skeletons for each class (e.g. _ShapeListImplBase) proxy classes (e.g. _ShapeListStub) a Java class for each struct e.g. Rectangle, GraphicalObject helper classes (narrow method) and holder classes (for out arguments) 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

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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?

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

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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 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

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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

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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

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The 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

The CORBA architecture is designed to allow clients to 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 17.6

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 interface In 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 CORBA’s alternative. (we will discuss it later with the Interface Repository)

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 Fig. 5.6–more about the object adapter later.

Skeletons–skeleton classes (for OO languages) are generated in the language of the server by an IDL compiler.–remote method invocations are dispatched via the appropriate skeleton to a particular servant, –the skeleton unmarshals the arguments in request messages and 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 to generate one of the following:

for object-oriented languages the class of a proxyfor procedural languages a set of stub procedures.

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

Implementation repositoryactivates registered servers on demand and locates running servers uses the object adapter name to register and activate servers.more about this later

Interface repository the interface repository provides information about registered IDL interfaces to clients and servers that require it. More about this later.

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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 Fig. 5.6.

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.

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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 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.

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, 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.

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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

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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

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

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

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we saw raises in the newShape method of ShapeList

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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

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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. };

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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 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 the object adapter

name to activate the object, then gives the server address to the client– the client sends subsequent invocations to the server

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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 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

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

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Summary

CORBA addresses heterogeneity:– RMI between a client and a remote remote object in different languages. – GIOP

specifies an external data representation called CDR – clients and servers can have different hardware.

specifies OS independent operations for request-reply protocol specifies a standard form for remote object references.

– IIOP implements the request-reply protocol over TCP/IP.

Object adapter– relates request messages to implementations of CORBA objects

Implementation repository– enables CORBA objects to be activated on demand

Interface repository– allows dynamic invocation of CORBA objects

IDL for defining interfaces•