Sockets stop sending messages but not timing out

Question

Problem:

I'm making an app that has instant messaging. For the most part, the messages are sent and received without any issues. But after some period of time, somewhere between 10 and 40 minutes, the messages no longer reach the server if the client's socket hasn't been used in a while. For example, if I chat with someone, leave the app open, take a nap, and come back to chat again, then the messages won't send. I am 99% sure it is not a problem on the server side because I print out everything it receives (and it's not receiving anything) and it works fine if I log in with another phone.

What I've tried:

At first I thought it was a timeout issue on the client side because on the server side I kept getting an error saying, "Connection reset by peer", somewhere in that inverval of 10-40 minutes. I (almost) resolved this by using Java.net.socket's connect() method with a timeout value of 0 which gives an infinite timeout:

clientSocket = new Socket();
clientSocket.connect( new InetSocketAddress(ServerInfo.IP, ServerInfo.PORT_NUMBER), 0 ); 

I rarely get the "Connection reset by peer" error message now, but the mysterious problem of messages not sending after some time goes by still remains.

Code:

This is my sendMessage function which always logs "SOCKET OPERATOR SENDING MESSAGE: 'message'":

public boolean sendMessage(String message) 
{                                                       
       PrintWriter out = null;

        try 
        { 
            out = new PrintWriter( clientSocket.getOutputStream(), true );
            Log.i( "MY_TAG", "SOCKET OPERATOR SENDING MESSAGE: " + message );

        } 
        catch (IOException e) 
        {
            e.printStackTrace();
            Log.i( "MY_TAG", "SOCKET OPERATOR FAILEDD TO SEND MESSAGE WITH EXCEPTION: " + e.getMessage() );

            return false;                   
        }


        out.println(message);

        return true;            
}

Question:

What on earth could possibly be the problem or how could I further debug this?

Answer 1

Logging at the point you do is futile. Nothing has happened yet.

If the connection is dead, sooner or later a send will cause an IOException: connection reset. But not the first time, due to socket buffering.

And when you do get this exception, don't just return false. Close the connection.

HOWEVER the problem here is the PrintWriter. It swallows exceptions. See the Javadoc. Either call checkError(), which returns a boolean indicating whether there has been an exception, or, better still, don't use PrintWriter at all: use BufferedWriter.write() and .newLine(), and .flush(), all of which can throw IOExceptions. This is better because you can see what the exception actually was. You'll have to move all that into the try block of course, ahead of the presently misleading log message.

And don't use a new PrintWriter or BufferedWriter per message. Use the same one for the life of the socket.

Answer 2

Before sending any message you must check if the connection still exists b/w client and server.

There are chances that if net connection becomes slow or is off in between, the connection will break or sometimes it becomes a dead connection from the server side because there was no communication b/w both.

you may try two things:

1. Make a general method which on sending message checks if: the connection exists then just send the message else : first make the connection and send that message.

We faced the same problem in Websockets and used the same startegy.

2. Ask server side team to keep sending some heartbeat packets from their side, this way your connection will not be dead.

For us, first strategy worked out and is more better.

Give it a try. :)

Answer 3

I believe the problem didn't necessarily lie with the client device or with the server, but rather with what's between those 2 things, my NAT router. The router is most likely dropping mappings in its table due to inactivity.

My solution is to use a heartbeat function (along with modifying my use of PrintWriter as EJP suggested), sending from the client to the server every 45 seconds. I'll most likely randomize the time somewhat just to handle the case in which a ton of people log in at the same time and affect the server's performance.