Windows Service Application in Delphi

What is Windows Service?

Windows service is an application that runs on background. It is usually used to check system updates continuously, to work on system resources. Service applications take requests from client applications, process those requests, and return information to the client applications. They typically run in the background, without much user input.

Why do we need it?

Suppose we have system that needs to respond 24/7 days or if we have software that needs to update frequently, then we can create service which will check the new updates and download it. Then update to new software. In such situations we can use Window Services.

What are the limits?

We cannot use any visual components in service applications. You can add just any nonvisual component to the new service. But you’re restricted to nonvisual components; you'll get an exception Controls Cannot Be Added To A Service message if you try to drop a visual component

How to create Windows Service in Delphi?

To create a service application in Delphi start Delphi IDE and select File->New->Other. Then select "Service Application". Then you can see application which includes a Datamodule descendent Tservice class. The TService class is where we do our stuff and it has a number of properties which you can see in the Object Inspector.

Note that if you have the Standard edition of Delphi "Service Application" may not be available. You need at least the Professional edition.

Configure service application - check object inspector for property and events. 

Properties

AllowPause and AllowStop properties used to allow Start and Stop service in Service Control Manager. You need to set these to true.

DisplayName specifies the name of the service in the Service Control Manager or Task Manager; enter any name you want in here.

Interactive property specifies whether the Service should be able to communicate with the desktop (for example, with ShowMessage).

The ServiceStartName and Password properties specify the user account detail to allow operating the service.

The ServiceType is set to stWin32 for a Win32 Service, and the StartType property can be stAuto, stBoot, stDisabled, stManual, or stSystem

Events

Once you’ve set up the properties, switch to the events tab. Let's go through the event options:

Before Install, After Install and Uninstall allow you to create actions that will take place during these events.

Start, Stop, Pause, and Continue are all key events that let you interact with the Service from the Service Control Manager or Task Manager.

You can start, stop or pause the service in the Service Control Manager.

OnStartEvent, OnExecute is an event that is called when the Service is started. This is where we will write our code to perform some task.

Install & Uninstall Service

Note that you need administrator rights to install and uninstall service applications, because it is necessary to write or delete registry entries in HKEY_LOCAL_MACHINE, and that requires permissions that normal or restricted users do not have.

To install the service: Click Start and select run, then type: PATHTOYOURSERVICE\yourservicename.exe /INSTALL /silent then press Enter

To uninstall: Click on Start and select run, then type: PATHTOYOURSERVICE\yourservicename.exe /UNINSTALL / silent then press Enter You will see a confirmation dialog when the service has been successfully uninstalled or an error message if it failed. It can fail if you do not have sufficient rights.

Note*: /silent If you do not want to see the confirmation dialog you can add the /silent switch like this:

Open the Control Panel; go into Administration and into the Services applet. You will see a list of currently installed services. Locate your service name in the list, right click on it and select "Start" menu item in the popup menu. Notice that the Status for the service changes to Started. You can press the F5 key or select the Refresh command to update the list to make sure that the service keeps running.

You can also see your service in the Task Manager. Right click on an open area of your task bar at the bottom of your screen, then select "Task Manager", select the Processes tab. Here you can see a list of currently running processes

Writing codes in service application to do some task

There are basically two places where you can put your service code: OnExecute or the OnStart event.

OnExecute Event:

Put your code in the TService.OnExecute method to perform some task when service is executing. Here you can also create a thread with your code if you want.

-      Occurs when the thread associated with the service starts up.

-      When the OnExecute event handler finishes, the service thread terminates.

-      Most OnExecute event handlers contain a loop that calls the service thread’s ProcessRequests method so that other service requests are not locked out."

Example

procedure TCompanySqlDatabaseSpecialSomething.ServiceExecute(

  Sender: TService);

const

  SecBetweenRuns = 10;

var

  Count: Integer;

begin

  Count := 0;

  while not Terminated do

  begin

    Inc(Count);

    if Count >= SecBetweenRuns then

    begin

      Count := 0;

      { place your service code here }

      { this is where the action happens }

      SomeProcedureInAnotherUnit;

    end;

    Sleep(1000);

    ServiceThread.ProcessRequests(False);

  end;

end;

OnStart event:

Used in situation if we want to handle each service request in separate threads. We can create a new thread (TThread) that contains your code and start the thread in the TService.OnStart event.

-      OnStartup occurs when the service first starts up, before the OnExecute event.

-      This event should be used to initialize the service. For example, if each service request is handled in a separate thread (a good idea if handling the request takes much time) the thread for a request is spawned in an OnStart event handler.

Example

Define a thread variable in your TService's private section like this:

  private

    { Private declarations }

    MyServiceThread: TMyServiceThread;

Now you need to create and fill in the OnStart and OnStop events like this:

procedure TCompanySqlDatabaseSpecialSomething.ServiceStart(

  Sender: TService; var Started: Boolean);

begin

  { Create an instance of the secondary thread where your service code is placed }

  MyServiceThread := TMyServiceThread.Create;

  { Set misc. properties you need (if any) in your thread }

  //MyServiceThread.Property1 := whatever;

  // and so on

  MyServiceThread.Resume;

end;

procedure TCompanySqlDatabaseSpecialSomething.ServiceStop(Sender: TService;

  var Stopped: Boolean);

begin

  MyServiceThread.Terminate;

end;

Note* - If you want to create Windows Service application that should response a client application's request with data communication between them, then its better to look for DataSnap server application. As it provides much flexibility to communicate and pass data between client and server. We can deploy a Datasnap application as Service in server also.