Tuesday, April 24, 2007

CORBA


CORBA “wraps” program code into a bundle containing information about the capabilities of the code and how to invoke it। The wrapped objects can then be invoked from other programs or CORBA objects across a network.

CORBA uses an interface definition language (IDL) to specify the interfaces that objects will present to the world. CORBA then specifies a “mapping” from IDL to a specific implementation language like C++ or Java. Standard mappings exist for Ada, C, C++, Lisp, Smalltalk, Java, COBOL, PL/I and Python. There are also non-standard mappings for Perl, Visual Basic, Ruby, and Tcl implemented by ORBs written for those languages.

This diagram illustrates how the generated code is used within the CORBA infrastructure:

This picture does not reflect all typically used possibilities. Normally the server side has the Portable Object Adapter (POA) that redirects calls either to the local servants or (to balance the load) to the other servers. Also, both server and client parts often have interceptors that are described below.

In addition to providing users with a language and a platform-neutral remote procedure call specification, CORBA defines commonly needed services such as transactions and security.

Key फ़ीचर्स


Objects by Value (OBV)

Apart from remote objects, the CORBA and RMI-IIOP define the concept of the OBV. The code inside the methods of these objects is executed locally by default. If the OBV has been received from the remote side, the needed code must be either a priori known for both sides or dynamically downloaded from the sender. To make this possible, the record, defining OBV, contains the Code Base that is a space separated list of URLs from where this code should be downloaded. The OBV can also have the remote methods.

The OBV's may have fields that are transferred when the OBV is transferred. These fields can be OBV's themselves, forming lists, trees or arbitrary graphs. The OBV's have a class hierarchy, including multiple inheritance and abstract classes.

CORBA Component Model (CCM)

CORBA Component Model (CCM) is an addition to the family of CORBA definitions. It was introduced with CORBA 3, and it describes standard application framework for CORBA components. It is an extension of "language independent Enterprise Java Beans (EJB)". It provides an abstraction of entities that can provide and accept services through well-defined named interfaces called ports.

The CCM has a component container, where software components can be deployed. The container offers a set of services that the components can use. These services include (but are not limited to) notification, authentication, persistence and transaction management. These are the most-used services any distributed system requires, and, by moving the implementation of these services from the software components to the component container, the complexity of the components is dramatically reduced.

External links

Portable interceptors

Portable interceptors are the "hooks", used by CORBA and RMI-IIOP to mediate the most important functions of the CORBA system. CORBA standard defines the following types of interceptors:

  1. IOR interceptors mediate the creation of the new references to the remote objects, presented by the current server.
  2. Client interceptors usually mediate the remote method calls on the client (caller) side. If the object Servant (CORBA) exists on the same server where the method is invoked, they also mediate the local calls.
  3. Server interceptors mediate the handling of the remote method calls on the server (handler) side.

The interceptors can attach the specific information to the messages being sent and IORs being created. This information can be later read by the corresponding interceptor on the remote side. Interceptors can also throw forwarding exceptions, redirecting request to another target.

General InterORB Protocol (GIOP)

The GIOP is an abstract protocol by which Object request brokers (ORBs) communicate. Standards associated with the protocol are maintained by the Object Management Group (OMG.). The GIOP architecture provides several concrete protocols:

  1. Internet InterORB Protocol (IIOP) - The Internet Inter-Orb Protocol, is a protocol for communication between CORBA ORBs that has been published by the Object Management Group. IIOP is an implementation of the GIOP for use over an internet, and provides a mapping between GIOP messages and the TCP/IP layer.
  2. SSL InterORB Protocol (SSLIOP) - SSLIOP is IIOP over SSL, providing encryption and authentication.
  3. HyperText InterORB Protocol (HTIOP) - HTIOP is IIOP over HTTP, providing transparent proxy bypassing.
  4. and many more...

Data Distribution Service (DDS)

The Object Management Group (OMG) has a related standard known as the Data Distribution Service (DDS) standard. DDS is a publish-subscribe data distribution model, in contrast to the CORBA remotely-invoked object model.

VMCID (Vendor Minor Codeset ID)

Each standard CORBA exception includes a minor code to designate the subcategory of the exception. Minor exception codes are of type unsigned long and consist of a 20-bit “Vendor Minor Codeset ID” (VMCID), which occupies the high order 20 bits, and the minor code which occupies the low order 12 bits.

Minor codes for the standard exceptions are prefaced by the VMCID assigned to OMG, defined as the unsigned long constant CORBA::OMGVMCID, which has the VMCID allocated to OMG occupying the high order 20 bits. The minor exception codes associated with the standard exceptions that are found in Table 3-13 on page 3-58 are or-ed with OMGVMCID to get the minor code value that is returned in the ex_body structure (see Section 3.17.1, “Standard Exception Definitions,” on page 3-52 and Section 3.17.2, “Standard Minor Exception Codes,” on page 3-58).

Within a vendor assigned space, the assignment of values to minor codes is left to the vendor. Vendors may request allocation of VMCIDs by sending email to tagrequest@omg.org.

The VMCID 0 and 0xfffff are reserved for experimental use. The VMCID OMGVMCID (Section 3.17.1, “Standard Exception Definitions,” on page 3-52) and 1 through 0xf are reserved for OMG use.


Corba Location

CorbaLoc means Corba Location and refers to a stringified object reference for a Corba object that looks similar to a URL.

All CORBA products must support two OMG-defined URLs: "corbaloc:" and "corbaname:". The purpose of these is to provide a human readable/editable way to specify a location where an IOR can be obtained.

An example of corbaloc is shown below:

corbaloc::160.45.110.41:38693/StandardNS/NameServer-POA/_root

A CORBA product may optionally support the "http:", "ftp:" and "file:" formats. The semantics of these is that they provide details of how to download a stringified IOR (or, recursively, download another URL that will eventually provide a stringified IOR).

References

External links

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