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The architecture implemented by Windows Forms is a very good way of implementing a producer-consumer architecture. The reason why it s good is because it s elegant and selfcontaining. Following is the source code that implements a generic producer-consumer architecture: interface IProducerConsumer { void Invoke( Delegate @delegate); void Invoke( Delegate @delegate, Object[] arguments); } class ThreadPoolProducerConsumer : IProducerConsumer { class Executor { public readonly Delegate _delegate; public readonly Object[] _arguments;

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Some command-line applications available for the Kerberos server include the following: kdcsetup creates the first admin account. kerberosautoconfig creates the keytab files.

public Executor( Delegate @delegate, Object[] arguments) { _delegate = @delegate; _arguments = arguments; } } private Queue< Executor> _queue = new Queue<Executor>(); private void QueueProcessor( Object obj) { MonitorEnter( _queue); while( _queueCount == 0) { MonitorWait( _queue, -1); } Executor exec = _queueDequeue(); MonitorExit( _queue); ThreadPoolQueueUserWorkItem( new WaitCallback( QueueProcessor)); exec_delegateDynamicInvoke( exec_arguments); } public SingleThreaderProducerConsumer() { ThreadPoolQueueUserWorkItem( new WaitCallback( QueueProcessor)); } public void Invoke( Delegate @delegate, Object[] arguments) { MonitorEnter( _queue); _queueEnqueue( new Executor( @delegate, arguments)); MonitorPulse( _queue); MonitorExit( _queue); } } ThreadPoolProducerConsumer has a single public method, Invoke, which is used in the same fashion as the Windows Forms Invoke method What makes the generic producer-consumer work is its use of the Monitor synchronization class Monitor is a special type of synchronization class that has the ability to indicate something has changed.

The ability to manage preferences is one of the most powerful aspects of directory services. In previous sections, we showed how to set up Open Directory to have a centralized repository of usernames and passwords. You can further secure the server itself (and any other servers in our environment) using Kerberos and password policies. We have also showed how to secure Open Directory accounts by enabling password policies and restricting the types of communications with the server. Now, we ll cover enforcing policies, known as managed preferences, on client systems. To access and configure policies in Tiger systems (which we will use as an example because it is the dominant operating system in the wild today), open Workgroup Manager, click Preferences, and change to the directory service for Open Directory. Then click a computer, user, or group on the left column, and click the type of policy in the right column (see Figure 13-17). Each policy has options for First or Always. The First option pushes managed preferences out to end users the first time they log in. The Always option pushes managed preferences out to end users every time they log in. These policies include the following: Applications: Chooses the applications that users can access or denies access to specified applications. Classic: Controls users ability to use Classic applications and controls how they are handled.

Figure 12-24. Directional voltage control using two relays Although you could use the point-to-point wiring schemes shown so far, it s better to build the circuit shown in Figure 12-24 on a printed circuit board (PCB). Figure 12-25 shows how you can connect the relays and diodes in a sturdy and compact way on a prototype PCB. Speaker wires are used to input the 12V and NXT signals and output the load voltage.

To understand how Monitor works, let s consider the overall workings of a producer-consumer implementation The consumer thread (QueueProcessor) executes, constantly waiting for items in the queue (_queue) To check the queue, the method MonitorEnter is called, which says, I want exclusive control for a code block that ends with the method call MonitorExit To check the queue, a while loop is started, which waits until something appears in the queue The thread could execute, constantly waiting for something to be added, but while the thread is looping, it has control of the lock This means a producer thread can t add anything to the queue The consumer needs to give up the lock, but also be able to check whether anything is available in the queue The solution is to call Monitor.