The way to go, in the previous scenario, is to develop a Windows service. In this recipe, we'll see how to write a good Windows service scaffold, and this can be the skeleton for many other services. So, feel free to use this code as a "template" to create all the services that you will need.

The project has been created starting from the default project template accessible by going to File | New | Other | Delphi Projects | Service Application and then has been integrated with a set of functionalities to make it "real."

All the low-level interfacing with the Windows Service Manager is done by the TService class. In ServiceU.pas, there is the actual descendant of TService that represents the Windows service we are implementing. Its event handlers are used to communicate with the operating system.

Usually, a service needs to respond to the Windows Service Controller commands independently of what it is doing. So, we need a background thread to do the actual work, while the TService.OnExecute event should not do any real work (this is not a must, but usually is the way to go). The unit named WorkerThreadU.pas contains the thread and the main service needed to hold a reference to the instance of this thread.

The background thread starts when the service is started (the OnStart event) and stops when the service is stopped (the OnStop event). The OnExecute event waits and handles ServiceController commands but doesn't do any actual functional work. This is done using ServiceThread.ProcessRequests(false); in a while loop.

Usually the OnExecute event handler is like this:

procedure TSampleService.ServiceExecute(Sender: TService);
begin
  while not Terminated do
  begin
    ServiceThread.ProcessRequests(false);
    TThread.Sleep(1000); 
  end;
end;

The wait of 1000 milliseconds is not a must, but consider that the wait time should be not too high because the service needs to be responsive to the Windows Service Controller messages. It should not be too low because otherwise the thread context switch may waste resources.

The background thread writes a line in a logfile once a second. While it is in a Paused state, the service stops writing. When the service continues, the thread will restart writing the log line. In the service event handlers, there is the logic to implement this change of state:

procedure TSampleService.ServiceContinue(Sender: TService; var Continued: Boolean);
begin
FWorkerThread.Continue;
Continued := True;
end;

procedure TSampleService.ServicePause(Sender: TService; var Paused: Boolean);
begin
FWorkerThread.Pause;
Paused := True;
end;

In the thread, there is the actual logic to implement the Paused state, and in this case, it is fairly simple: we've to pause the writing of the logfile:

Here's an extract:

    Log := TStreamWriter.Create(
      TFileStream.Create(LogFileName, fmCreate or fmShareDenyWrite));
    try
      while not Terminated do
      begin
        if not FPaused then
        begin
          Log.WriteLine('Message from thread: ' + TimeToStr(now));
        end;
        TThread.Sleep(1000);
      end;
    finally
      Log.Free;
    end;

The Boolean instance variable FPaused can be considered thread safe for this use.

Delphi services don't have a default description under the Windows Service Manager. If we want to give a description, we have to write a specific key in the Windows registry. Usually, this is done in the AfterInstall event. In our service, this is the code to write in the AfterInstall event handler:

procedure TSampleService.ServiceAfterInstall(
Sender: TService);
var
Reg: TRegistry; //declared in System.Win.Registry;
begin
Reg := TRegistry.Create(KEY_READ or KEY_WRITE);
try
  Reg.RootKey := HKEY_LOCAL_MACHINE;
  if Reg.OpenKey(
    '\SYSTEM\CurrentControlSet\Services\' + name, 
    False {do not create if not exists}) then
  begin
    Reg.WriteString('Description', 
      'My Fantastic Windows Service');
    Reg.CloseKey;
  end;
finally
  Reg.Free;
end;
end;

It is not necessary to delete this key in the AfterUnInstall event because Windows deletes all the keys related to the service (under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<MyServiceName>) when it is actually uninstalled.

Let's try an installation. Build the project, open the Windows command prompt, and go to the folder where the project has been built. Then, run this command:

C:\<ExeProjectPath>\WindowsService.exe /install

If everything is ok, you should see this message:

Figure 11.1: The service installation is ok

Now, you can check under the Windows Services Console and you should find the service installed. Click on Start, wait for the confirmation, and the service should start to write its logfile.

Play with Pause and Continue and check the file activity.

Windows Services are very powerful. Using the abstractions that Delphi provides, you can also create an application that, reading a parameter on the command line, can act as a normal GUI application or as a Windows Service.

In the respective recipe folder, there is another recipe called 20_WindowsServiceOrGUI.

This application can be used as a normal Windows Service using the normal command line switches used so far, but if launched with /GUI, it acts as a GUI application and can use the same application code (not TService). In our example, the GUI version uses the same worker thread as the service version. This can be very useful also for debugging purposes.

Run the application with the following command:

C:\<ExeProjectPath>\WindowsServiceOrGUI.exe /GUI

You will get a GUI version of the service, as shown here:

Figure 11.2: The GUI version of the Windows Service

LogMessage('Your message goes here for SUCCESS', EVENTLOG_SUCCESS, 0, 1);