Delphi Indy TCP Client/Server communication best approach

Question

I have a client and a server application that is communicating just fine, there is a TIdCmdTCPServer in the server and a TIdTCPClient in the client.

The client has to authenticate in the server, the client asks the server for the newest version information and downloads any updates, and other communications. All this communication with TIdTCPClient.SendCmd() and TIdTCPClient.LastCmdResult.Text.Text.

The way it is, the server receives commands and replies, the clients only receives replies, never commands, and I would like to implement a way to make the client receives commands. But as I heard, if the client uses SendCmd it should never be listening for data like ReadLn() as it would interfere with the reply expected in SendCmd. I thought of making a command to check for commands, for example, the client would send a command like "IsThereCommandForMe" and the server would have a pool of commands to each client and when the client asks, the server send it in the reply, but I think it would not be a good approach as there would be a big delay between the commands being available and the client asking for it. I also thought of making a new connection with new components, for example a TIdCmdTcpClient, but then there would be 2 connections for each client, I don't like that idea as I think it could easily give problems in the communication.

The reason I want this, is that I want to implement a chat functionality in the client, and it should be receiving messages from the server without asking for it all the time, imagine all clients continually asking the server if there is message for them. And I would like to be able to inform the client when there is an update available instead the client being asking if there is any. And with this I could send more commands to the client too.

what are your thoughts about this ? how can I make the server receiving commands from the clients, but also sends them ?

Answer 1

TCP sockets are bidirectional by design. Once the connection between 'client' and 'server' has been established, they are symmetric and data can be sent at any time from any side over the same socket.

It only depends on the protocol (which is just written 'contract' for the communication) which communication model is used. HTTP for example uses a request/reply model. With Telnet for example, both sides can initate data transmissions. (If you take a look at the Indy implementation for Telnet, you will see that it uses a background thread to listen for server data, but it uses the same socket connection in the main thread to send data from client to server).

A "full duplex" protocol which supports both request/response and server push, and also is firewall-friendly, is WebSockets . With WebSockets (a HTTP upgrade), the server can send data to the connected client(s) any time. This would meet your 'chat' requirement.

If you use TIdTCPClient / TIdCmdTCPServer, corporate firewalls might block the communication.