예제 #1
0
  /** Used to monitor address changes on the the interface. */
  private void handleInterfaceCheck() {
    try {
      int port = super.getAdvertisementPort();
      if (port < 0) {
        // can't compose a nodel address yet
        _logger.info("(nodel server port still not available; will wait.)");

        return;
      }

      InetAddress localIPv4Address = getLocalIPv4Address();
      String nodelAddress = "tcp://" + localIPv4Address.getHostAddress() + ":" + port;

      if (nodelAddress.equals(_nodelAddress))
        // nothing to do
        return;

      // the address has changed so should update advertisements
      _logger.info(
          "An address change has been detected. previous={}, current={}",
          _nodelAddress,
          nodelAddress);

      _nodelAddress = "tcp://" + localIPv4Address.getHostAddress() + ":" + port;
      _httpAddress =
          "http://"
              + localIPv4Address.getHostAddress()
              + ":"
              + Nodel.getHTTPPort()
              + Nodel.getHTTPSuffix();

      Nodel.updateHTTPAddress(_httpAddress);

      synchronized (_lock) {
        // recycle receiver which will in turn recycle sender
        _recycleReceiver = true;

        // "signal" thread
        Stream.safeClose(_receiveSocket);
      }

    } catch (Exception exc) {
      _logger.warn("'handleInterfaceCheck' did not complete cleanly; ignoring for now.", exc);
    }
  } // (method)
예제 #2
0
/** Used for registering / unregistering nodes for discovery / lookup purposes. */
public class NodelAutoDNS extends AutoDNS {

  /** IPv4 multicast group */
  public static final String MDNS_GROUP = "224.0.0.252";

  /** IPv6 multicast group (not used here but reserved) */
  public static final String MDNS_GROUP_IPV6 = "FF02::FB";

  /** Multicast port */
  public static final int MDNS_PORT = 5354;

  /** The period between probes when actively probing. (millis) */
  private static final int PROBE_PERIOD = 45000;

  /** Expiry time (allow for at least one missing probe response) */
  private static final long STALE_TIME = 2 * PROBE_PERIOD + 10000;

  /**
   * How long to allow the multicast receiver to be silent. Will reinitialise socket as a
   * precaution.
   */
  private static final int SILENCE_TOLERANCE = 3 * PROBE_PERIOD + 10000;

  /** (instrumentation) */
  private static AtomicLong s_multicastOutOps = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_multicastOutOpsMeasurement =
      new AtomicLongMeasurementProvider(s_multicastOutOps);

  /** Multicast in operations. */
  public static AtomicLongMeasurementProvider MulticastOutOpsMeasurement() {
    return s_multicastOutOpsMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_multicastOutData = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_multicastOutDataMeasurement =
      new AtomicLongMeasurementProvider(s_multicastOutData);

  /** Multicast out data. */
  public static AtomicLongMeasurementProvider MulticastOutDataMeasurement() {
    return s_multicastOutDataMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_multicastInOps = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_multicastInOpsMeasurement =
      new AtomicLongMeasurementProvider(s_multicastInOps);

  /** Multicast in operations. */
  public static AtomicLongMeasurementProvider MulticastInOpsMeasurement() {
    return s_multicastInOpsMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_multicastInData = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_multicastInDataMeasurement =
      new AtomicLongMeasurementProvider(s_multicastInData);

  /** Multicast in data. */
  public static AtomicLongMeasurementProvider MulticastInDataMeasurement() {
    return s_multicastInDataMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_unicastOutOps = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_unicastOutOpsMeasurement =
      new AtomicLongMeasurementProvider(s_unicastOutOps);

  /** Unicast in operations. */
  public static AtomicLongMeasurementProvider UnicastOutOpsMeasurement() {
    return s_unicastOutOpsMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_unicastOutData = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_unicastOutDataMeasurement =
      new AtomicLongMeasurementProvider(s_unicastOutData);

  /** Unicast out data. */
  public static AtomicLongMeasurementProvider UnicastOutDataMeasurement() {
    return s_unicastOutDataMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_unicastInOps = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_unicastInOpsMeasurement =
      new AtomicLongMeasurementProvider(s_unicastInOps);

  /** Unicast in operations. */
  public static AtomicLongMeasurementProvider UnicastInOpsMeasurement() {
    return s_unicastInOpsMeasurement;
  }

  /** (instrumentation) */
  private static AtomicLong s_unicastInData = new AtomicLong();

  /** (instrumentation) */
  private static AtomicLongMeasurementProvider s_unicastInDataMeasurement =
      new AtomicLongMeasurementProvider(s_unicastInData);

  /** Unicast in data. */
  public static AtomicLongMeasurementProvider UnicastInDataMeasurement() {
    return s_unicastInDataMeasurement;
  }

  /**
   * Returns '127.0.0.99' as a dummy address, normally when no network services are available. Using
   * '99' to know what's going on if it happens to come up.
   */
  private static InetAddress s_dummyInetAddress = parseNumericalIPAddress("127.0.0.99");

  /** (logging) */
  private Logger _logger = LoggerFactory.getLogger(NodelAutoDNS.class);

  /** General purpose lock / signal. */
  private Object _serverLock = new Object();

  /** General purpose lock / signal. */
  private Object _clientLock = new Object();

  /** Used to micro-stagger responses. */
  private Random _random = new Random();

  /** Thread-pool for IO operations. */
  private ThreadPool _threadPool = new ThreadPool("Discovery", 24);

  /** This class' timer thread. */
  private Timers _timerThread = new Timers("Discovery");

  /** The thread to receive the multicast data. */
  private Thread _multicastHandlerThread;

  /** The thread to receive the multicast data. */
  private Thread _unicastHandlerThread;

  /** (for graceful clean up) */
  private List<TimerTask> _timers = new ArrayList<TimerTask>();

  /** (permanently latches false) */
  private volatile boolean _enabled = true;

  /** (used in 'incomingQueue') */
  private class QueueEntry {

    /** For logging purposes */
    public final String source;

    /** The packet */
    public final DatagramPacket packet;

    public QueueEntry(String source, DatagramPacket packet) {
      this.source = source;
      this.packet = packet;
    }
  }

  /** The incoming queue. (self locked) */
  private Queue<QueueEntry> _incomingQueue = new LinkedList<QueueEntry>();

  /** Used to avoid unnecessary thread overlapping. */
  private boolean _isProcessingIncomingQueue = false;

  /** Incoming queue processor runnable. */
  private Runnable _incomingQueueProcessor =
      new Runnable() {

        @Override
        public void run() {
          processIncomingPacketQueue();
        }
      };

  /** Used in '_services'. */
  private class ServiceItem {

    SimpleName _name;

    public ServiceItem(SimpleName name) {
      _name = name;
    }
  } // (class)

  /** Holds the registered service items. */
  private ConcurrentMap<SimpleName, ServiceItem> _services =
      new ConcurrentHashMap<SimpleName, ServiceItem>();

  /** Holds the collected advertisements. */
  private ConcurrentMap<SimpleName, AdvertisementInfo> _advertisements =
      new ConcurrentHashMap<SimpleName, AdvertisementInfo>();

  /** (as an InetAddress; will never be null) */
  private InetAddress _group = parseNumericalIPAddress(MDNS_GROUP);

  /** (as an InetSocketAddress (with port); will never be null) */
  private InetSocketAddress _groupSocketAddress = new InetSocketAddress(_group, MDNS_PORT);

  /**
   * Whether or not we're probing for client. It will probe on start up and then deactivate. e.g.
   * 'list' is called. (nanos)
   */
  private AtomicLong _lastList = new AtomicLong(System.nanoTime());

  /** The last time a probe occurred. (nanos) */
  private AtomicLong _lastProbe = new AtomicLong(0);

  /**
   * Whether or not client resolution is being used. This affects whether polling should take place.
   * (one way switch)
   */
  private volatile boolean _usingResolution = false;

  /**
   * The time before probing can be suspended if there haven't been any recent 'list' or 'resolve'
   * operation. 5 minutes.
   *
   * <p>(millis)
   */
  private static final long LIST_ACTIVITY_PERIOD = 5 * 60 * 1000;

  /** General purpose lock for sendSocket, receiveSocket, enabled, recycle*flag */
  private Object _lock = new Object();

  /** For multicast sends and unicast receives on arbitrary port. (locked around 'lock') */
  private MulticastSocket _sendSocket;

  /** (socket label) */
  private static final String s_sendSocketLabel = "[multicastSender_unicastSenderReceiver]";

  /** (flag; locked around 'lock') */
  private boolean _recycleSender;

  /** For multicast receives on the MDNS port. */
  private MulticastSocket _receiveSocket;

  /** (socket label) */
  private static String s_receiveSocketlabel = "[multicastReceiver]";

  /** (flag; locked around 'lock') */
  private boolean _recycleReceiver;

  /** Used if direct "multicast" (using unicast) is enabled. */
  private DatagramSocket _hardLinksSocket;

  /** (socket label) */
  private static String s_hardLinksSocketlabel = "[unicastHardLinksSenderReceiver]";

  /** (will never be null after being set) */
  private String _nodelAddress;

  /** Will be a valid address. */
  private String _httpAddress =
      "http://"
          + getLocalIPv4Address().getHostAddress()
          + ":"
          + Nodel.getHTTPPort()
          + Nodel.getHTTPSuffix();

  /**
   * Holds a safe list of resolved addresses and ports that should be "directly" multicast to (i.e.
   * using unicast). Can be used if multicasting is unreliable or inconvenient. (is either null or
   * has at least one element)
   */
  private List<InetSocketAddress> _hardLinksAddresses = composeHardLinksSocketAddresses();

  /**
   * Returns immediately.
   *
   * <p>(Private constructor)
   */
  private NodelAutoDNS() {
    // (no blocking code can be here)

    // create the receiver thread and start it
    _multicastHandlerThread =
        new Thread(
            new Runnable() {

              @Override
              public void run() {
                multicastReceiverThreadMain();
              }
            },
            "autodns_multicastreceiver");
    _multicastHandlerThread.setDaemon(true);
    _multicastHandlerThread.start();

    // create the receiver thread and start it
    _unicastHandlerThread =
        new Thread(
            new Runnable() {

              @Override
              public void run() {
                unicastReceiverThreadMain();
              }
            },
            "autodns_unicastreceiver");
    _unicastHandlerThread.setDaemon(true);
    _unicastHandlerThread.start();

    // kick off the client prober to start
    // after 10s - 15s (randomly chosen)
    _timers.add(
        _timerThread.schedule(
            new TimerTask() {

              @Override
              public void run() {
                handleProbeTimer();
              }
            },
            (long) (10000 + _random.nextDouble() * 5000),
            PROBE_PERIOD));

    // kick off the cleanup tasks timer
    _timers.add(
        _timerThread.schedule(
            new TimerTask() {

              @Override
              public void run() {
                handleCleanupTimer();
              }
            },
            60000,
            60000));
    ;

    // monitor interface changes after 10s delay, then every 2 mins
    _timers.add(
        _timerThread.schedule(
            new TimerTask() {

              @Override
              public void run() {
                handleInterfaceCheck();
              }
            },
            10000,
            120000));

    _logger.info(
        "Auto discovery threads and timers started. probePeriod:{}, stalePeriodAllowed:{}",
        DateTimes.formatShortDuration(PROBE_PERIOD),
        DateTimes.formatShortDuration(STALE_TIME));
  } // (init)

  /** Creates a new socket, cleaning up if anything goes wrong in the process */
  private MulticastSocket createMulticastSocket(String label, InetAddress intf, int port)
      throws Exception {
    MulticastSocket socket = null;

    try {
      _logger.info(
          "Preparing {} socket. interface:{}, port:{}, group:{}",
          label,
          (intf == null ? "default" : intf),
          (port == 0 ? "any" : port),
          _group);

      // in previous versions the interface was selected using constructor instead of
      // 'socket.setInterface(intf)'
      // but that uncovered side-effect in OSX which caused 'cannot assign address' Java bug

      socket = new MulticastSocket(port); // (port '0' means any port)

      if (intf != null) socket.setInterface(intf);

      // join the multicast group
      socket.joinGroup(_group);

      _logger.info("{} ready. localAddr:{}", label, socket.getLocalSocketAddress());

      return socket;

    } catch (Exception exc) {
      Stream.safeClose(socket);

      throw exc;
    }
  }

  /** (thread entry-point) */
  private void multicastReceiverThreadMain() {
    while (_enabled) {
      MulticastSocket socket = null;

      try {
        synchronized (_lock) {
          // clear flag regardless
          _recycleReceiver = false;
        }

        socket = createMulticastSocket(s_receiveSocketlabel, s_interface, MDNS_PORT);

        synchronized (_lock) {
          // make sure not flagged since reset
          if (_recycleReceiver) {
            Stream.safeClose(socket);
            continue;
          }

          _receiveSocket = socket;
        }

        while (_enabled) {
          DatagramPacket dp = UDPPacketRecycleQueue.instance().getReadyToUsePacket();

          // ('returnPacket' will be called in 'catch' or later after use in thread-pool)

          try {
            socket.receive(dp);

          } catch (Exception exc) {
            UDPPacketRecycleQueue.instance().returnPacket(dp);

            throw exc;
          }

          InetAddress recvAddr = dp.getAddress();

          if (recvAddr.isMulticastAddress()) {
            s_multicastInData.addAndGet(dp.getLength());
            s_multicastInOps.incrementAndGet();
          } else {
            s_unicastInData.addAndGet(dp.getLength());
            s_unicastInOps.incrementAndGet();
          }

          // check whether it's external i.e. completely different IP address
          // (local multicasting would almost always be reliable)

          MulticastSocket otherLocal = _sendSocket;
          boolean isLocal = (otherLocal != null && recvAddr.equals(otherLocal.getLocalAddress()));

          // update counter which is used to detect silence
          if (!isLocal) _lastExternalMulticastPacket = System.nanoTime();

          enqueueForProcessing(dp, s_receiveSocketlabel);
        } // (inner while)

      } catch (Exception exc) {
        // (timeouts and general IO problems)

        // clean up regardless
        Stream.safeClose(socket);

        synchronized (_lock) {
          if (!_enabled) break;

          if (_recycleReceiver)
            _logger.info(s_receiveSocketlabel + " was gracefully closed. Will reinitialise...");
          else
            _logger.warn(
                s_receiveSocketlabel
                    + " receive failed; this may be a transitional condition. Will reinitialise... message was '"
                    + exc.toString()
                    + "'");

          // set flag
          _recycleSender = true;
          // "signal" other thread
          Stream.safeClose(_sendSocket);

          // stagger retry
          Threads.waitOnSync(_lock, 333);
        }
      }
    } // (outer while)

    _logger.info("This thread has run to completion.");
  } // (method)

  /** (thread entry-point) */
  private void unicastReceiverThreadMain() {
    while (_enabled) {
      MulticastSocket socket = null;

      try {
        synchronized (_lock) {
          // clear flag regardless
          _recycleSender = false;
        }

        socket = createMulticastSocket(s_sendSocketLabel, s_interface, 0);

        // make sure a recycle request hasn't since occurred
        synchronized (_lock) {
          if (_recycleSender) {
            Stream.safeClose(socket);
            continue;
          }

          _sendSocket = socket;
        }

        while (_enabled) {
          DatagramPacket dp = UDPPacketRecycleQueue.instance().getReadyToUsePacket();

          try {
            socket.receive(dp);

          } catch (Exception exc) {
            UDPPacketRecycleQueue.instance().returnPacket(dp);

            throw exc;
          }

          if (dp.getAddress().isMulticastAddress()) {
            s_multicastInData.addAndGet(dp.getLength());
            s_multicastInOps.incrementAndGet();
          } else {
            s_unicastInData.addAndGet(dp.getLength());
            s_unicastInOps.incrementAndGet();
          }

          enqueueForProcessing(dp, s_sendSocketLabel);
        } // (inner while)

      } catch (Exception exc) {
        boolean wasClosed = (socket != null && socket.isClosed());

        // clean up regardless
        Stream.safeClose(socket);

        synchronized (_lock) {
          if (!_enabled) break;

          if (wasClosed)
            _logger.info(
                s_sendSocketLabel + " was signalled to gracefully close. Will reinitialise...");
          else _logger.warn(s_sendSocketLabel + " receive failed; will reinitialise...", exc);

          // stagger retry
          Threads.waitOnSync(_lock, 333);
        }
      }
    } // (outer while)

    _logger.info("This thread has run to completion.");
  }

  private void enqueueForProcessing(DatagramPacket dp, String label) {
    // place it in the queue and make it process if necessary
    synchronized (_incomingQueue) {
      QueueEntry qe = new QueueEntry(label, dp);
      _incomingQueue.add(qe);

      // kick off the other thread to process the queue
      // (otherwise the thread will already be processing the queue)
      if (!_isProcessingIncomingQueue) {
        _isProcessingIncomingQueue = true;
        _threadPool.execute(_incomingQueueProcessor);
      }
    }
  }

  /** Processes whatever's in the queue. */
  private void processIncomingPacketQueue() {
    while (_enabled) {
      QueueEntry entry;

      synchronized (_incomingQueue) {
        if (_incomingQueue.size() <= 0) {
          // nothing left, so clear flag and return
          _isProcessingIncomingQueue = false;

          return;
        }

        entry = _incomingQueue.remove();
      }

      DatagramPacket dp = entry.packet;
      String source = entry.source;

      try {
        // parse packet
        NameServicesChannelMessage message = this.parsePacket(dp);

        // handle message
        this.handleIncomingMessage(source, (InetSocketAddress) dp.getSocketAddress(), message);

      } catch (Exception exc) {
        if (!_enabled) break;

        // log nested exception summary instead of stack-trace dump
        _logger.warn(
            "{} while handling received packet from {}: {}",
            source,
            dp.getSocketAddress(),
            Exceptions.formatExceptionGraph(exc));

      } finally {
        // make sure the packet is returned
        UDPPacketRecycleQueue.instance().returnPacket(entry.packet);
      }
    } // (while)
  }

  private boolean _suppressProbeLog = false;

  /** The last time an external multicast packet was received. (nano-based) */
  private volatile long _lastExternalMulticastPacket = System.nanoTime();

  /** The client timer; determines whether probing is actually necessary (timer entry-point) */
  private void handleProbeTimer() {
    if (_usingResolution) {
      // client names are being resolved, so stay probing
      _suppressProbeLog = false;
      sendProbe();

    } else {
      // the time difference in millis
      long listDiff = (System.nanoTime() - _lastList.get()) / 1000000L;

      if (listDiff < LIST_ACTIVITY_PERIOD) {
        _suppressProbeLog = false;
        sendProbe();

      } else {
        if (!_suppressProbeLog) {
          _logger.info(
              "Probing is paused because it has been more than {} since a 'list' or 'resolve' (total {}).",
              DateTimes.formatShortDuration(LIST_ACTIVITY_PERIOD),
              DateTimes.formatShortDuration(listDiff));
        }

        _suppressProbeLog = true;
      }
    }
  }

  /** Performs the probe asynchronously. */
  private void sendProbe() {
    final NameServicesChannelMessage message = new NameServicesChannelMessage();

    message.agent = Nodel.getAgent();

    List<String> discoveryList = new ArrayList<String>(1);
    discoveryList.add("*");

    List<String> typesList = new ArrayList<String>(2);
    typesList.add("tcp");
    typesList.add("http");

    message.discovery = discoveryList;
    message.types = typesList;

    _lastProbe.set(System.nanoTime());

    // IO is involved so use a thread-pool
    _threadPool.execute(
        new Runnable() {

          @Override
          public void run() {
            sendMessage(_sendSocket, s_sendSocketLabel, _groupSocketAddress, message);

            // check if hard links (direct "multicasting") are enabled for some hosts
            if (_hardLinksAddresses != null && _hardLinksSocket != null) {
              for (InetSocketAddress socketAddress : _hardLinksAddresses) {
                sendMessage(_hardLinksSocket, s_hardLinksSocketlabel, socketAddress, message);
              }
            }
          }
        });
  }

  /** Handle clean-up tasks (timer entry-point) */
  private void handleCleanupTimer() {
    long currentTime = System.nanoTime();

    reapStaleRecords(currentTime);

    recycleReceiverIfNecessary(currentTime);
  } // (method)

  /** Checks for stale records and removes them. */
  private void reapStaleRecords(long currentTime) {
    LinkedList<AdvertisementInfo> toRemove = new LinkedList<AdvertisementInfo>();

    synchronized (_clientLock) {
      for (AdvertisementInfo adInfo : _advertisements.values()) {
        long timeDiff = (currentTime / 1000000) - adInfo.timeStamp;

        if (timeDiff > STALE_TIME) toRemove.add(adInfo);
      }

      // reap if necessary
      if (toRemove.size() > 0) {
        StringBuilder sb = new StringBuilder();

        for (AdvertisementInfo adInfo : toRemove) {
          if (sb.length() > 0) sb.append(",");

          _advertisements.remove(adInfo.name);
          sb.append(adInfo.name);
        }

        _logger.info(
            "{} stale record{} removed. [{}]",
            toRemove.size(),
            toRemove.size() == 1 ? " was" : "s were",
            sb.toString());
      }
    }
  } // (method)

  /**
   * Check if the main receiver has been silent for some time so recycle the socket for best
   * resilience.
   */
  private void recycleReceiverIfNecessary(long currentTime) {
    long timeDiff = (currentTime - _lastExternalMulticastPacket) / 1000000;

    if (timeDiff > SILENCE_TOLERANCE) {
      _logger.info(
          "There appears to be external silence on the multicast receiver (this may or may not be expected); the socket will be recycled to ensure resilience.");

      synchronized (_lock) {
        // recycle receiver which will in turn recycle sender
        _recycleReceiver = true;

        // "signal" the other thread
        Stream.safeClose(_receiveSocket);
      }
    }
  }

  /** Parses the incoming packet. */
  private NameServicesChannelMessage parsePacket(DatagramPacket dp) {
    String packetString = new String(dp.getData(), 0, dp.getLength(), UTF8Charset.instance());

    return (NameServicesChannelMessage)
        Serialisation.coerceFromJSON(NameServicesChannelMessage.class, packetString);
  }

  /** One responder per recipient at any stage. */
  private class Responder {

    public InetSocketAddress _recipient;

    public LinkedList<ServiceItem> _serviceSet;

    private Iterator<ServiceItem> _serviceIterator;

    public Responder(InetSocketAddress recipient) {
      _recipient = recipient;

      synchronized (_serverLock) {
        // get a snap-shot of the service values
        _serviceSet = new LinkedList<ServiceItem>(_services.values());
      }

      // the service iterator
      _serviceIterator = _serviceSet.iterator();
    }

    /**
     * Starts the responder. Wait a random amount of time to before actually sending anything.
     * Staggered responses assists with receiver buffer management by the remote receivers.
     */
    public void start(int randomResponseDelay) {
      // use a random minimum delay of 333ms
      int delay = 333;

      if (randomResponseDelay > 333) delay = randomResponseDelay;

      int timeToWait = _random.nextInt(delay);

      _timerThread.schedule(
          new TimerTask() {

            @Override
            public void run() {
              completeResponse();
            }
          },
          timeToWait);
    }

    /** Complete the necessary response, also a timer entry-point. */
    private void completeResponse() {
      // prepare a message
      final NameServicesChannelMessage message = new NameServicesChannelMessage();
      message.present = new ArrayList<String>();

      // try keep the packets relatively small by roughly guessing how much space
      // its JSON advertisement packet might take up

      // account for "present"...
      // and
      // "addresses":["tcp://136.154.27.100:65017","http://136.154.27.100:8085/index.htm?node=%NODE%"]
      // so start off with approx. 110 chars
      long roughTotalSize = 110;

      while (_serviceIterator.hasNext()) {
        ServiceItem si = _serviceIterator.next();

        String name = si._name.getOriginalName();

        // calculate size to be the name, two inverted-commas, and a comma and a possible space in
        // between.
        int size = name.length() + 4;
        roughTotalSize += size;

        message.present.add(name);

        // make sure we're not going anywhere near UDP MTU (64K),
        // in fact, squeeze them into packets similar to size of Ethernet MTU (~1400 MTU)
        if (roughTotalSize > 1200) break;
      } // (while)

      message.addresses = new ArrayList<String>();
      message.addresses.add(_nodelAddress);
      message.addresses.add(_httpAddress);

      // IO is involved so use thread-pool
      // (and only send if the 'present' list is not empty)

      if (message.present.size() > 0) {
        _threadPool.execute(
            new Runnable() {

              @Override
              public void run() {
                sendMessage(_sendSocket, s_sendSocketLabel, _recipient, message);
              }
            });
      }

      // do we need this completed in the very near future?
      // if so, space out by at least 333ms.
      if (_serviceIterator.hasNext()) {

        _timerThread.schedule(
            new TimerTask() {

              @Override
              public void run() {
                completeResponse();
              }
            },
            333);
      } else {
        synchronized (_responders) {
          _responders.remove(_recipient);
        }
      }
    } // (method)
  } // (class)

  /** Sends the message to a recipient */
  private void sendMessage(
      DatagramSocket socket,
      String label,
      InetSocketAddress to,
      NameServicesChannelMessage message) {
    if (isSameSocketAddress(socket, to))
      _logger.info("{} sending message. to=self, message={}", label, message);
    else _logger.info("{} sending message. to={}, message={}", label, to, message);

    if (socket == null) {
      _logger.info("{} is not available yet; ignoring send request.", label);
      return;
    }

    // convert into bytes
    String json = Serialisation.serialise(message);
    byte[] bytes = json.getBytes(UTF8Charset.instance());

    DatagramPacket packet = new DatagramPacket(bytes, bytes.length);
    packet.setSocketAddress(to);

    try {
      socket.send(packet);

      if (to.getAddress().isMulticastAddress()) {
        s_multicastOutData.addAndGet(bytes.length);
        s_multicastOutOps.incrementAndGet();
      } else {
        s_unicastOutData.addAndGet(bytes.length);
        s_unicastOutOps.incrementAndGet();
      }

    } catch (IOException exc) {
      if (!_enabled) return;

      if (socket.isClosed())
        _logger.info(s_sendSocketLabel + " send() ignored as socket is being recycled.");
      else _logger.warn(s_sendSocketLabel + " send() failed. ", exc);
    }
  } // (method)

  /** The map of responders by recipient address. */
  private ConcurrentMap<SocketAddress, Responder> _responders =
      new ConcurrentHashMap<SocketAddress, Responder>();

  /** Handles a complete packet from the socket. */
  private void handleIncomingMessage(
      String label, InetSocketAddress from, NameServicesChannelMessage message) {
    if (isSameSocketAddress(_sendSocket, from))
      _logger.info("{} message arrived. from=self, message={}", label, message);
    else _logger.info("{} message arrived. from={}, message={}", label, from, message);

    // discovery request?
    if (message.discovery != null) {
      // create a responder if one isn't already active

      if (_nodelAddress == null) {
        _logger.info("(will not respond; nodel server port still not available)");

        return;
      }

      synchronized (_responders) {
        if (!_responders.containsKey(from)) {
          Responder responder = new Responder(from);
          _responders.put(from, responder);

          int delay = message.delay == null ? 0 : message.delay.intValue();

          responder.start(delay);
        }
      }
    } else if (message.present != null && message.addresses != null) {
      for (String name : message.present) {
        synchronized (_clientLock) {
          SimpleName node = new SimpleName(name);
          AdvertisementInfo ad = _advertisements.get(node);
          if (ad == null) {
            ad = new AdvertisementInfo();
            ad.name = node;
            _advertisements.put(node, ad);
          }

          // refresh the time stamp and update the address
          ad.timeStamp = System.nanoTime() / 1000000;
          ad.addresses = message.addresses;
        }
      }
    }
  } // (method)

  @Override
  public NodeAddress resolveNodeAddress(SimpleName node) {
    // indicate client resolution is being used
    _usingResolution = true;

    AdvertisementInfo adInfo = _advertisements.get(node);
    if (adInfo != null) {
      Collection<String> addresses = adInfo.addresses;

      for (String address : addresses) {
        try {
          if (address == null || !address.startsWith("tcp://")) continue;

          int indexOfPort = address.lastIndexOf(':');
          if (indexOfPort < 0 || indexOfPort >= address.length() - 2) continue;

          String addressPart = address.substring(6, indexOfPort);

          String portStr = address.substring(indexOfPort + 1);

          int port = Integer.parseInt(portStr);

          NodeAddress nodeAddress = NodeAddress.create(addressPart, port);

          return nodeAddress;

        } catch (Exception exc) {
          _logger.info("'{}' node resolved to a bad address - '{}'; ignoring.", node, address);

          return null;
        }
      }
    }

    return null;
  }

  @Override
  public void registerService(SimpleName node) {
    synchronized (_serverLock) {
      if (_services.containsKey(node))
        throw new IllegalStateException(node + " is already being advertised.");

      ServiceItem si = new ServiceItem(node);

      _services.put(node, si);
    }
  } // (method)

  /** Used to monitor address changes on the the interface. */
  private void handleInterfaceCheck() {
    try {
      int port = super.getAdvertisementPort();
      if (port < 0) {
        // can't compose a nodel address yet
        _logger.info("(nodel server port still not available; will wait.)");

        return;
      }

      InetAddress localIPv4Address = getLocalIPv4Address();
      String nodelAddress = "tcp://" + localIPv4Address.getHostAddress() + ":" + port;

      if (nodelAddress.equals(_nodelAddress))
        // nothing to do
        return;

      // the address has changed so should update advertisements
      _logger.info(
          "An address change has been detected. previous={}, current={}",
          _nodelAddress,
          nodelAddress);

      _nodelAddress = "tcp://" + localIPv4Address.getHostAddress() + ":" + port;
      _httpAddress =
          "http://"
              + localIPv4Address.getHostAddress()
              + ":"
              + Nodel.getHTTPPort()
              + Nodel.getHTTPSuffix();

      Nodel.updateHTTPAddress(_httpAddress);

      synchronized (_lock) {
        // recycle receiver which will in turn recycle sender
        _recycleReceiver = true;

        // "signal" thread
        Stream.safeClose(_receiveSocket);
      }

    } catch (Exception exc) {
      _logger.warn("'handleInterfaceCheck' did not complete cleanly; ignoring for now.", exc);
    }
  } // (method)

  @Override
  public void unregisterService(SimpleName node) {
    synchronized (_serverLock) {
      if (!_services.containsKey(node))
        throw new IllegalStateException(node + " is not advertised anyway.");

      _services.remove(node);
    }
  }

  @Override
  public Collection<AdvertisementInfo> list() {
    long now = System.nanoTime();

    _lastList.set(now);

    // check how long it has been since the last probe (millis)
    long timeSinceProbe = (now - _lastProbe.get()) / 1000000L;

    if (timeSinceProbe > LIST_ACTIVITY_PERIOD) sendProbe();

    // create snap-shot
    List<AdvertisementInfo> ads = new ArrayList<AdvertisementInfo>(_advertisements.size());

    synchronized (_clientLock) {
      ads.addAll(_advertisements.values());
    }

    return ads;
  }

  @Override
  public AdvertisementInfo resolve(SimpleName node) {
    // indicate client resolution is being used
    _usingResolution = true;

    return _advertisements.get(node);
  }

  /** Permanently shuts down all related resources. */
  @Override
  public void close() throws IOException {
    // clear flag
    _enabled = false;

    // release timers
    for (TimerTask timer : _timers) timer.cancel();

    Stream.safeClose(_sendSocket, _receiveSocket, _hardLinksSocket);
  }

  /** Creates or returns the shared instance. */
  public static AutoDNS create() {
    return Instance.INSTANCE;
  }

  /** (singleton, thread-safe, non-blocking) */
  private static class Instance {

    private static final NodelAutoDNS INSTANCE = new NodelAutoDNS();
  }

  /** Returns the singleton instance of this class. */
  public static NodelAutoDNS instance() {
    return Instance.INSTANCE;
  }

  /** Returns the first "sensible" IPv4 address. */
  public static InetAddress getLocalIPv4Address() {
    InetAddress selectedInterface = s_interface;

    // if an interface is being used and IP addresses don't necessarily match, use this candidate
    // anyway
    InetAddress candidate = null;

    try {
      Enumeration<NetworkInterface> en = NetworkInterface.getNetworkInterfaces();
      if (en == null) return s_dummyInetAddress;

      while (en.hasMoreElements()) {
        NetworkInterface intf = en.nextElement();
        for (Enumeration<InetAddress> enumIpAddr = intf.getInetAddresses();
            enumIpAddr.hasMoreElements(); ) {
          InetAddress inetAddress = enumIpAddr.nextElement();
          if (!inetAddress.isLoopbackAddress() && inetAddress instanceof Inet4Address) {

            // is an IPv4 address that is not a loopback address

            // if interface binding isn't being used, return the found address
            if (selectedInterface == null) {
              return inetAddress;

            } else {
              // make first one candidate
              if (candidate != null) candidate = inetAddress;

              if (selectedInterface.equals(inetAddress)) return inetAddress;

              // otherwise keep iterating, will return the candidate at the end
            }
          }
        }
      }
    } catch (IOException exc) {
      // ignore
    }

    if (candidate != null) return candidate;

    return s_dummyInetAddress;
  }

  /**
   * Parses a dotted numerical IP address without throwing any exceptions. (convenience function)
   */
  private static InetAddress parseNumericalIPAddress(String dottedNumerical) {
    try {
      return InetAddress.getByName(dottedNumerical);

    } catch (Exception exc) {
      throw new Error("Failed to resolve dotted numerical address - " + dottedNumerical);
    }
  }

  /**
   * Safely returns true if a packet has the same address and a socket. Used to determine its own
   * socket.
   */
  private static boolean isSameSocketAddress(DatagramSocket socket, InetSocketAddress addr) {
    if (socket == null || addr == null) return false;

    SocketAddress socketAddr = socket.getLocalSocketAddress();

    return socketAddr != null && socketAddr.equals(addr);
  }

  /**
   * Turns the list of addresses into "resolved" InetSocketAddresses. (should only be called once)
   */
  private List<InetSocketAddress> composeHardLinksSocketAddresses() {
    List<InetAddress> addresses = Nodel.getHardLinksAddresses();

    if (addresses.size() <= 0) return null;

    List<InetSocketAddress> socketAddresses = new ArrayList<InetSocketAddress>();
    for (InetAddress address : addresses)
      socketAddresses.add(new InetSocketAddress(address, MDNS_PORT));

    // at least one address is enabled, so initialise a general purpose UDP socket and
    // receiver thread.

    Thread thread =
        new Thread(
            new Runnable() {

              @Override
              public void run() {
                hardLinksReceiverThreadMain();
              }
            },
            s_hardLinksSocketlabel);
    thread.setDaemon(true);
    thread.start();

    return socketAddresses;
  }

  /** (thread entry-point) */
  private void hardLinksReceiverThreadMain() {
    _logger.info("Instructed to use hardlinks. address:{}", _hardLinksAddresses);

    DatagramSocket socket = null;
    try {
      // initialise a UDP socket on an arbitrary port
      _hardLinksSocket = new DatagramSocket();

      socket = _hardLinksSocket;

      while (_enabled) {
        DatagramPacket dp = UDPPacketRecycleQueue.instance().getReadyToUsePacket();

        // ('returnPacket' will be called in 'catch' or later after use in thread-pool)

        try {
          socket.receive(dp);

          s_unicastInData.addAndGet(dp.getLength());
          s_unicastInOps.incrementAndGet();

          enqueueForProcessing(dp, s_hardLinksSocketlabel);

        } catch (Exception exc) {
          UDPPacketRecycleQueue.instance().returnPacket(dp);

          // ignore
        }
      } // (while)

    } catch (Exception exc) {
      _logger.warn("Failed to initialise [" + s_hardLinksSocketlabel + "] socket.", exc);
    } finally {
      _logger.info("[" + s_hardLinksSocketlabel + "] thread run to completion.");

      // close for good measure
      Stream.safeClose(socket);
    }
  }
} // (class)