COM/DCOM Implementation Basics of: Object creation and access Object Reuse Interface referencing

4 Basic Problems Basic interoperability – ensuring different vendor components can interoperate Version Control – upgrading separate components without upgrading entire system Language independence – different components can be developed in different languages Transparency – transparency across processes

Creating an Object Invoke an API to create the new object - CoCreateInstance COM locates object implementation and initiates the server process Object created and pointer to interface returned Object has to be initialized once it is created - load data from file, stream or any other data store. Alternatively, use monikers.

Accessing Objects In-process server - client and server execute in the same process. Local Object Proxy - lightweight Remote Procedure Call. Remote Object Proxy - DCOM is used for this. Requires marshalling and unmarshalling of data

Object Reuse Two forms: Containment - one object holds an exclusive reference to another (outer object holds and inner object) But performance problems can arise due to hierarchies in containment. Aggregation - defined in DCOM. Need collaboration between inner and outer objects. Mainly used for nested constructions. Both have to maintain transparency.

Interface referencing Specified by IDL, have a unique IID. Provides a small set of methods. COM objects can implement multiple interfaces. They can be derived from other interfaces using concept of inheritance - most inherit from IUnknown, also from IDispatch and IPersist.

Interface referencing (contd..) IUnknown has three methods: QueryInterface Addref Release Addref and Release are reference counting methods.

Interface referencing (contd..) QueryInterface - determines if interface is supported by object (at runtime) If it is, pointer to the interface is returned., else returns an error. Invoked when application needs a function of an object. This mechanism is provided by the Component Object library

Object Creation in DCOM Enhanced version of mechanism in COM - allows objects to be created on different machines. Library to needs to know server name (on which object to be created) and CLSID. 2 mechanisms for providing clients to indicate server name for object creation: Fixed configuration in system registry Remote server name can be explicitly defined Alternatively, use monikers.

Interface referencing (DCOM) DCOM has made an extension to definition of a new primitive data type. This is the interface reference to OBJREF. Variations: NULL STANDARD CUSTOM HANDLER