Netty isWriteable()

Question

I am running a Netty server for an online game, however I am experiencing some weird behavior:

Randomly, the channel just stops being able to write from the server, ie the user is still connected and incoming data is received but outgoing data doesn't reach the client.

I have took the time to debug this and I discovered that channel.isWriteable() is returning false for connecting client with the problem.

Can anyone shine any light on why this might be happening?

What are the reasons for a channel not to become writeable anmore?

By the way, this also happens on a localhost connection too.

    public ChannelFuture write(Packet message) {
    ioWrite++;

    if (!channel.isConnected()) {
        System.err.println("Trying to write to bad channel");

        return null;
    }

    if (!channel.isWritable()) {
        System.err.println("Channel buffer is full?");
    }

    if ((channel != null) && channel.isConnected()) {
        return channel.write(message);
    }

    return null;
}

Encoder:

public class GameProtocolEncoder extends OneToOneEncoder {



private ChannelBuffer response;


protected Object encode(ChannelHandlerContext channelHandlerContext, Channel channel, Object o) throws Exception {
    Packet p = (Packet) o;

    try {
        byte[] data       = p.getData();
        int    dataLength = p.getLength();

        if (p.getLength() > 5000) {
            System.err.println("unable to write data chunk to large");

            return null;
        }

        if (!p.isBare()) {
            int siz = 0;

            siz      += (p.getSize() == Packet.Size.VariableShort)
                        ? 2
                        : 1;
            siz      += (p.getId() > 128)
                        ? 2
                        : 1;
            response = ChannelBuffers.buffer(siz + p.getLength());

            int id          = p.getId();

            if (id< 128) {
                response.writeByte(id);
            } else {
                response.writeByte((byte) ((id >> 8) + 128));
                response.writeByte((byte) (id & 0xFF));
            }

            if (p.getSize() != Packet.Size.Fixed) {    // variable length
                if (p.getSize() == Packet.Size.VariableByte) {
                    if (dataLength > 255) {            // trying to send more data then we can represent with 8 bits!
                        throw new IllegalArgumentException("Tried to send packet length " + dataLength
                                                           + " in 8 bits [pid=" + p.getId() + "]");
                    }

                    response.writeByte((byte) dataLength);
                } else if (p.getSize() == Packet.Size.VariableShort) {
                    if (dataLength > 5000) {
                        throw new IllegalArgumentException("Tried to send packet length to big: " + id);
                    }

                    response.writeShort((short) dataLength);
                }
            }
        } else {
            response = ChannelBuffers.buffer(dataLength);
        }

        response.writeBytes(p.getData());

        return response;
    } catch (Exception e) {
        Logger.err("Error handling message: " + p);
        Logger.err(e);
    }

    return null;
}

Answer 1

I think the problem is here that you not flush the Channel. Try to replace channel.write(...) with channel.writeAndFlush(...).

Answer 2

A channels stops being writeable when its socket send buffer fills up, which in turn happens when the sender outruns the receiver.

If it happens a lot you may want to look into speeding up your receiving code, but in general it can happen any time, and all you can do is defer the write until the channel becomes writeable again.