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with Borland Czz Builder, was built as a standard executable as well as a COM ( component object model) for the. Microsoft Windows environment. COM is a. Component Object Model: An Overview & Practical Abstract- The Component Object Model (COM) is a system .. 9/pdf. [MS-COM]. Component Object Model Plus (COM+) Protocol. Intellectual Property Rights Notice for Open Specifications Documentation.
Microsoft Technologies. A model that. DCOM is the distributed component model. One of those models is distributed component object model DCOM which is a protocol that enables software. COM is Microsoft's framework for developing.. Download Books.. Broker Architecture are two popular distributed object models. In this paper, we make. The consequence of having these interfaces implemented as pointers means that they can only be used to invoke methods on DCOM components and not used to edit the data within them.
DCOM also uses interfaces as a method for components to interact with one another.
The architecture was designed in such a manner as to prevent DCOM objects from ever having direct access to one another. Due to the lack of direct access between components, the model is able to pre- serve the encapsulation of data as well as the data processing.
This design decision was not implemented by accident. The developers of DCOM decided that in order to make the model more exible it should preserve the encapsula- tion of the components. One important concept to remember about interfaces is that they can never be instantiated. The reason is because there is no implementation associated with it. DCOM components must implement the interface and as a result the component can then be created and used.
DCOM is also capable of providing polymorphism by providing the ability for two or more components to implement an interface di erently. The only condi- tion needed to achieve polymorphism is for the DCOM components to preserve the behaviour of the interface.
It is also possible for a DCOM component to implement multiple interfaces. A typical example of this might be a component which is responsible for dealing with a car rental booking.
The DCOM component might provide an interface for dealing with a booking with a visa card as well as dealing with a booking using a cheque. Another key component that the DCOM architecture addresses as was men- tioned in section 1 is that of versioning.
In DCOM all interfaces are immutable, meaning that once an interface has been implemented it can not be changed. If a change has to be made to the interface, then a new interface is created. There is no concept of versioning interfaces. This is done so as to provide backward compatibility. A new interface would be responsible for adding new functional- ity to the interface but also preserving the original functionality which existed before the upgrade.
In order to achieve this, most developers implement the old functionality in terms of the new functionality added to the interface.
This interface, when implemented for DCOM compliance allows a component to make a dynamic look up on a component to see if an interface is supported. If the 9 interface is supported then QueryInterface returns a pointer to that interface. If that interface is not supported the QueryInterface returns a NULL pointer indicating that this component doesn't support that feature.
This approach enables the evolution of functionality in an application as all the components which the application uses are connected to and bound dynamically. This means that as new components are added to the system the applications can make direct use of them. All that happens is that the application tries to nd a component and if it does not exist yet, the application falls back to one that it knows how to use by default.
The problem of using the same name for interfaces is avoided in DCOM through the use of generated a unique identi er which signi es each interface.
From a network transparency perspective, the use of interfaces makes it much easier for DCOM to intercept any local interface call and substitute it with a network protocol. This kind of approach requires little if no intervention from the local architecture and hence makes the integration totally seamless. Interfaces, even though they are only pointers, provide developers with the choice of selecting a language which is relevant to them.
With DCOM the only restriction on language choice is that it must be capable of dealing with pointers and must conform to the speci cations regarding the interface map and method invocation.
DCOM Microsoft Distributed Component Object Model Solutions Manual
This is explained in section 3. The next section discusses the three major interface methods which need to be implemented in order to conform to being a DCOM component. The rst method and probably the most important method is QueryInterface. The aim of this function is to provide clients with a means of being able to determine what functionality a component possess.
The QueryInterface takes an interface name to search for as well as a reference point to start the search from. If the QueryInterface method can nd a match then the QueryInterface will add one to the reference counter for that interface and return the pointer to the component. If the QueryInterface is unable to nd what it was searching for it returns a null pointer.
The QueryInterface method only serves as a lookup function as DCOM compo- 10 nents are not able to directly access one another. The second method which is indirectly called from QueryInterface if the result is successful is AddRef. This method is responsible for incrementing the reference count which is controlling how many clients are using this component. The third and nal method which belongs to the IUnknown interface is Re- lease.
This method is responsible for decrementing and checking the number of references there are to any one component. When the reference count for a component returns to zero the component is dynamically unloaded. The binary standard prescribes a certain layout which an object must take in order to conform with DCOM.
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The layout of the objects refers to the way in which the binary memory map which points to the functions is represented in memory. In DCOM the equivalent is the interface map. By following these guidelines a developer has the choice of any language which is capable of making use of pointers. DCOM components make extensive use of pointers in the interface map and this is why it is important for the language to provide support for pointers.
Below is a pictorial representation of how the pointers link up between the component, the interface map and the actual function itself.
Instead of having two di erent components with two di erent interface maps pointing to the same functions, it is possible to have the second component point to the interface map of the rst component.
This in itself introduces problems regarding reference counting but the DCOM concept of interfaces manages to solve this problem. It speci es the representation and communication of object refer- ences Microsoft and Digital Equipment Corporation b.
The protocol supports versioning of interfaces through universally unique identi ers UUID's. Each version of a published interface will have a unique identi er.
An IPID is a bit identi er which represents a particular interface of a particular object in a particular server. An IPID however does not indicate the binding information necessary to carry out the actual invocation. This information is represented in the object export identi er OXID. Each machine has a unique object export service to coordinate the management of all OXID's on that machine.
It caches and returns to clients on request the string bindings necessary to connect to OXIDs of exported objects. It also receives pings described below from remote clients to keep its own objects alive. The DCOM protocol provides for remote reference counting as well as pinging.
Reference counting will be inadequate in cases where clients terminate abnor- mally. Remote abnormal termination is detected by periodic pings. The output les include a header le, server stub le, client stub le, and auxiliary les. This allows a DCOM interface to be represented in a lan- guage independent manner. IAccInfo , the interface name is a valid data type and as such can be used as a parameter within a method declarations, this provides for static type checking during the MIDL compile phase.
As clients gain access to an object, AddRef is called, this increments the reference counter. When a client no longer requires access, Release is called, decrementing the counter. The QueryInterface method is used for interrogating the server to nd out which interfaces it supports.
The client code can invoke QueryInterface for a particular interface name and nd out if the named interface is implemented by the server. Once the a client has a pointer to the object's IUnknown interface, it can use QueryInterface to request pointers to other interfaces.
The IID is used for globally unique run-time identi cation of the interface. The facility for asynchronous method calls is provided as part of the DCOM standard. Each interface has its own dynamic dispatch table, consisting of a few standard methods function pointers whose calling conventions and semantics are defined by COM, followed by an arbitrary number of custom methods whose calling conventions and semantics are specific to that particular interface.
Although the COM specification defines a few basic interfaces, anyone can independently define new COM interfaces, and in fact the OSKit defines quite a number of such interfaces. COM interfaces are identified by bit globally unique identifiers GUIDs , which are algorithmically generated through various standardized mechanisms; this avoids all the accidental collisions that can easily occur when human-readable names are used as identifiers.
Derived interfaces cannot relax or weaken the requirements of the base interface, since that would violate the whole principle of subtyping. Multiple inheritance of COM interfaces cannot be implemented simply by extending dispatch tables this way, since in this case there would be multiple mutually conflicting dispatch tables to extend; however, the effect of multiple inheritance can be achieved by making the object support multiple independent interfaces using the querying mechanism described below.
This minimal COM interface contains only three standard methods, query, addref, and release, which provide the basic administrative facilities that all COM objects are expected to implement. These basic facilities, which essentially provide run-time type determination and reference counting, are described in the following sections.
To determine if a COM object supports a particular interface, the client simply calls the standard query method on the object, passing the GUID of the desired interface as a parameter. If the object supports the requested interface, it returns a pointer to that interface; this returned pointer is generally, though not always, a different pointer from the one the client already had, since it points to a different interface, even though the interface refers to the same underlying logical object.
The client can then interact with the object through the methods defined by this interface. Since interfaces are identified only by simple GUIDs and do not directly contain any detailed information about the methods defined by the interface, the client must already know before it requests the interface what methods the interface supports and what their semantics are.
In other words, the basic COM query mechanism only allows the client and to determine the common set of interfaces both it and the object already understand; it does not directly enable the client to learn about other arbitrary unknown interfaces the object might support.
Semantics of the Query Operation The COM standard specifies that all COM objects must support the standard query operation, and furthermore, the query operation must have certain well-defined semantics. Symmetric: A query on an interface for the same interface must succeed. Reflexive: If a query on interface A for interface B succeeds, then a query on interface B for interface A must also succeed.
Transitive: If a query on interface A for interface B succeeds, and a query on interface B for interface C succeeds, then a query on interface C for interface A must succeed. However, note that subsequent queries for a given interface identifier on a given object are not required always to return the same pointer. This allows objects to create interfaces dynamically upon request and free them later when they are no longer in use, independently of the lifetime of the object itself.
As a special exception to this rule, queries on an object for the IUnknown interface must always return the same pointer; this allows clients to perform a reliable object identity test between two arbitrary COM interface pointers by querying each for their IUnknown interfaces and comparing the pointers. However, as an approximate object identity test, in which occasional false negative answers can be tolerated i.For this purpose the PC station has to be configured.
In designing a suite of interfaces, the designers can spend their time in the essence of the design--the contracts between the parties--without having to think about the underlying communication mechanisms for any interoperability scenario. This can be. Traditionally, developers have had little choice when 3 choosing the language of implementation due to the software bus prescrib- ing a certain language and a certain methodology.
From observation, the problem with implementation inheritance is that it is significantly easier for programmers not to be clear about the actual interface between a base and derived class than it is to be clear. To determine if a COM object supports a particular interface, the client simply calls the standard query method on the object, passing the GUID of the desired interface as a parameter.
NET components in different languages. Note The reverse application case — using a. Platforms Is the application only to communicate with Windowsbased systems no Linux. The rate of change of data can be set via DB10 byte 0.