Introduction to CORBAThe Object Management Group (OMG) was formed in 1989. Its aims were: to make better use of distributed systemsto use object-oriented programmingto allow objects in different programming languages to communicate with one anotherThe object request broker (ORB) enables clients to invoke methods in a remote objectCORBA is a specification of an architecture supporting this. CORBA 1 in 1990 and CORBA 2 in 1996.

Introduction to CORBAThe main components of CORBAs RMI framework are:An interface definition language known as IDL.An architecture.The General Inter-ORB protocol (GIOP) defines an external data representation, called CDRspecifies 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 GIOP implemented in TCP/IP CORBA services - generic services useful in distributed applications e.g. Naming Service, Event Service.Don't be put off by GIOP and IIOPThey are just names for familiar thingsGIOP is just about external data representationand a Request-reply protocol allowing for objects to be activatedThe architecture allows for mixed languagesand object activation (added to Figure 5.6)IIOP is just about remote object references

CORBA RMICORBA 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 modelsimilar 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.

CORBA IDL interfaces Shape and ShapeListstruct 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 referenceslong getVersion() ;};Figure 17.1Differences from a Java remote inteface?why are argument types defined in the IDL?(not necerssary in Java remote interfaces)

Parameters in CORBA IDLPassing 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 struct GraphicalObject (argument of newShape and result of getAllState)Note: the method allShapes returns an array of remote object references as follows:typedef sequence 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 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 CORBAThis 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

Illustration of programming CORBA We illustrate CORBA with a Java client and serverThe interface compiler is called idltojavawhen given an IDL interface, it producesserver skeletons for each class (e.g. _ShapeListImplBase)proxy classes (e.g. _ShapeListStub)a Java class for each struct e.g. Rectangle, GraphicalObjecthelper classes (narrow method) and holder classes (for out arguments)the equivalent Java interfaces (e.g. ShapeList below)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 17.3A Java server has classes for its IDL interfaces (e.g. Shape and ShapeList). Here is the class ShapeListServantimport 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() { ... }}CORBA objects are instances of servant classes.In non-OO languages implementations of CORBA objects cant be classes. What might they be in C?This class has to create CORBA objects of type Shape. How does it do that?

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

Java client program for CORBA interfaces Shape and ShapeListimport 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

The main components of the CORBA architectureThe 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 5.6object adapter, implementation repository and interface repositoryFigure 17.6

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. 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. The OA has a remote object reference and a method to be invoked on itWhat does dispatch do in C++/Java?What does dispatch in C?

Implementation repositoryImplementation repositoryit 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:- not all CORBA objects (e.g. call backs) need be activated on demand- access control information can be stored in an implementation repository

Interface repositoryit provides information about registered IDL interfacesfor 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 typesthe 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 referencesthis type identifier is called the repository IDbecause the interface repository stoes interfaces against their IDsapplications that use static invocation with client proxies and IDL skeletons do not require an interface repository. Not all ORBs provide an interface repository.

CORBA IDLIDL 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.there are no constructors in IDL,so how do we create CORBA objects?Suppose there was a constructor,what would be the problem for a servere.g. being asked to create an instance of a given class?

IDL module WhiteboardModules allow interfaces and associated definitions to be grouped.A module defines a naming scope.

IDL method signatures[oneway] (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 usedat-most-once call semantics is the defaultInheritance - IDL interfaces may extend one or more interfacesall 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 Servicean extended interface may add new methods, types, constants and exceptions It may redefine types, constants and exceptions but not methods

Figure 17.8IDL constructed types 1this figure continues on the next slideSee Fig 5.1 for an example of string

Figure 17.8 IDL constructed types 2

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 objectThe 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 activationsthey contain the address of the implementation repositorythe implementation repository receives the request and uses the object adapter name to activate the object, then gives the server address to the clientthe client sends subsequent invocations to the server

Figure 17.9Naming graph in CORBA Naming Serviceinitial naming contextShapeListCDEBinitial naming contextPRSTVQUinitial naming contextXX

Figure 17.10Part of the CORBA Naming Service NamingContext interface in IDLstruct NameComponent { string id; string kind; };

typedef sequence 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

CORBA services include the followingNaming Service (it would be a good idea to study it!)Event Service and Notification Service:in ES suppliers and consumers communicate via an event channelNS extends this to allow filtering and typed eventsSecurity service:authentication of principals and access control of CORBA objects with policiesauditing by servers, facilities for non-repudiationTrading service: allows CORBA objects to be located by attributeTransaction service and concurrency control serviceTS provides flat or nested transactions CCS provides locking of CORBA objectsPersistent object service:for storing the state of CORBA objects in a passive form and retrieving it

SummaryCORBA 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 adapterrelates request messages to implementations of CORBA objectsImplementation repositoryenables CORBA objects to be activated on demandInterface repositoryallows dynamic invocation of CORBA objectsIDL for defining interfaces

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.Viewing: These slides must be viewed in slide show mode.Teaching material based on Distributed Systems: Concepts and Design, Edition 3, Addison-Wesley 2001.

Exercises for Chapter 17: CORBA CASE STUDYFrom Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and DesignEdition 3, Addison-Wesley 2001

Exercise 17.1The Task Bag is an object that stores pairs of (key and value). A key is a string and a value is a sequence of bytes. Its interface provides the following remote methods:pairOut: with two parameters through which the client specifies a key and a value to be stored.pairIn: whose first parameter allows the client to specify the key of a pair to be removed from the Task Bag. The value in the pair is supplied to the client via a second parameter. If no matching pair is available, an exception is thrown. readPair: is the same as pairIn except that the pair remains in the Task Bag.Use CORBA IDL to define the interface of the Task Bag. Define an exception that can be thrown whenever any one of the operations cannot be carried out. Your exception should return an integer indicating the problem number and a string describing the problem. The Task Bag interface should define a single attribute giving the number of tasks in the bag. page 680

Exercise 17.2Define an alternative signature for the methods pairIn and readPair, whose return value indicates when no matching pair is available. The return value should be defined as an enumerated type whose values can be ok and wait. Discuss the relative merits of the two alternative approaches. Which approach would you use to indicate an error such as a key that contains illegal characters?page 680

Exercise 17.3Which of the methods in the Task Bag interface could have been defined as a oneway operation? Give a general rule regarding the parameters and exceptions of oneway methods. In what way does the meaning of the oneway keyword differ from the remainder of IDL?page 680

Exercise 17.4The IDL union type can be used for a parameter that will need to pass one of a small number of types. Use it to define the type of a parameter that is sometimes empty and sometimes has the type Value.page 683

Exercise 17.5In Figure 17.1 the type All was defined as a sequence of a fixed length. Redefine this as an array of the same length. Give some recommendations as to the choice between arrays and sequences in an IDL interface.page 683

Exercise 17.6The Task Bag is intended to be used by cooperating clients, some of which add pairs (describing tasks) and others remove them (and carry out the tasks described). When a client is informed that no matching pair is available, it cannot continue with its work until a pair becomes available. Define an appropriate callback interface for use in this situation.page 677

Exercise 17.7Describe the necessary modifications to the Task Bag interface to allow callbacks to be used. page 677

Exercise 17.8Which of the parameters of the methods in the TaskBag interface are passed by value and which are passed by reference?page 672

Exercise 17.9Use the Java IDL compiler to process the interface you defined in Exercise 17.1. Inspect the definition of the signatures for the methods pairIn and readPair in the generated Java equivalent of the IDL interface. Look also at the generated definition of the holder method for the value argument for the methods pairIn and readPair. Now give an example showing how the client will invoke the pairIn method, explaining how it will acquire the value returned via the second argument.page 685

Exercise 17.10Give an example to show how a Java client will access the attribute giving the number of tasks in the Task bag object. In what respects does an attribute differ from an instance variable of an object?page 682

Exercise 17.11Explain why the interfaces to remote objects in general and CORBA objects in particular do not provide constructors. Explain how CORBA objects can be created in the absence of constructors.Chapter 5 and page 684

Exercise 17.12Redefine the Task Bag interface from Exercise 17.1 in IDL so that it makes use of a struct to represent a Pair, which consists of a Key and a Value. Note that there is no need to use a typedef to define a struct.page 683

Exercise 17.13Discuss the functions of the implementation repository from the point of view of scalability and fault tolerance.page 679 and page 684

Exercise 17.14To what extent may CORBA objects be migrated from one server to another?page 679 and page 684

Exercise 17.15Discuss the benefits and drawbacks of the two-part names or NameComponents in the CORBA naming service. page 688

Exercise 17.16Give an algorithm that describes how a multipart name is resolved in the CORBA naming service. A client program needs to resolve a multipart name with components A, B and C, relative to an initial naming context. How would it specify the arguments for the resolve operation in the naming service?page 688

Exercise 17.17A virtual enterprise consists of a collection of companies who are cooperating with one another to carry out a particular project. Each company wishes to provide the others with access to only those of its CORBA objects relevant to the project. Describe an appropriate way for the group to federate their CORBA Naming Services.page 690

Exercise 17.18Discuss how to use directly connected suppliers and consumers of the CORBA event service in the context of the shared whiteboard application. The PushConsumer and PushSupplier interfaces are defined in IDL as follows:interface PushConsumer {void push(in any data) raises (Disconnected);void disconnect_push_consumer();}interface PushSupplier {void disconnect_push_supplier();}Either the supplier or the consumer may decide to terminate the event communication by calling disconnect_push_supplier() or disconnect_push_consumer() respectively. page 690

Exercise 17.19 (first part)Describe how to interpose an Event Channel between the supplier and the consumers in your solution to Exercise 17.18. An event channel has the following IDL interface:interface EventChannel {ConsumerAdmin for_consumers();SupplierAdmin for_suppliers();};where the interfaces SupplierAdmin and ConsumerAdmin, which allow the supplier and the consumer to get proxies are defined in IDL as follows:interface SupplierAdmin {ProxyPushConsumer obtain_push_consumer();---};interface ConsumerAdmin {ProxyPushSupplier obtain_push_supplier();---};

Exercise 17.19 continuedThe interface for the proxy consumer and procy supplier are defined in IDL as follows:interface ProxyPushConsumer : PushConsumer{void connect_push_supplier (in PushSupplier supplier) raises (AlreadyConnected);};interface ProxyPushSupplier : PushSupplier{void connect_push_consumer (in PushConsumer consumer) raises (AlreadyConnected);};What advantage is gained by the use of the event channel?page 690

This set of slides took about 1 hour 15 minutes at to present - but was little rushed. A little longer would be usefuldiffers from Java in that Java has classes but IDL does not.CORBA must define anything that will be passed as argument or returned as result.in Java the argument types are classes which can be defined in the language. Object is useful as argument or result where any type of remote object reference might be used. e.g. in lookup and bind.e,g, of argumnent - reference to ShapeListnarrow method is for casting down e.g. from a reference of type Object to one of type ShapeList

Java only has a single result, no out arguments.it uses a factory method to create CORBA objectsCORBA objects in C are a set of data e.g. in a struct and a set of functions to access it.animation goes through the components

in both cases, the dispatcher knows a method and some arguments as well as the reference to a CORBA object.

dispatcher in C++/Java invokes a method in the local object whose reference is obtained from the CORBA object reference.In C, the object reference is passed as an extra argument to a function that is called.The server might not have the code for a class mentioned in a constructor.These are specifications of interfaces to services, they may be implementaed in a variety of ways, for example the POS may use files or a database to store object state.