CODEWITHSUNDEEP
delphidatasnapdelphi-10-seattle
Freddie Bell
Freddie Bell

Reputation: 2287

Delphi 10 Seattle Datasnap error: "Operation failed. Connection was closed."

I created a stand-alone Datasnap TCP/IP server using the Wizard. I selected sample methods (echostring and reversestring). I saved the server and ran it. Then I created a client application, and using the file-new-other, added a ClientModule to that client project, along with the ClientClasses unit. On the main form. I added a button. On the button's onclick event handler, I added the following code:

procedure TForm1.Button1Click(Sender: TObject);
begin
  if ClientModule1.SQLConnection1.Connected then
  begin
    Button1.Text := 'Open';
    ClientModule1.SQLConnection1.Close;
  end
  else
  begin
    Button1.Text := 'Close';
    // ClientModule1.SQLConnection1.Open;
    ClientModule1.ServerMethods1Client.ReverseString('myteststring');
  end;
end;

The purpose here is to simulate a situation where the client is logging into and logging out of the server regularly rather than keeping a connection. This is especially important on apps deployed to mobile.

You can see I commented out the Connection.Open, because the first call to the ServerMethods1client opens the connection. The generated code is shown here:

function TClientModule1.GetServerMethods1Client: TServerMethods1Client;
begin
  if FServerMethods1Client = nil then
  begin
    SQLConnection1.Open;
    FServerMethods1Client := TServerMethods1Client.Create(SQLConnection1.DBXConnection, FInstanceOwner);
  end;
  Result := FServerMethods1Client;
end;

Now the problem arises. On first click to the button, the connection is opened, and the method is called. On the second click to the button, the connection is closed. On the 3rd click, an exception is raised "Operation Failed. Connection was Closed" is raised from with the TDBXCommand code.

As a workaround, I tried this:

procedure TForm1.Button1Click(Sender: TObject);
begin
  if ClientModule1.SQLConnection1.Connected then
  begin
    Button1.Text := 'Open';
    ClientModule1.SQLConnection1.Close;
    ClientModule1.ServerMethods1Client := nil;
  end
  else
  begin
    Button1.Text := 'Close';
    // ClientModule1.SQLConnection1.Open;
    ClientModule1.ServerMethods1Client.ReverseString('myteststring');
  end;
end;

This does sort-of solve the problem, since the ClientModule1's FServerMethods1Client instance is reset so the create code runs again like it did on the first run.

The only other problem now, is (I am using Eurekalog) it creates a memory leak.

What am I doing wrong? What's the right way to connected/disconnect from a Datasnap server repeatedly without restarting the app?

Upvotes: 1

Views: 1274

Answers (2)

Freddie Bell
Freddie Bell

Reputation: 2287

In my Android FMX implementation, I only call servermethods to get stuff done. (ie I don't use Datasnap data components). There's too much uncontrolled data transmission overhead to the Datasnap architecture to contemplate anything else realistically on a mobile device... To get around it (and not have memory leaks), I now create local instances of the TServermethods1Client as and when I need them and free them in context: 

function TClientModule1.PostTheLog: Boolean;
var
  Server: TServerMethods1Client;
begin

  Server := TServerMethods1Client.Create(ClientModule1.SQLConnection1.DBXConnection);
  try
      UserID := Server.GetUserID; 
      ...
  finally
      Server.Free;
  end;
end;

Now the ClientModule1.SQLConnection1 can be connected and disconnected at will (preferably connected just before any call to a servermethod, and disconnected thereafter) and no further issues arise.

Which then begs the question: In which ideal world would the publicly accessible ServerMethods1Client actually be useful?

Upvotes: -1

Jon Robertson
Jon Robertson

Reputation: 604

The reason for the first error is that the code that binds the client side proxy (which allows server methods to be called) is tied to the local SQL connection. Note the call to create the proxy class:

FServerMethods1Client := TServerMethods1Client.Create(SQLConnection1.DBXConnection, ...)

The underlying DBExpress connection is passed by reference, and the proxy class uses that connection to call the server. You closed and re-opened the connection, but the underlying DBExpress connection that ServerMethodsClient1 was using has been destroyed. Thus, you receive the "Connection was closed" exception. The connection that ServerMethodsClient1 was using has been closed. You have to recreate ServerMethodsClient1 as you did in your second example.

I can't answer your second question, as I believe it is ARC specific. For a VCL DataSnap app, I would call ServerMethodsClient1.Free rather than setting it to nil. Based on my very, very limited understanding of Delphi's ARC implementation (which is all from the newsgroups), I believe you should call ServerMethodsClient1.DisposeOf, since the class descends from TComponent

But I'm not sure about that. I'm sure someone will jump on here that understands ARC and the proper solution to destroy the object rather than having a memory leak.

Upvotes: 2

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