RNSO - A Remote Notifiers,Subjects and Observers Server over DCOM

RNSO is a server where the well known Observer Design Pattern is implemented with Distributed COM.

The Observer Design Pattern helps you to build applications when a change on a data need to be notified to another of your app.
The Datas are called Subjects, and parts of applications interrested by the changes is (are) Observers.
Changes are notified from Subjects to Observers by notifications, throught a Server (called the Manager). Only the manager knows about the Observers who want to be notified. It maintains the connections called Subscriptions.

Generally, Observers indicates they want to be notified by a method :
Observer1.Subscribe( pSubject ) ;
Observer2.Subscribe( pSubject ) ;
Subject prevents changes by another method :
Subject.Notify( ..something ) ;

Here, the 2 observers Observer1 and Observer2 will be notified ..

This design pattern is very usefull for GUI applications (MDI..), but this can be used in several other cases. As an example, if you build a DLL to access to a hardware equipment and  you’d like to use it on remote Machine, you ‘ll be able with RNSO.

RNSO allows to build client-server applications over DCOM, without knowing about DCOM or the Observer Design Pattern, and is independant of applications. The Server can serve several applications at the same time, even they have no relations.

How does RNSO Work?

 


Subject1 notifies about changes. The server (RNSO.EXE, is automatically launched by the OS) notify the Observers who subscribed to the subject.

Observers have a sink interface used by server to notify them. This is also a component, with a mailbox. The mailbox is read by a thread of the component. So the server is not blocked by clients.

Datas that a subject can notify are binary. So you can transmit all you want.

Binaries of RNSO

I made several binaries for RNSO :

RNSO.exe                           The server
RNSOSupport.DLL             Dll used both by the server and the clients.
RNSOClient.DLL                Dll used to build the observer clients (need also RNSOSupport.DLL). You don't need of it if you just build a Subject.

The binaries are build with Visual C++ 6.0.

The Sample Application

The sample application shows you:
- how to make a CDialog as an Observer and a Subject.
- how to use classes and ready-to-use components .

The following figure shows 2 instances of the application.
1) On the left, we declared the name of the object for the dialog: Subject1.
2) On the rite, we declare the dialog as an observer to Subject1.
3) On the left, the subject notify a change  'My Message'
4) On the rite, the subscriber received the notification.
 

You can start several apps and subscribe to 'Subject1'. All of them will be notified. When a subject is going down, subscribers receive a notification.

In my sample, I derived my CDialog from CDialog an from RnsoNotificationRout. This class has 2 virtuals methods. Overload them to handle notifications:


class AFX_EXT_CLASS	RnsoNotificationRout
{
	public:
	RnsoNotificationRout(); 
	virtual	~RnsoNotificationRout(); 
	virtual void OnNotifyNOTIFY(ObserverNotification*	pNotification); 
	virtual void OnNotifyNOTIFY_SUBJECTBROKEN(DWORD dwSubjectID);
};

You aren't obliged to have a dialog or a window to build an Observer or a Subject. You can also make you class just as an Observer, or just a Subject.

This is very simple.

Why to use this framework?

You dont have to maintain subscriptions yourself with lists,  arrays.. etc.

You can build interprocess / intermachines apps.

Download demo project - 197 KB