data matrix code generator c# Figure 8-5. Using the client as the server for message-based callbacks in Visual C#.NET

Generation DataMatrix in Visual C#.NET Figure 8-5. Using the client as the server for message-based callbacks

Figure 8-5. Using the client as the server for message-based callbacks
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CHAPTER 8 HOSTING AND COMMUNICATIONS
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When you re comparing MSMQ to the ASMX style of message processing, the big difference is the wire format and protocol. While you can pack SOAP into the body of MSMQ messages, you still need the MSMQ stack on both the client and the server. This necessitates Windows-to-Windows communication (or the use of some translation layer). ASMX, on the other hand, does not know and does not care what platform is on the other end of the communication because it is based on open, industry-standard specifications (see 6). Conversely, you have much more control over what goes into the message with MSMQ, liberating you from the constraints of SOAP and XML Schema. When you re not using Queued Components, you must write code to create messages and place them in queues. You must also create a listener process, one that is listening for messages arriving in the queue, pulls them out, and processes them. This alone is a strong argument in favor of using Component Services. Many features of COM+ can be leveraged automatically via adherence to some design principles and some details of configuration (thread pools and object pools, for instance). See 7 for a detailed look at Queued Components. Let s take a look at a custom message pump for MSMQ. Here s the premise: The enterprise you work for has decided it needs a complete inventory of all of the Word documents that exist on all of the workstations. You re going to push an application to the users machines that will scan their hard drive for Word docs, and send them to the server, which will insert them into the database. Because this application will run first thing in the morning when users log in, you know there will be a huge spike in traffic doing inserts into this database table. In order to avoid coupling the efficiency of the document search to the availability of the database, you ll have the client post messages into a queue, and then they can be pulled out and processed as the server can get to them, without forcing the update to succeed before the client can continue looking for more documents. The first thing you ll need is a Windows Service that acts as a listener for incoming messages. You ll start with the Windows Service Visual Studio .NET project type, which gives you a template for a class that uses System.ServiceProcess.ServiceBase as its base class. This class gives you the plumbing you need to host your process as a service running in the background. Note that this code needs a reference to the System.Messaging.dll assembly. using System.Messaging; namespace QListener { public partial class ListenerSvc : ServiceBase { private MessageQueue queue; bool bDone = false; protected override void OnStart(string[] args) { if (!MessageQueue.Exists(QLibrary.DocDescription.QueueName)) { MessageQueue.Create(QLibrary.DocDescription.QueueName); } queue = new MessageQueue(QLibrary.DocDescription.QueueName);
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Thread t = new Thread(MonitorQueue); t.Start(); } protected override void OnStop() { bDone = true; } //Class Definition Continues The overridden onStart method is called when the Service is started, either at system startup or when an administrator starts it manually from the Services applet. This code ensures that the queue you ll be using exists, and then starts a listener thread using the address of the MonitorQueue method. You ll look at the code for monitor queue shortly. The QLibrary.DocDescription type contains some types that you ll use from both the client and the server, so it lives in its own assembly. The definition of the string QueueName (used above) looks like this: public const string PrivateQ = @"\private$\"; public const string LocalQName = "queuetest"; public static readonly string QueueName = string.Format("{0}{1}{2}", System.Net.Dns.GetHostName(), PrivateQ, LocalQName); This builds up the full name of the queue you ll be monitoring. The overridden OnStart method instructs your code to create a private queue named queuetest that you ll monitor from the service. MonitorQueue is the process that waits for messages to come into the queue, and then processes them. There are a couple of different ways you can monitor a queue. You can set up a trigger, which relies on another Windows Service to monitor the queue and then pass the messages to a component of your choosing. Using this results in code that looks more like event trapping code. The downside of this approach is that you must set up the trigger on the server hosting the queue, resulting in more complex deployment and configuration. The other method is to write your listener as a polling application. The algorithms used to do this can become quite complex. You can create your own thread pools, and you re also in control of the polling frequency used on each thread. Further, you may have many queues involved. What you need will depend upon the type of processing the listener will be doing and the expected load patterns of messages coming into the queue. The example here is quite simple: It uses a single thread that s listening for incoming messages and then processes them (from Program.cs in the ConsoleHost project of the Code08 solution). private void MonitorQueue() { Message msg; while (!bDone) { try {
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