Stop method after certain time in Delphi Datasnap Server Application - windows

Question

I've build a datasnap server application for handling data between a windows application and mobile apps.
One method can take a while, and I want to be able to stop it after a certain time(Timeout).
How can I achieve this?

Answer 1

The code below shows one way to provide a server method with timeout behaviour.
The task which may take too long is executed in a secondary thread which is
started in the server method. This method uses a TSimpleEvent object (see the online help) to enable the
secondary thread to signal back to the server method's thread that it has completed. The value (in milliseconds) you specify in the call to Event.WaitFor defines how long to wait before the call times out.
If the call to WaitFor on the SimpleEvent times out, you can take whatever action you
like to notify the server's client. If the call to WaitFor returns wsSignaled, that means that the DBThread must have called SetEvent on the Event object before the period specified when calling WaitFor expired.
Btw, this example was written for D7, so might require minor adaptation for
Seattle. Also it uses a TForm descendant as the "server", but should work equally well in a DataSnap server method, since the principle is the same.
It doesn't address the issue of how exactly to stop whatever task you kick off in the secondary thread, because whether that is possible and how to do it if it is depends on exactly what the task is. Because of that, and the fact that you probably wouldn't want to delay the server method by waiting for the DBThread to complete, it does not attempt to free the DBThread, though in the real world that should of course be done.
type
TServer = class;
TDBThread = class(TThread)
private
FServer: TServer;
FEvent: TSimpleEvent;
FCancelled : Boolean;
function GetCancelled: Boolean;
procedure SetCancelled(const Value: Boolean);
public
procedure Execute; override;
constructor Create(AServer : TServer);
property Server : TServer read FServer;
property Event : TSimpleEvent read FEvent;
property Cancelled : Boolean read GetCancelled write SetCancelled;
end;
TServer = class(TForm)
// ignore the fact that in this case, TServer is a descendant of TForm
Button1: TButton;
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
private
protected
CS : TCriticalSection;
Event : TSimpleEvent;
public
procedure DoServerMethod;
end;
[...]
{ TDBThread }
constructor TDBThread.Create(AServer: TServer);
begin
inherited Create(True); // create suspended
FreeOnTerminate := False;
FServer := AServer;
FEvent := FServer.Event;
end;
procedure TDBThread.Execute;
var
StartTime : Cardinal;
begin
Cancelled := False;
// Following is for illustration ONLY, to simulate a process which takes time.
// Do not call Sleep() in a loop in a real thread
StartTime := GetTickCount;
repeat
Sleep(100);
until GetTickCount - StartTime > 5000;
if not Cancelled then begin
{ TODO : Transfer result back to server thread }
Event.SetEvent;
end;
end;
function TDBThread.GetCancelled: Boolean;
begin
FServer.CS.Enter;
try
Result := FCancelled;
finally
FServer.CS.Leave;
end;
end;
procedure TDBThread.SetCancelled(const Value: Boolean);
begin
FServer.CS.Enter;
try
FCancelled := Value;
finally
FServer.CS.Leave;
end;
end;
procedure TServer.DoServerMethod;
var
DBThread : TDBThread;
WaitResult : TWaitResult;
begin
DBThread := TDBThread.Create(Self);
DBThread.Resume;
WaitResult := Event.WaitFor(1000);
case WaitResult of
wrSignaled : begin
// the DBThread completed
ShowMessage('DBThread completed');
end;
wrTimeOut : begin
// the DBThread time out
DBThread.Cancelled := True;
ShowMessage('DBThread timed out');
// Maybe use PostThreadMessage here to tell the DBThread to abort (if possible)
// whatever task it is doing that has taken too long.
end;
end; {case}
{ TODO : Terminate and dispose of the DBThread }
end;
procedure TServer.FormCreate(Sender: TObject);
begin
CS := TCriticalSection.Create;
Event := TSimpleEvent.Create;
end;
procedure TServer.Button1Click(Sender: TObject);
begin
DoServerMethod;
end;