Start Communication from server first in delphi by Indy 10

Question 1

You are using TIdCmdTCPServer. By definition, it sends responses to client-issued commands. For what you are asking, you should use TIdTCPServer instead, then you can do whatever you want in the TIdTCPServer.OnExecute event.

What you ask for is doable, but its implementation depends on your particular needs for your protocol.

If you just want to send unsolicited server-to-client messages, and never responses to client-to-server commands, then the implementation is fairly straight forward. Use TIdContext.Connection.IOHandler when needed. You can loop through existing clients in the TIdTCPServer.Contexts list, such as inside the TIdTCPServer.OnConnect and TIdTCPServer.OnDisconnect events. On the client side, you need a timer or thread to check for server messages periodically. Look at TIdCmdTCPClient and TIdTelnet for examples of that.

But if you need to mix both client-to-server commands and unsolicited server-to-client messages on the same connection, you have to design your protocol to work asynchronously, which makes the implementation more complex. Unsolicited server messages can appear at anytime, even before the response to a client command. Commands need to include a value that is echoed in the response so clients can match up responses, and the packets need to be able to differentiate between a response and an unsolicited message. You also have to give each client its own outbound queue on the server side. You can use the TIdContext.Data property for that. You can then add server messages to the queue when needed, and have the OnExecute event send the queue periodically when it is not doing anything else. You still need a timer/thread on the client side, and it needs to handle both responses to client commands and unsolicited server messages, so you can't use TIdConnection.SendCmd() or related methods, as it won't know what it will end up reading.

I have posted examples of both approaches in the Embarcadero and Indy forums many times before.

Question 2

Clients initiate communication. That is the definition of a client–the actor that initiates the communication. Once the connection is established though, both sides can send data. So, the clients connect to the server. The server maintains a list of all connected clients. When the server wants to send out communications it just sends the data to all connected clients.

Since clients initiate communication, it follows that, in the event of broken communication, it is the client's job to re-establish connection.

Question 3

If you want to see working code examples where server sends data, check out Indy IdTelnet: the telnet client uses a thread to listen to server messages. There is only one socket, created by the client, but the server uses the same socket for its messages to the client, at any time.

The client starts the connection, but does not have to start a conversation by saying 'HELLO' or something like that.

Technically, the client only needs to open the socket connection, without sending any additional data. The client can remain quiet as long as he wants, even until the end of the connection.

The server has a socket connection to the client as soon as the client has connected. And over this socket, the server can send data to the client.

Of course, the client has to read from the connection socket to see the server data. This can be done in a loop in a background thread, or even in the main thread (not in a VCL application of course as it would block).

Question 4

Finally, this is the code that I used to solve my problem:

// Thread at client-side
procedure FNotifRecieverThread.Execute;
var
  str: string;
  MID: Integer;
  TCP1: TIdTCPClient;
begin
  if frmRecieverMain.IdTCPClient1.Connected then
  begin
    TCP1 := TIdTCPClient.Create(nil);
    TCP1.Host := frmRecieverMain.IdTCPClient1.Host;
    TCP1.Port := frmRecieverMain.IdTCPClient1.Port;
    TCP1.ConnectTimeout := 20000;

    while True do
    begin
      try
        TCP1.Connect;

        while True do
        begin
          try
            str := '';
            TCP1.SendCmd('checkmynotif');

            TCP1.Socket.WriteLn(IntToStr(frmRecieverMain.UserID));
            str := TCP1.Socket.ReadLn;

            if Pos('showmessage_', str) = 1 then
            begin
              MID := StrToInt(Copy(str, Pos('_', str) + 1, 5));
              frmRecieverMain.NotifyMessage(MID);
            end
            else
            if str = 'updateusers' then
            begin
              LoadUsers;

              frmRecieverMain.sgMsgInbox.Invalidate;
              frmRecieverMain.sgMsgSent.Invalidate;
              frmRecieverMain.cbReceipent.Invalidate;
            end
            else
              if str = 'updatemessages' then
            begin
              LoadMessages;
              frmRecieverMain.DisplayMessages;
            end;
          except
            // be quite and try next time :D
          end;          

          Sleep(2000);
        end;
      finally
        TCP1.Disconnect;
        TCP1.Free;
      end;

      Sleep(5000);
    end;
  end;
end;

// And command handlers at server-side
procedure TfrmServer.cmhCheckMyNotifCommand(ASender: TIdCommand);
var
  UserID, i: Integer;
  str: string;
begin
  str := 'notifnotfound';
  UserID := StrToIntDef(ASender.Context.Connection.Socket.ReadLn, -1);

  for i := 0 to NotificationStack.Count - 1 do
    if NotificationStack.Notifs[i].Active and
      (NotificationStack.Notifs[i].UserID = UserID)
    then
    begin
      NotificationStack.Notifs[i].Active := False;
      str := NotificationStack.Notifs[i].NotiffText;
      Break;
    end;

  ASender.Context.Connection.Socket.WriteLn(str);
end;

// And when i want to some client notificated from server, I use some methodes like this:
procedure TfrmServer.cmhSetUserOnlineCommand(ASender: TIdCommand);
var
  UserID, i: Integer;
begin
  UserID := StrToIntDef(ASender.Context.Connection.Socket.ReadLn, -1);

  if UserID <> -1 then
  begin
    for i := 0 to OnLineUsersCount - 1 do // search for duplication...
      if OnLineUsers[i].Active and (OnLineUsers[i].UserID = UserID) then
        Exit; // duplication rejected!    

    Inc(OnLineUsersCount);
    SetLength(OnLineUsers, OnLineUsersCount);

    OnLineUsers[OnLineUsersCount - 1].UserID := UserID;
    OnLineUsers[OnLineUsersCount - 1].Context := ASender.Context;
    OnLineUsers[OnLineUsersCount - 1].Active := True;

    for i := 0 to OnLineUsersCount - 1 do      // notify all other users for refresh users list
      if OnLineUsers[i].Active and (OnLineUsers[i].UserID <> UserID) then
      begin
        Inc(NotificationStack.Count);
        SetLength(NotificationStack.Notifs, NotificationStack.Count);

        NotificationStack.Notifs[NotificationStack.Count - 1].UserID := OnLineUsers[i].UserID;
        NotificationStack.Notifs[NotificationStack.Count - 1].NotiffText := 'updateusers';
        NotificationStack.Notifs[NotificationStack.Count - 1].Active := True;
      end;
  end;
end;