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 services that you will need.

The project has been created starting from the default project template accessible from 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 Windows Service Manager is done by the TService class. In the ServiceU.pas component, 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 ServiceController 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 the ServiceController commands but doesn't do any actual functional work. This is done using the ServiceThread.ProcessRequests(false); event in a while loop.

Usually, the OnExecute event handler looks like the following:

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

The waiting time 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, and not 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 a 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 as a thread safe for this use.

Delphi services don't have a default description under 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, write the following code 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 and run this command:

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

If all is okay, you should see this message:

The service installation is okay

Now, you can check in the Windows Services Console. 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, Continue, and check the file activity.

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

In the 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 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 in the following screenshot:

The GUI version of the Windows service

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