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A Primer of CORBA A Framework for Distributed,Applications in Defence. Communications Division,Electronics and Surveillance Research Laboratory. DSTO GD 0192, Based on object technology the OMG defines an Object Management Architecture. OMA for the support of interoperable applications across heterogeneous computing. platforms The communication core of this underlying model is the Common Object. Request Broker Architecture CORBA that provides a framework for flexible and. transparent communication between distributed objects The adoption of this approach. eases software development by allowing interaction between reusable components. through well defined interfaces In particular applying CORBA technology to C4I. problems in the military environment provides simple integration of legacy software. and COTS software This report provides an overview of the OMA and describes in. detail each component of CORBA,RELEASE LIMITATION,Approved for public release. DEPARTMENT OF DEFENCE,DEFENCE SCIENCE TECHNOLOGY ORGANISATION DSTO.
Published by, DSTO Electronics and Surveillance Research Laboratory. PO Box 1500,Salisbury South Australia 5108 Australia. Telephone 08 82595555,Fax 08 8259 6567,Commomuealth of Australia 1999. AR 010 622,March 1999,APPROVED FOR PUBLIC RELEASE,A Primer of CORBA A Framework for. Distributed Applications in Defence,Executive Summary.
Distributed object computing is a promising technology that is fast becoming the. dominant computing paradigm of the future Inevitably a critical part of software. development is integrating present and future software so that legacy systems can still. remain fully operational in new environments Using an object oriented approach the. Object Management Group OMG defines an Object Management Architecture OMA. for the support of interoperable applications across heterogeneous hardware platforms. and operating systems By adopting interface and protocol specifications the OMA is. expected to provide an underlying model for all software components embracing. those which were previously developed or which are not necessarily object oriented. The Common Object Request Broker Architecture CORBA is the communication. heart of the OMG OMA which provides a framework for flexible and transparent. communication between distributed objects in heterogeneous computing. environments CORBA is a well established and widely adopted standard that. specifies the attached components and how they interoperate Essentially distributed. CORBA objects provide scalable and flexible solutions for heterogeneous environments. and for the Internet and intranets The adoption of this approach eases software. development by allowing interaction between reusable components through well. defined interfaces, Owing to the increasing dominance of joint military operations in the future the. interoperability of military communications and information systems within and. across individual Services Navy Army and Air Force will be more prominent. Applying CORBA technology to C4I problems in the military environment provides. simple integration of these systems Such integration greatly increases the value of. information that spread across various defence applications and different computer. platforms thereby fulfilling the operational requirements of modern battlefield. interoperability, This report is intended to serve as a tutorial on CORBA covering from the OMA to. each component of CORBA The OMG is also striving at a high level of abstraction to. define various services and facilities necessary for distributed object computing These. components will provide fundamental object interfaces necessary for building object. frameworks towards specific application domains such as telecommunications. medical systems finance manufacturing and C4I systems. Communications Division, Andrew Au is a Research Scientist in Netxoork Integration Group. with interests in tra c control and performance guarantees for. high speed netioorks He has been investigating ATM signalling. and communications issues in distributed computing. environments,1 INTRODUCTION 1,2 INCREASING DEGREES OF INTEROPERABILITY 2. 3 THE OBJECT MANAGEMENT ARCHITECTURE 4,3 1 OMA Component Definitions 4.
3 2 Object Frameworks 7,4 THE CORBA OBJECT MODEL 8. 4 1 Requests 8,4 3 Interfaces 9,4 4 Operations 9,4 5 Object Implementation 10. 5 COMMON OBJECT REQUEST BROKER ARCHITECTURE 10,5 1 Object Request Broker 12. 5 1 1 Example ORBs 12,5 2 Object References 14,5 3 Clients 14. 5 4 Client Stubs 15,5 5 Dynamic Invocation Interface 16.
5 6 Interface Repository 16,5 7 Object Implementations 16. 5 8 Implementation Skeleton 17,5 9 Dynamic Skeleton Interface 17. 5 10 Implementation Repository 18,5 11 Object Adaptors 18. 5 12 ORB Interface 19,6 OMG INTERFACE DEFINITION LANGUAGE 19. 6 1 IDL Specifications 20,6 2 Programming Language Mappings 21.
7 STATIC METHOD INVOCATION 22,7 1 Stubs and Skeletons 23. 7 2 Generating Interface Stubs and Skeletons 23,7 3 Activating Static Invocation 24. 8 DYNAMIC INVOCATION AND DISPATCH 25,8 1 Dynamic Invocation Interface 26. 8 1 1 Obtaining an Object Reference 26,8 1 2 Constructing a Request 27. 8 1 3 Invoking the Request 27,8 2 Dynamic Skeleton Interface 28.
9 THE INTERFACE REPOSITORY 29, 9 1 The Containment Hierarchy of Interface Repository Classes 29. 9 2 Interface Retrieval 31,9 3 Federated Interface Repositories 32. 9 3 1 OMGIDL Format 32, 9 3 2 DCE Universal Unique Identifier UUID Format 32. 9 3 3 Local Format 32,10 THE ORB INTERFACE 33,10 1 Converting Object References to Strings 33. 10 2 Object Reference Operations 33,11 THE BASIC OBJECT ADAPTOR 34.
12 INTER ORB ARCHITECTURE 37,13 CONCLUDING REMARKS 38. REFERENCES 40,APPENDLX ORB PSEUDO OBJECTS 41,DSTO GD 0192. 1 Introduction, The prevalence of computers in routine business functions has created a heterogeneous. information processing environment embracing vast networks of autonomous and. distributed computing resources Clearly there is a growing need for technology to. flexibly coordinate these diverse computing resources to fully integrate distributed. systems These efforts are expected to facilitate portability of applications and. interoperability of systems and networks in support of challenging new information. processing requirements, In recent years distributed computing has made significant advances to support. objects distributed across a network showing dominance of the object oriented. paradigm An object is a unique instance of a data structure abstract data type. encapsulated with a set of routines called methods which operate on that data This. approach entails the transparent distribution of applications across networks of. heterogeneous computers from different vendors For large organisations such as. defence legacy systems pose a serious problem because these systems are mission. critical and must remain fully operational at all times even in new computing. environments, One of the main problems facing distributed computing is software component.
integration To achieve this integration the Object Management Group OMG has. undertaken definition of a distributed object computing platform for inter. communication of application objects in widely distributed heterogeneous. environments With over 800 member companies the OMG is an international industry. consortium formed to develop adopt and promote standards for distributed object. computing The OMG Object Management Architecture OMA aims to define at a. high level of abstraction various facilities necessary for distributed object computing. Within this architectural framework the technology adopted for Object Request. Brokers ORBs is known as the Common Object Request Broker Architecture. CORBA for transparent communication between application objects Indeed this is. an OMG specification based on the standard interface definition between OMG. compliant objects, Military communications and information systems have been characterised by the. development of expensive purpose built and non interoperable systems Owing to the. increasingly dominance of joint military operations in the future the interoperability of. these systems within and across Services will be more prominent Object technology. particularly the CORBA offers several benefits for military systems including support. for application diversity technology insertion system evolution and distribution The. integration of interoperable military systems greatly increases the value of information. spread across various defence applications over different computer platforms thereby. fulfilling the operational requirements of modern battlefield interoperability. DSTO GD 0192, This report is intended to provide an overview of OMA and a snapshot of CORBA in. achieving interoperability across platforms and applications Section 2 discusses. increasing degrees of interoperability from basic interconnectivity to business. collaboration Section 3 introduces the OMG OMA Section 4 explains the CORBA. Object Model and Section 5 provides an overview of CORBA Section 6 describes the. OMG Interface Definition Language IDL that bridges diverse programming. languages operating systems networks and object systems CORBA supports both. static and dynamic method invocations as discussed in Sections 7 and 8 respectively. Section 9 presents the Interface Repository IR that stores the interface specifications of. each object on the CORBA object bus The interface to the ORB functions is described in. Section 10 and the Basic Object Adaptor BOA is discussed in Section 11 A general. ORB interoperability architecture is introduced in Section 12 for the support of. distributed objects across heterogeneous ORBs Finally concluding remarks are drawn. in Section 13, For an introductory overview of the CORBA readers may choose to skip Sections 6 11. which are involved in more technical detail,2 Increasing Degrees of Interoperability. There are many degrees of interoperability in a computing environment Basic. interconnectivity only allows simple data transfer whereas application level. interoperability enables applications running in any environment to exchange. information and perform processing even if they were developed at different times by. different developers To this end we identify three phases which are predominantly. involved in creating integrated large scale distributed information systems. 1 Network Interconnectivity, To guarantee basic communication computing resources are first interconnected to.
exchange messages Based on the prevalent client server model a distributed. system is organised as a number of distributed server processes that offer various. services to client processes across a network 1 By means of interprocess. communication mechanisms such as remote procedure calls RPCs servers provide. clients with access to general system defined services Examples are file storage. printing authentication and naming services,2 Software Interoperability. A more ambitious goal is to execute tasks jointly among interoperable computing. resources Such interoperability involves intricate interactions through the use of. prograrnming capabilities such as an RPC mechanism extended with a data. translation facility 1 Examples are the integration of heterogeneous information in. advanced multimedia applications and information storage in integrated. repositories Although the basic client server model does support a certain level of. DSTO GD 0192, interoperability the complexity of migrating from locally distributed systems to. more global systems demands new tools and techniques In addition greater. interoperability is required to allow various programming languages and. development tools to work together to reuse functions across platforms. 3 Business Collaboration, Business collaboration goes beyond software interoperability simply by expanding. service boundaries of the software component infrastructure This infrastructure. provides application level collaboration in the form of application frameworks. allowing developers to customise implementation methods based on business. requirements 2 At this application level software components are created to. behave like business objects1 They collaborate more than just interoperate at the. semantic level to accomplish a business process For example in a car reservation. system four business objects can be defined to represent customer invoice car and. car lot with some agreed upon semantics for communicating with each other to. perform business transactions, In recent years the trend towards higher levels of interoperability is evidenced by the. rapid growth in middleware technology This category of software mediates between. an application program and a network so that the specifics of the operating. environment are isolated from the application In addition middleware provides. interfaces between clients and servers by managing the interaction between disparate. applications across heterogeneous computing platforms An example of a middleware. program is the Object Management Group OMG s Common Object Request Broker. Architecture CORBA that manages communication between objects. Through the development of standards for distributed object computing the OMG. attempts to provide a common architectural framework for portability and. interoperability in heterogeneous computing environments By modelling a distributed. system as a collection of interacting objects distributed components can communicate. with each other only using messages addressed to well defined interfaces This object. concept produces a natural model for integrating distributed computing resources. which can be effectively applied to both distributed computing and. telecommunications environments As object oriented computing becomes more. mature and moves into the mainstream the work of the OMG will increase in. importance, A business object is a piece of information which is of interest and recognisable to the business.
A set of different business objects working together is able to define a business enterprise For. instance one business object represents the accounts receivable while another business object. represents manufacturing,DSTO GD 0192,3 The Object Management Architecture. The OMG has developed is a high level conceptual model of a complete distributed. environment called the Object Management Architecture OMA This model specifies. an architectural framework of distributed objects that helps reduce the complexity. lower the costs and hasten the introduction of new software applications Every. component in the architecture is defined in terms of an object oriented interface Only. through these interfaces can an object request services from any other object The OMA. is supported by detailed interface specifications that drive the industry towards. interoperable reusable portable software components based on open standard object. oriented interfaces Hence this architecture becomes an underlying model for all. software components including those which were previously developed or which are. not necessarily object oriented, The OMG Object Model provides an organised presentation of object concepts and. terminology in which common object semantics are defined in a standard. implementation independent way These semantics specify the externally visible. interfaces that are used to interact with object state and object behaviour Clients issue. requests to object services only through these well defined interfaces specified in the. OMG Interface Definition Language IDL The request carries information including. an operation the object reference of the service object and parameters if any. Typically the implementation and location of each object are hidden from the. requesting client thus ensuring interoperability between software components and. portability of applications In each potential request an interface to the target object is a. description of a set of possible operations that are specified in the OMG IDL. The OMA Reference Model identifies and characterises components interfaces and. protocols but does not in itself define them in detail A particular computing or. business problem space is partitioned into practical high level architectural. components that can be addressed by various technologies This model therefore. provides the means to build interoperable software systems distributed across all. hardware and software environments,3 1 OMA Component Definitions. The OMA Reference Model identifies five components in heterogeneous environments. Object Request Broker Object Services Common Facilities Domain Interfaces and. Application Objects The OMA can also be viewed as three major segments consisting. of these five critical components, 1 Application Oriented Segment Application Objects and Common Faculties are. solution specific components which are located closest to the end user. DSTO GD 0192, non standardised application application domain specific horizontalfacility.
specific interfaces interfaces interfaces,Interfaces. Object Request Broker software bus,general service. interfaces, Figure 1 Object Management Architecture Reference Model. 2 System Oriented Segment the underlying infrastructure of distributed object. computing environments comprises of Object Request Broker the communications. heart and Object Services, 3 Vertical Market Oriented Segment Domain Interfaces provide vertical extensions. to specific applications or domains applicable for a variety of industries. Figure 1 illustrates the Object Request Broker component and its interactions with the. other four categories of object interfaces These five OMA components are defined as. Object Request Broker ORB, The ORB is the broker which lets applications request use of another object.
without knowing where that other object is located on the system or network It. provides the basic object interaction capabilities which are necessary for any of the. components to communicate independent of the specific platforms and. implementation techniques The ORB finds the requested object wherever it is. located and passes the requested information to the requested object It also passes. information back to the requester as necessary The ORB component guarantees. portability and interoperability of objects over a network of heterogeneous systems. The ORB is commercially referred to as the Common Object Request Broker. Architecture CORBA in which the programming interfaces to the ORB component. are defined,DSTO GD 0192,Object Services, The Object Services component standardises the life cycle management of objects. which are low level system type services necessary for developing applications. Functions are provided to create objects to control access to objects to keep track of. relocated objects and to consistently maintain the relationship between groups of. objects These general purpose services are used by many distributed object. programs to provide for application consistency and to increase programmer. productivity They are indeed the basic building blocks for distributed object. applications from which higher level facilities and object frameworks can be. constructed for interoperability across multiple platform environments Adopted. OMG Object Services are collectively called CORBAservices Typical examples are. the naming service and the trading service for the discovery of other available. services Other services include life cycle management security transactions and. event notification Specifications for Object Services are contained in CORBAservices. Common Object Services Specification,Common Facilities. The Common Facilities component provides a set of generic application functions. that can be configured to the requirements of a specific configuration leading to. uniformity in generic operations and to better options for end users for configuring. their working environments These are higher level services which are semantically. closer to the application objects For example printing document management. database and electronic mail facilities are readily usable by many applications The. availability of such capabilities allows applications to be created quickly in a. portable and interoperable way Adopted OMG Common Facilities are collectively. called CORBAfacilities in which an OpenDoc based Distributed Document. Component Facility DDCF is included The OMG intends to collect information. about Common Facilities in CORBAfacilities Common Facilities Architecture. Domain Interfaces, These domain specific interfaces are oriented towards many separate applications. domains Examples are finance manufacturing telecommunications and medical. Application Objects, To perform specific tasks for users new classes of Application Objects can be built. by modifying existing classes through generalisation or specialisation provided by. Object Services Application Objects are actually business objects and applications. These are the ultimate consumers of the CORBA infrastructure A particular. application can be constructed from a large number of basic object classes partly. OpenDoc is a compound document architecture that enables embedding of features from. different application programs into a single working document. DSTO GD 0192,Application Domain Facility,Objects Objects Objects.
Object Request Broker,AI Application interface CF Common facilities. DI Domain interfaces OS Object services,Figure 2 OMA Reference Model Interface Usage. specific for the application partly from the set of Common Facilities thereby. improving developer productivity and configuration flexibility. 3 2 Object Frameworks, Object Frameworks are groups of higher level components that interact to provide. functionality of direct interest to end users in particular application or technology. domains 3 These are collections of cooperating objects typically oriented towards. domains such as telecommunications health care finance and manufacturing The. cooperating objects can also be categorised into application domain facility and. service objects Each object or component in an object framework supports some. combination of application domain common facility and object services interfaces as. illustrated in Figure 2 Some components support all types of interfaces while others. support a subset of these interfaces In general service objects support object services. interfaces whereas facility objects support common facilities interfaces and potentially. inherited object services interfaces Domain objects support domain interfaces and. potentially inherited common facility and object services interfaces Likewise. application objects support application interfaces and potentially all the other. interfaces,DSTO GD 0192, The basic client server model is used to coordinate the interactions between two. objects In order to provide the overall functionality of the object framework objects. are able to make requests to all other objects on a peer to peer basis Each component. uses the ORB to communicate with other components through the supported. interfaces Depending on the occasion an object can therefore act as either a client or a. 4 The CORBA Object Model, The OMG Object Model in the Object Management Architecture OMA is an abstract.
object model that is not directly realised by any particular technology The CORBA. Object Model on the other hand is a concrete object model based on the OMG Object. Model which provides the underlying definitions for a particular technology This. model encompasses more detailed specifications defined from the abstract concepts in. particular the interactions between clients and servers. 4 1 Requests, A client of a service is any entity capable of requesting the service A request is an. event mat associates a set of information at a particular time consisting of an. operation a target object zero or more actual parameters and an optional request. context In addition a request form is supported as a description or pattern that can be. evaluated or performed multiple times to issue requests. A client may request one or more services from an object For the purpose of. identifying the request an object may be identified by a value called an object name. An object reference however reliably identifies a particular object that may be denoted. by multiple distinct object references Additional information about the request may. be provided by a request context A request causes a service to be performed on behalf. of the client and results if any may be returned to the client An abnormal condition. may generate an exception to the client carrying additional return parameters. particular to that exception, A signature defines the types of the parameters for a given operation These types are. used to restrict a possible parameter or to characterise a possible result The set of. values that satisfy a type is called the extension of the type A type whose members are. objects is an object type Values in a request are restricted to values that satisfy these. type constraints Legal values of the OMG type hierarchy are shown in Figure 3 The. set of basic values includes various forms of integers floating points characters. booleans and any Constructed values are structured types such as records called.


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