Indy Write Buffering / Efficient TCP communication

Question

I know, I'm asking a lot of questions...but as a new delphi developer I keep falling over all these questions :)

This one deals with TCP communication using indy 10. To make communication efficient, I code a client operation request as a single byte (in most scenarios followed by other data bytes of course, but in this case only one single byte). Problem is that

var Bytes : TBytes;
...
SetLength (Bytes, 1);
Bytes [0] := OpCode;
FConnection.IOHandler.Write (Bytes, 1);
ErrorCode := Connection.IOHandler.ReadByte;

does not send that byte immediately (at least the servers execute handler is not invoked). If I change the '1' to a '9' for example everything works fine. I assumed that Indy buffers the outgoing bytes and tried to disable write buffering with

FConnection.IOHandler.WriteBufferClose;

but it did not help. How can I send a single byte and make sure that it is immediatly sent? And - I add another little question here - what is the best way to send an integer using indy? Unfortunately I can't find function like WriteInteger in the IOHandler of TIdTCPServer...and

WriteLn (IntToStr (SomeIntVal))

seems not very efficient to me. Does it make a difference whether I use multiple write commands in a row or pack things together in a byte array and send that once?

Thanks for any answers!

EDIT: I added a hint that I'm using Indy 10 since there seem to be major changes concerning the read and write procedures.

Note that sending 1 byte is not more efficient than sending more bytes. You should probably read up on TCP/IP, packet size, transfer overhead and everything. There's a reason most of the old internet protocols use text, even though data size is larger than with binary data. — mghie
This command is an exception. Most commands add more bytes for the command parameters. I thought it couldn't be bad to pack things as close together as possible. Or am I wrong here? — jpfollenius
As long as your complete data frame fits into one TCP packet it shouldn't make much difference. Text based protocols OTOH are a big help when it comes to debugging stuff. — mghie
I posted a question at stackoverflow.com/questions/2440926/… that may be related somehow, so I figured I'd mention it here to in case anyone has seen something similar. — Jim

Darian Miller Darian Miller · Accepted Answer · 2009-03-04T03:38:53

Write buffering is disabled by default. You can check write buffering to see if it's active in your code by testing the fConnection.IOHandler.WriteBufferingActive property.

As far as the best way to send an integer... 'it depends' on your protocol and overall goals. Specifically, use FConnection.IOHandler.Write() as there are overloaded methods to write just about any type of data, including an integer.

Taken from IdIOHandler:

// Optimal Extra Methods
//
// These methods are based on the core methods. While they can be
// overridden, they are so simple that it is rare a more optimal method can
// be implemented. Because of this they are not overrideable.
//
//
// Write Methods
//
// Only the ones that have a hope of being better optimized in descendants
// have been marked virtual
procedure Write(const AOut: string; const AEncoding: TIdEncoding = enDefault); overload; virtual;
procedure WriteLn(const AEncoding: TIdEncoding = enDefault); overload;
procedure WriteLn(const AOut: string; const AEncoding: TIdEncoding = enDefault); overload; virtual;
procedure WriteLnRFC(const AOut: string = ''; const AEncoding: TIdEncoding = enDefault); virtual;
procedure Write(AValue: TStrings; AWriteLinesCount: Boolean = False; const AEncoding: TIdEncoding = enDefault); overload; virtual;
procedure Write(AValue: Byte); overload;
procedure Write(AValue: Char; const AEncoding: TIdEncoding = enDefault); overload;
procedure Write(AValue: LongWord; AConvert: Boolean = True); overload;
procedure Write(AValue: LongInt; AConvert: Boolean = True); overload;
procedure Write(AValue: SmallInt; AConvert: Boolean = True); overload;
procedure Write(AValue: Int64; AConvert: Boolean = True); overload;
procedure Write(AStream: TStream; ASize: Int64 = 0; AWriteByteCount: Boolean = False); overload; virtual;

Another question you had was "Does it make a difference whether I use multiple write commands in a row or pack things together in a byte array and send that once?" For the majority of cases, yes it makes a difference. For highly stressed servers you are going to have to get more involved in how bytes are sent back and forth, but at this level you should abstract out your sends into a separate protocol type class that builds the data to be sent and sends it in a burst and have a receiving protocol that receives a bunch of data and processes it as a complete unit instead of breaking things down to sending/receiving an integer, character, byte array, etc..

As a very rough quick example:

TmyCommand = class(TmyProtocol)
private
  fCommand:Integer;
  fParameter:String;
  fDestinationID:String;
  fSourceID:String;
  fWhatever:Integer;
public
  property Command:Integer read fCommand write fCommand;
  ...

  function Serialize;
  procedure Deserialize(Packet:String);
end;

function TmyCommand.Serialize:String;
begin
  //you'll need to delimit these to break them apart on the other side
  result := AddItem(Command) + 
            AddItem(Parameter) + 
            AddItem(DestinationID) + 
            AddItem(SourceID) + 
            AddItem(Whatever);
end; 
procedure TMyCommand.Deserialize(Packet:String);
begin
   Command := StrToInt(StripOutItem(Packet));
   Parameter := StripOutItem(Packet);
   DesintationID := StripOutItem(Packet); 
   SourceID := StripOutItem(Packet);
   Whatever := StrToInt(StripOutItem(Packet));
end;

Then send this via:

  FConnection.IOHandler.Write(myCommand.Serialize());

On the other side you can receive the data via Indy and then

  myCommand.Deserialize(ReceivedData);

Indy Write Buffering / Efficient TCP communication

3 Answers