Client server design question

Question

I'm working on creating a multiplayer game, with the client in ActionScript and the server written in Java. I'm using XMLSocket in the client to connect to the server. The server code looks something like:

ServerSocket server = new ServerSocket(port);

while(true)
{
    Socket client = server.accept();
    ClientHandlingProcess child = new ClientHandlingProcess(client);
    child.start(); // starts new thread for client
}

The client handling process looks something like:

while(true)
{
    byte[] arr = new byte[1024];
    if(socket.getInputStream().read(arr) == -1)
    {
        break;
    }

    handleInputMethod(arr);

    while(!message_queue.isEmpty())
    {
        socket.getOutputStream().write(message_queue.get(0).getBytes());
        message_queue.remove(0);
    }
}

The issue is that the read method is blocking, meaning I can't send any data while I'm waiting for data to come in. What would be a good way to fix this? Have two processes: one for reading and one for writing, have the client send empty requests when it has no proper messages to send, or is there another way I can proceed?

Answer 1

Never rely on a client to do ANYTHING to make your server run properly. If you do, then malicious users can attack your game simply by refusing to perform whatever is required of them.

Use non-blocking IO or threads for this task. Threads is fine if you have a small number of players (eg, < 10) and you are confident with passing data safely between threads. Otherwise, go for the non-blocking IO route.

Answer 2

You have basically these two options:

• Use blocking IO with one reading thread per client
• Use non blocking IO

The first option has the disadvantage that it does not scale for large number of clients.

While non blocking IO can be done by checking inputstream.available(), it may be a good idea to look at the New I/O API. Wikipedia has a good article on NIO with sample code. You can find additional information in the JavaDoc for the NIO package. And on a quick glance this tutorial seems helpful.

You might get away with doing smart things on the client side, such as sending a dummy message on every "turn". This, however, means that a rouge client can freeze your server. So you should not go into this direction.

Answer 3

Java's NIO package (as of Java6), provides support for non-blocking I/O only, via Selectors.

Answer 4

I agree with the other answers that NIO provides better results. However using NIO isn't that trivial. You will definitely learn something when you code the NIO network code yourself. Or you just use an asynch network package like netty or Apache Mina. I prefer netty, since I find it better documented. However if you understand one of them, you understand the other one as well.

But, if you want to keep the design you have, you can split the sending and receiving code. Create some "PlayerSession" or "PlayerConnection" Object that provides the methods for sending messages to the client and contains one thread that blocks on the read and handles input. Maybe you want to move the sending stuff to an own thread, just try it.