TCPserver without OnExecute event

Question

I want to make a TCPserver and send/receive message to clients as needed, not OnExecute event of the TCPserver.

Send/receive message is not a problem; I do like that:

procedure TFormMain.SendMessage(IP, Msg: string);
var
  I: Integer;
begin
  with TCPServer.Contexts.LockList do
  try
    for I := 0 to Count-1 do
      if TIdContext(Items[I]).Connection.Socket.Binding.PeerIP = IP then
      begin
        TIdContext(Items[I]).Connection.IOHandler.WriteBuffer(Msg[1], Length(Msg));
        //  and/or Read 
        Break;
      end;
  finally
    TCPServer.Contexts.UnlockList;
  end;
end;

Note 1: If I don't use OnExecute, the program raise an exception when a client connects.
Note 2: If I use OnExecute without doing anything, the CPU usage goes to %100
Note 3: I don't have a chance to change the TCP clients.

So what should I do?

Answer 1

Use OnExecute and if you have nothing to do, Sleep() for a period of time, say 10 milliseconds. Each connection has its own OnExecute handler so this will only affect each individual connection.

Answer 2

TIdTCPServer requires an OnExecute event handler assigned by default. To get around that, you would have to derive a new class from TIdTCPServer and override its virtual CheckOkToBeActive() method, and should also override the virtual DoExecute() to call Sleep(). Otherwise, just assign an event handler and have it call Sleep().

This is not an effective use of TIdTCPServer, though. A better design is to not write your outbound data to clients from inside of your SendMessage() method directly. Not only is that error-prone (you are not catching exceptions from WriteBuffer()) and blocks SendMessage() during writing, but it also serializes your communications (client 2 cannot receive data until client 1 does first). A much more effective design is to give each client its own thread-safe outbound queue, and then have SendMessage() put the data into each client's queue as needed. You can then use the OnExecute event to check each client's queue and do the actual writing. This way, SendMessage() does not get blocked anymore, is less error-prone, and clients can be written to in parallel (like they should be).

Try something like this:

uses
  ..., IdThreadSafe;

type
  TMyContext = class(TIdServerContext)
  private
    FQueue: TIdThreadSafeStringList;
    FEvent: TEvent;
  public
    constructor Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TThreadList = nil); override;
    destructor Destroy; override;
    procedure AddMsgToQueue(const Msg: String);
    function GetQueuedMsgs: TStrings;
  end;

constructor TMyContext.Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TThreadList = nil);
begin
  inherited;
  FQueue := TIdThreadSafeStringList.Create;
  FEvent := TEvent.Create(nil, True, False, '');
end;

destructor TMyContext.Destroy;
begin
  FQueue.Free;
  FEvent.Free;
  inherited;
end;

procedure TMyContext.AddMsgToQueue(const Msg: String);
begin
  with FQueue.Lock do
  try
    Add(Msg);
    FEvent.SetEvent;
  finally
    FQueue.Unlock;
  end;
end;

function TMyContext.GetQueuedMsgs: TStrings;
var
  List: TStringList;
begin
  Result := nil;
  if FEvent.WaitFor(1000) <> wrSignaled then Exit;
  List := FQueue.Lock;
  try
    if List.Count > 0 then
    begin
      Result := TStringList.Create;
      try
        Result.Assign(List);
        List.Clear;
      except
        Result.Free;
        raise;
      end;
    end;
    FEvent.ResetEvent;
  finally
    FQueue.Unlock;
  end;
end;

procedure TFormMain.FormCreate(Sender: TObject);
begin
  TCPServer.ContextClass := TMyContext;
end; 

procedure TFormMain.TCPServerExecute(AContext: TIdContext);
var
  List: TStrings;
  I: Integer;
begin
  List := TMyContext(AContext).GetQueuedMsgs;
  if List = nil then Exit;
  try
    for I := 0 to List.Count-1 do
      AContext.Connection.IOHandler.Write(List[I]);
  finally
    List.Free;
  end;
end;

procedure TFormMain.SendMessage(const IP, Msg: string); 
var 
  I: Integer; 
begin 
  with TCPServer.Contexts.LockList do 
  try 
    for I := 0 to Count-1 do 
    begin
      with TMyContext(Items[I]) do
      begin
        if Binding.PeerIP = IP then 
        begin 
          AddMsgToQueue(Msg); 
          Break; 
        end;
      end; 
    end;
  finally 
    TCPServer.Contexts.UnlockList; 
  end; 
end; 

Answer 3

In the OnExecute handler, you can use thread communication methods like TEvent and TMonitor to wait until there is data for the client.

TMonitor is available since Delphi 2009 and provides methods (Wait, Pulse and PulseAll) to send / receive notifications with mininmal CPU usage.

Answer 4

The Indy component set is designed to emulate blocking operation on a network connection. You're supposed to encapsulate all your code in the OnExecute event handler. That's supposed to be easier, because most protocols are blocking any way (send command, wait for response, etc).

You apparently don't like it's mode of operation, you'd like something that works without blocking. You should consider using a component suite that's designed for the way you intend to use it: give the ICS suite a try! ICS doesn't use threads, all the work is done in event handlers.

Answer 5

I had similar situation taking 100% CPU and it solved by adding IdThreadComponent and:

void __fastcall TForm3::IdThreadComponent1Run(TIdThreadComponent *Sender)
{
    Sleep(10);
}

Is it right? I am not sure.