Re: Hashtable updates to disk

On Wed, 16 Jun 2010, Boris Punk wrote:


Loads.

How do you want to store the hashtable?

Let's assume serialisation. Not tested, and obviously not ready for real
use:

public class MapDumper {
  public static <K, V> Map<K, V> makeDumpingMap(Map<K, V> m, File file, long interval) {
  Serializable s = (Serializable)m;
  Map<K, V> sm = Collections.synchronizedMap(m);
  new PeriodicDumper(s, sm, file, interval).start();
  return sm;
  }
}

public class PeriodicDumper implements Runnable {
  private final Serializable obj;
  private final Object lock;
  private final File file;
  private final long interval;
  private volatile Thread t;

  public PeriodicDumper(Serializable obj, Object lock, File file, long interval) {
  this.obj = obj;
  this.lock = lock;
  this.file = file;
  this.interval = interval;
  }

  public void run() {
  while (t != null) {
  try {
  Thread.sleep(interval);
  } catch (InterruptedException e) {
  // just treat an interrupt as an early exit from the sleep
  }
  try {
  dump();
  } catch (IOException e) {
  // do something
  }
  }
  }

  public void dump() throws IOException {
  // go via a buffer to avoid doing IO while holding the lock
  ByteArrayOutputStream buf = new ByteArrayOutputStream();
  ObjectOutputStream oout = new ObjectOutputStream(buf);
  synchronized (lock) {
  oout.writeObject(obj);
  oout.close();
  }
  OutputStream fout = new FileOutputStream(file);
  try {
  buf.writeTo(fout);
  }
  finally {
  fout.close();
  }
  }

  public void start() {
  synchronized (this) {
  if (t == null) {
  t = new Thread(this);
  }
  t.setDaemon(true);
  t.start();
  }
  }

  public void stop() {
  synchronized (this) {
  if (t != null) {
  Thread t = this.t;
  this.t = null;
  t.interrupt();
  }
  }
  }
}

Also, if you could get access to the magic cookie inside the map used to
detect concurrent modifications, you could easily skip dumps when no
change has occurred.

You should do the dump a bit more cleverly than this, too, so you're never
in a state where the data on disk is incomplete. Dump to a second file,
then atomically rename over the first.

tom

--
In the long run, we are all dead. -- John Maynard Keynes