This question appears in Mumbai University > Parallel And Distributed System subject

Marks: 10 M

Year: May 14

• CORBA allows object implementations to vary widely were multiple IDL interfaces may be implemented by a single server program, while in other cases an IDL interface may be implemented by a series of shell scripts, one for each operation.
• The Object Request Broker provides the flexibility to permit integration of existing applications without locking new objects into a limited set of implementation criteria.
• An object adapter (OA) provides the means by which various types of object implementations utilize ORB services such as object reference generation, object method invocation, security, activation and deactivation of objects and implementations.
• Depending on the underlying ORB, an OA may choose to provide these services by delegating to the ORB or by performing the work itself.
• In either case, object implementations are not aware of the difference because they interface only to the OA. Each ORB is expected to provide a general OA, called the Basic Object Adapter (BOA), which is intended to support objects implemented as separate programs.

• An adapter performs the following functions:

  • It registers servant objects when they are created or installed on the server.
  • It creates object references for the servants.
  • It activates a servant object when it is called.

  There are three main ways of installing a servant object on a server.

  • The first way consists of creating the servant, using an object factory.
  • The second way consists of importing the servant from another site, by moving or copying it to the server site.
  • The third way consists of constructing the object from its elementary parts, i.e. a state and a set of functions (methods) operating on the state.
  • This latter mode is used when an existing application, not written in object style, must be reused in an object-oriented setting.
  • In this case, the existing application needs to be "wrapped up", were its contents needs to be encapsulated to make only visible an object-style interface.
  • Several different adapters may coexist on a server, to provide different policies for process activation.
• A servant object may now be identified by providing identification for its adapter together with an internal identification of the object within the adapter. This information is part of the object's reference.
• Starting from the stub, the call is directed to the adapter, providing the internal identification of the servant object within the adapter.
• The adapter must locate the object's skeleton, activate the object if needed, and forward the call to the skeleton.
• Except for persistent implementations, the BOA will automatically activate an object implementation if a request comes in for it and it is not yet executing. Object Adapters other than the BOA may exist.
• The CORBA specification mentions a Library Object Adapter (LOA) for use with lightweight object implementations that are co-resident with client applications, and an object-oriented database adapter that provides an interface from object-oriented databases to the ORB.