Question

I have a few test clients that are encountering the same issue each time. The clients can connect, and they can send their first message, but after that the server stops responding to that client. I suspect that the problem is related to s.accept(), but I’m not sure exactly what is wrong or how to work around it.

def startServer():
    host = ''
    port = 13572
    backlog = 5
    size = 1024

    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.bind((host,port))
    s.listen(backlog)

    print "Close the command prompt to stop Gamelink"

    while 1:
        try:
            client, address = s.accept()
            data = client.recv(size)
            if data:
                processData(data)
                client.send("OK")
            else:
                print "Disconnecting from client at client's request"
                client.close()
        except socket.error, (value, message):
            if s:
                print "Disconnecting from client, socket issue"
                s.close()
            print "Error opening socket: " + message
            break
        except:
            print "Gamelink encountered a problem"
            break
        print "End of loop"
    client.close()
    s.close()

The server is intended to be accessed across a local network, and it needs to be light weight and very quick to respond, so if another implementation (such as thread based) would be better for meeting those requirements please let me know. The intended application is to be used as a remote gaming keyboard, thus the need for low resource use and high speed.

Answer 1

Writing a server using socket directly will be hard. As Keith says, you need to multiplex the connections somehow, like with select or poll or threads or fork. You might think you need only one connection, but what will you do when something hiccups and the connection is lost? Will your server be able to respond to reconnection attempts from the client if it hasn’t yet realized the connection is lost?

If your networking needs are basic, you might be able to let something else handle all the listening and accepting and forking stuff for you. You don’t specify a platform, but examples of such programs are launchd on Mac OS and xinetd on Linux. The details differ between these tools, but basically you configure them, in some configuration file, to listen for a connection on some port. When they get it, they take care of setting up the connection, then they exec() your program with stdin and stdout aimed at the socket, so you can simply use all the basic IO you probably already know like print and sys.stdin.read().

The trouble with solutions like xinitd and launchd is that for each new connection, they must fork() and exec() a new instance of your program. These are relatively heavy operations so a large number of connections or a high rate of new connections might hit the limits of your server. But worse, since each connection is in a separate process, sharing data between them is hard. Also, most solutions you might find to communicate between processes involve a blocking API, and now you are back to the problem of multiplexing with select or threads or similar.

If that doesn’t meet your needs, I think you are better off learning to use a higher-level networking framework which will handle all the problems you will inevitably encounter if you go down the path of socket. One such framework I’d suggest is Twisted. Beyond handling the mundane details of handling connections, and the more complex task of multiplexing IO between them, you will also have a huge library of tools that will make implementing your protocol much easier.