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CHAPTER 10 FUNCTIONAL ROLES
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public void Stop() { lock(this) { foreach (Thread thread in threads.Values) thread.Abort(); threads.Clear(); } } public void HandlerCompleted(WorkerRequestHandler theHandler) { lock(this) { threads.Remove(theHandler); } FireRequestCompleted(); } public delegate void RequestCompletedHandler(); public event RequestCompletedHandler OnRequestCompleted; public void FireRequestCompleted() { if (OnRequestCompleted == null) return; // fire event on the UI thread lock(this) { uiControl.Invoke(OnRequestCompleted); } } } } Listing 10-8. A VB .NET Implementation of the Coordinator Imports System.Threading Public Class Coordinator Private uiControl As Control ' key is WorkerRequestHandler, value is Thread Dim threads As New Hashtable Public Sub New(ByVal theUiControl As Control) uiControl = theUiControl End Sub Public Sub Run() SyncLock (Me) Dim handler As New WorkerRequestHandler Dim thread As New Thread(AddressOf handler.Process) AddHandler handler.OnRequestCompleted, AddressOf HandlerCompleted threads.Add(handler, thread)
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CHAPTER 10 FUNCTIONAL ROLES
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thread.Start() End SyncLock End Sub Public Sub [Stop]() SyncLock (Me) For Each t As Thread In threads.Values t.Abort() Next threads.Clear() End SyncLock End Sub Public Sub HandlerCompleted(ByVal theHandler As WorkerRequestHandler) SyncLock (Me) threads.Remove(theHandler) End SyncLock FireRequestCompleted() End Sub Public Delegate Sub RequestCompletedHandler() Public OnRequestCompleted As RequestCompletedHandler Public Sub FireRequestCompleted() ' fire event on the UI thread SyncLock (Me) uiControl.Invoke(OnRequestCompleted) End SyncLock End Sub End Class Coordinator is the only class in the system with thread synchronization logic. In this example, there is type coupling between the various classes, as shown in Figure 10-11.
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Figure 10-11. The coupling between the classes in the sample implementation All the classes are in the same component, so the coupling wouldn t create any deployment problems. The coupling would, however, prevent you from testing WorkerRequestHandler and Coordinator separately. In the Binders section later in this chapter, I ll show you how to use binders to eliminate the type coupling.
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CHAPTER 10 FUNCTIONAL ROLES
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The idea behind coordinators is not new in software systems. In the design pattern literature, there are various names for classes that act in ways similar to coordinators: handlers, relayers, mediators, adapters, dispatchers, fa ades, liaisons, and others. How a coordinator is implemented internally, in terms of classes and design patterns, is not important for the purposes of this chapter. Coordinators have a specific purpose in life: to coordinate the actions of one or more workers assigned to them. They do so in two ways: by handling the events fired by workers, and by calling workers to carry out detailed operations as part of a broader task.
What Is a Coordinator
Coordinators are like managers in a small company department. They handle many of the infrastructure details, such as scheduling, resource allocation, and resource coordination. By having coordinators assume these tasks, workers can focus better on the business logic they must implement. The coordination task itself might be considered a sort of business rule or metarule. In a distributed system, coordinators fulfill tasks similar to Dispatching Servers (DSs) in JEDI,5 but they also contain coordination logic specific to the workers they manage, and are therefore application-specific. There is a difference in scope between coordinators and JEDI DSs. The latter are designed to control the events over a node in a distributed system. Coordinators are used to control a narrow group of workers, which may or may not be in the same address space as the coordinator. The purpose of a coordinator is to reduce the complexity of workers by offloading tasks that are not strictly part of the workers main mission. A coordinator controls one or more workers assigned to it. It does so by interacting with the workers, using events or direct method calls. Figure 10-12 shows two typical coordinator usages.
Coordinator <<Coordinator>>
Coordinator <<Coordinator>>
Multiple Workers
<<Worker>>
Figure 10-12. Coordinators can control one or more workers. The lines connecting the coordinators and workers aren t meant to imply a relationship or an association, in the UML sense. The fact that a coordinator can be assigned to handle just one worker is significant for reasons that may not be immediately obvious. After all, if the outside system needs to interact with a single worker, why introduce a middleman Besides being useful for managing
5. Gianpaolo Cugola, Elisabetta Di Nitto, and Alfonso Fuggetta, The JEDI Event-Based Infrastructure and Its Application to the Development of the OPSS WFMS, IEEE Transactions of Software Engineering, September 2001.
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