Datasnap and SocketError

Question

I have a datasnap server with a vcl forms client. From the client, how can I handle if the server has been say shutdown and restarted with existing client connections? This scenario raises a 10053 EIdSocketError exception.

To replicate, I run up both Server and Client, make a call to the server (I use methods exposed via the DataSnap proxy generator) which succeeeds. I then shut down the server (eg Close the application) and restart it. I then attempt to make a call to the server again.

eg: CLIENT call

    sm := TsvrPolicySearchClient.Create(datClientDB.SQLConnection1.DBXConnection);
    try
      ds := sm.SearchPolicyByPolicy(40, WCRef, '', 3);
      dspPolicyGroup.DataSet := ds;

      if cdsPolicyGroup.Active then
        cdsPolicyGroup.Refresh
      else
        cdsPolicyGroup.Open;

    finally
      sm.Free;
    end;

dspPolicyGroup is a TDataSetProvider and cdsPolicyGroup is a TClientDataSet (I just use it locally to "store" my TDataSet result).

SERVER

function TsvrPolicySearch.SearchPolicyByPolicy(AClientId: Integer; WCRefNum, ClientRef: string; SearchMethod: Integer): TDataSet;
begin
  spPolicyByWCRef.Close;
  spPolicyByWCRef.ParamByName('p_client').AsInteger := AClientId;
  spPolicyByWCRef.ParamByName('p_search_method').AsInteger := SearchMethod;
  spPolicyByWCRef.ParamByName('p_wc_refno').AsString := WCRefNum;
  spPolicyByWCRef.Open;
  Result := spPolicyByWCRef;
end;

I would think this is regularly encountered by people seeing that it's quite easy to replicate. Should I place a "Test Connection" call or something first (such as a method TestConnection) before each call to check for EIdSocketError (and equivalent) and handle? Or is it more a design flaw perhaps?

Thanks

Answer 1

My scenario:

• Client application connect to DataSnap Server (TCP/IP, remote server)
• Client request a DataSet (using DataSnap server methods). TClientDataSet
• Client downloaded dataset
• Server shutdown (taskkill, close application no matters)
• Client request a DataSet againt (ehhrrr Socket Error)

Solution:

• On client I catch exception (AppEvents.OnException)
• Recognize that is Socket error from connection to DataSnap server
• I show dialog window with information that connection is lost. User can click "RETRY" - if so,
  • Free and re-create DataSnap client module and initialize connection
  • Try to request dataset (special dataset for connection test) if application catch exception entire process works from the beginning.

After this operation my Client re-connect to DataSnap server and can request dataset using new TCP/IP connection. Of course user can close dialog, but then application is turn off.

I think this can help you. I tried many others solutions but this proved to be the best. Moreover algorithm also supports the loss of connection with the client's fault.

Answer 2

The easiest workaround for this problem is to change the TDSServerClass LifeCycle attribute to Invocation. This will cause you to have a stateless server, and the server will create a new session per request. But, you will be able to close and reconnect the server without interrupting the clients connection.