What is CORBA? CORBA (Common Object Request Broker Architecture) is a distributed object-oriented client/server platform.

Transcription

1 CORBA

2 What is CORBA? CORBA (Common Object Request Broker Architecture) is a distributed object-oriented client/server platform. It includes: an object-oriented Remote Procedure Call (RPC) mechanism object services (such as the Naming or Trading Service) language mappings for different programming languages interoperability protocols programming guidelines and patterns CORBA replaces ad-hoc special purpose mechanisms (such as socket communication) with an open, standardized, scalable, and portable platform

3 The Object Management Group (OMG) Founded 1989 to create specifications for open distributed computing. World s largest software consortium with 457 member organizations (2005) CORBA (Common Object Request Broker Architecture) CORBA 1.0 (1991): Object Request Broker, IDL, C CORBA 2.0 (1995): Interoperability, C++ CORBA 3.0 (2002): Components, Scripting, Real-time UML: Unified Modelling Language MDA: Model Driven Architecture

4 What is Client/Server Computing? A group of clients and servers cooperate in solving a problem Servers are passive entities,providing a service, and wating for requests from clients Clients are active entites, requesting a service from the server Clients and servers run (typically) as operating system processes on different computers Object-oriented client/server computing adds OO aspects: interfaces, messages, inheritance, and polymorphism.

5 Advantages & Disadvantages of CORBA Advantages: vendor-neutral and open standard, portable, wide variety of implementations, hardware platforms, operating systems, languages takes the grunt work out of distributed programming Disadvantages: no reference implementation specified by consensus and compromise not perfect

6 Heterogeneity CORBA works for homogenous and heterogenous environments. Characteristics of heterogenous environments are: Language transparency Location transparency Service transparency Implementation transparency Architecture transparency Operatingsystem transparency (Protocol transparency) (Transport transparency)

7 The Object Management Architecure (OMA) Application Interfaces Domain Interfaces Object Request Broker Object Services

8 Objects Object: encapsulated entity with immutable specific identity, interacts only through well-specified interfaces CORBA object Java object(language-specific object) A single Java object can implement multiple CORBA objects A CORBA object can successively be implemented (incarnated) through multiple Java objects Most simple case: one-to-one relationship between CORBA objects and Java objects (servants)

9 Interfaces Interface: Set of operations Object services: Interfaces required in many applications independently from application domain Horizontal services, infrastructure services Domain interfaces: standardized interfaces for services specific to an application area Vertical services Application interfaces: specific for the application, not standardized Interfaces are defined using IDL (Interface Definition Language)

10 ORB: Object Request Broker Client OBJ REF in args operation() out args + return Object (Servant) IDL SKEL DSI DII IDL STUBS ORB INTERFACE Object Adapter ORB CORE GIOP/IIOP/ESIOPS

11 Remote Object Invocation Client Application Server Object Stub interface (dynamic or IDL-generated) Object Request Broker (ORB) Skeleton interface (dynamic or IDL-generated) Object Adaptor Object Request Broker (ORB)

12 Request Invocation Client sends a request: Client ORB analyses object reference Connects to server if necessary Sends request: (target object, operation name, parameters) ORB receives request: Activates server (if no server active) Activates target servant (if no servant active) Invokes method, waits for completion Sends reply(if required) Method semantics: At-most-once: exception in case of error Best effort (oneway)

13 The Object Request Broker and the Object Adaptor The core of CORBA is the Object Request Broker (ORB) Each machine involved in a CORBA application must have an ORB running in order for processes on that machine to interact with CORBA objects running in remote processes. Object clients and servers make requests through their ORBs the ORB is responsible for making the requests happen or indicating why they can't. The client ORB provides a stub for a remote object. Requests made on the stub are transferred from the client's ORB to the ORB servicing the implementation of the target object. The request is passed on to the implementation through an object adaptor and the object's skeleton interface.

14 The Skeleton interface The skeleton interface is specific to the type of object that is exported remotely through CORBA. provides a wrapper interface that the ORB and object adaptor can use to invoke methods on behalf of the client or as part of the lifecycle management of the object. The object adaptor provides a general facility that "plugs" a server object into a particular CORBA runtime environment. (BOA vs. POA) All server objects can use the object adaptor to interact with the core functionality of the ORB, the ORB can use the object adaptor to pass along client requests and lifecycle notifications to the server object. Typically, an IDL compiler is used to generate the skeleton interface for a particular IDL interface; this generated skeleton interface will include calls to the object adaptor that are supported by the CORBA environment in use.

15 Interoperability IOR: Interoperable Object Reference CDR: Common Data Representation GIOP: General Inter-ORB Protocol IIOP: Internet Inter-ORB Protocol Alternative protocols: ESIOP: Environment-specific Inter-ORB protocols DCE-CIOP SCCP (Signalling Control Part) IOP Pluggable Protocols

16 Object Reference Every reference identifies exactly one object Different references may refer to the same object References may be nil (point nowhere) References may become invalid (dangle): The object referred-to has already disappeared References are opaque Note: Interoperable Object References (IOR) References are strongly typed. References allow late binding. References may be persistent.

17 Object Services Naming Service (CosNaming) Trader (CosTrading) Property Service (CosPropertyService) Event Service (CosEventComm, CosEventChannelAdmin) Notification Service (CosNotification)... (Transactions) (Persistency) (PSS: Persistent State Service)

18 Inter-CORBA Communication CORBA v.2.0 standard includes specifications for inter-orb communication protocols that can transmit object requests between various ORBs running on the network. independent of the particular ORB implementations running at either end of the communication link. An ORB implemented in Java can talk to another ORB implemented in C, as long as use the same CORBA communication protocol. The inter-orb protocol is responsible for delivering messages between two cooperating ORBs (method requests, return types, error messages, etc.) The inter-orb protocol also deals with differences between the two ORB implementations, like machine-level byte ordering and alignment. The Internet Inter-ORB Protocol (IIOP) is an inter-orb protocol based on TCP/IP. TCP /IP is by far the most commonly used network protocol on the Internet, so IIOP is the most commonly used CORBA communication protocol. There are other standard CORBA protocols defined for other network environments, however( e.g.; DCE Common Inter-ORB Protocol - DCE-CIOP),