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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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Public Class Receiver Private sharedMemory As Rendezvous Public terminationRequested As Boolean Public Sub New(ByVal theSharedMemory As Rendezvous) sharedMemory = theSharedMemory End Sub Public Sub Run() While Not terminationRequested WaitForNotifications() System.Threading.Thread.Sleep(10) End While End Sub Sub WaitForNotifications() If Not sharedMemory.IsNotificationPending Then Return ' notification found: get the payload Dim s As String = sharedMemory.RemoveNotification() ProcessNotification(s) End Sub Sub ProcessNotification(ByVal s As String) End Sub End Class The Rendezvous class uses C# locks to protect critical sections. Locks only work at the thread level, so if Sender and Receiver were in different processes, a semaphore or other process synchronization object would be required. To handle multiple receivers, you could change the Rendezvous object to manage a subscription process. When a receiver subscribes, the Rendezvous object might add the new subscriber to an internal list and then create a queue to which the subscriber s notifications are written. Each time the sender calls AddNotification to add a notification, the Rendezvous object would add it to the queues of all subscribers. The Rendezvous object could also handle notification filtering. Shared memory notifications are generally used only in a unicast mode, because they become cumbersome with multiple subscribers. The problem with multicast mode is due to the way subscribers typically wait for notifications, by monitoring a designated rendezvous area of shared memory. To support multiple subscribers, you would need to make sure that each subscriber had a different signaling flag. This entails that subscribers understand that they might not be alone in accessing the notification data, and must obey certain rules to prevent interfering with each other.
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Using files as a shared resource is a fairly simple, if inefficient, way to send notifications. You can use files to send notifications between processes on the same machine or on different machines. In the first case, you can use a single file. The simplest technique is to have the sender save the notification payload in a shared file. The subscriber must obviously know the name of the file and directory. The subscriber watches the directory, waiting for the file to appear. When it appears, the subscriber reads the file contents and then deletes the file. The diagram in Figure 8-16 shows the timing.
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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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See If Shared File Is Present()
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Figure 8-16. Using a shared file between processes on the same machine The sender writes the notification payload to a temporary file in the shared folder. Once the file has been written, its name is changed into the shared file s designated name. The file system executes the Rename command as an uninterruptible command, so the receiver doesn t see the shared file appear until the temp file has been completely written, closed, renamed, and is ready to be read. The renaming stratagem spares the sender and receiver from having to deal with file locks. A significant problem with this simplistic approach is that the sender can t send further notifications until the subscriber has read the previous one. There are two typical solutions. The simplest is to use multiple files, named according to a numbering scheme. The alternative is to use a single file, but with new rules, to allow the file to contain multiple notifications. Using multiple files, the sender uses the same approach shown in the previous sequence diagram, but creates a new shared file for each notification sent, using a predefined naming sequence. It might create files named SharedFile000, SharedFile001, etc. The receiver would delete each file after reading it. Listing 8-16 and Listing 8-17 show implementations in C# and VB .NET. Listing 8-16. Using Numbered Files to Fire Events in C# public class Sender { string sharedDirectory; int fileNumber = 0; public Sender(string theSharedDirectory) { sharedDirectory = theSharedDirectory; if (!System.IO.Directory.Exists(theSharedDirectory)) System.IO.Directory.CreateDirectory(theSharedDirectory); } public void FireEvent(string thePayload) { string tempFileName = sharedDirectory + @"\temp"; // create the temp file System.IO.StreamWriter writer = new System.IO.StreamWriter(tempFileName); writer.WriteLine(thePayload); writer.Close();
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