Question

What’s the most appropriate way to detect if a socket has been dropped or not? Or whether a packet did actually get sent?

I have a library for sending Apple Push Notifications to iPhones through the Apple gatways (available on GitHub). Clients need to open a socket and send a binary representation of each message; but unfortunately Apple doesn’t return any acknowledgement whatsoever. The connection can be reused to send multiple messages as well. I’m using the simple Java Socket connections. The relevant code is:

Socket socket = socket();   // returns an reused open socket, or a new one
socket.getOutputStream().write(m.marshall());
socket.getOutputStream().flush();
logger.debug("Message \"{}\" sent", m);

In some cases, if a connection is dropped while a message is sent or right before; Socket.getOutputStream().write() finishes successfully though. I expect it’s due to the TCP window isn’t exhausted yet.

Is there a way that I can tell for sure whether a packet actually got in the network or not? I experimented with the following two solutions:

1. Insert an additional socket.getInputStream().read() operation with a 250ms timeout. This forces a read operation that fails when the connection was dropped, but hangs otherwise for 250ms.

2. set the TCP sending buffer size (e.g. Socket.setSendBufferSize()) to the message binary size.

Both of the methods work, but they significantly degrade the quality of the service; throughput goes from a 100 messages/second to about 10 messages/second at most.

Any suggestions?

UPDATE:

Challenged by multiple answers questioning the possibility of the described. I constructed “unit” tests of the behavior I’m describing. Check out the unit cases at Gist 273786.

Both unit tests have two threads, a server and a client. The server closes while the client is sending data without an IOException thrown anyway. Here is the main method:

public static void main(String[] args) throws Throwable {
    final int PORT = 8005;
    final int FIRST_BUF_SIZE = 5;

    final Throwable[] errors = new Throwable[1];
    final Semaphore serverClosing = new Semaphore(0);
    final Semaphore messageFlushed = new Semaphore(0);

    class ServerThread extends Thread {
        public void run() {
            try {
                ServerSocket ssocket = new ServerSocket(PORT);
                Socket socket = ssocket.accept();
                InputStream s = socket.getInputStream();
                s.read(new byte[FIRST_BUF_SIZE]);

                messageFlushed.acquire();

                socket.close();
                ssocket.close();
                System.out.println("Closed socket");

                serverClosing.release();
            } catch (Throwable e) {
                errors[0] = e;
            }
        }
    }

    class ClientThread extends Thread {
        public void run() {
            try {
                Socket socket = new Socket("localhost", PORT);
                OutputStream st = socket.getOutputStream();
                st.write(new byte[FIRST_BUF_SIZE]);
                st.flush();

                messageFlushed.release();
                serverClosing.acquire(1);

                System.out.println("writing new packets");

                // sending more packets while server already
                // closed connection
                st.write(32);
                st.flush();
                st.close();

                System.out.println("Sent");
            } catch (Throwable e) {
                errors[0] = e;
            }
        }
    }

    Thread thread1 = new ServerThread();
    Thread thread2 = new ClientThread();

    thread1.start();
    thread2.start();

    thread1.join();
    thread2.join();

    if (errors[0] != null)
        throw errors[0];
    System.out.println("Run without any errors");
}

[Incidentally, I also have a concurrency testing library, that makes the setup a bit better and clearer. Checkout the sample at gist as well].

When run I get the following output:

Closed socket
writing new packets
Finished writing
Run without any errors

Answer 1

This not be of much help to you, but technically both of your proposed solutions are incorrect. OutputStream.flush() and whatever else API calls you can think of are not going to do what you need.

The only portable and reliable way to determine if a packet has been received by the peer is to wait for a confirmation from the peer. This confirmation can either be an actual response, or a graceful socket shutdown. End of story – there really is no other way, and this not Java specific – it is fundamental network programming.

If this is not a persistent connection – that is, if you just send something and then close the connection – the way you do it is you catch all IOExceptions (any of them indicate an error) and you perform a graceful socket shutdown:

1. socket.shutdownOutput();
2. wait for inputStream.read() to return -1, indicating the peer has also shutdown its socket