/*
GASH 2
DBLockSync.java
The GANYMEDE object storage system.
Created: 9 February 2000
Module By: Jonathan Abbey, jonabbey@arlut.utexas.edu
-----------------------------------------------------------------------
Ganymede Directory Management System
Copyright (C) 1996-2014
The University of Texas at Austin
Ganymede is a registered trademark of The University of Texas at Austin
Contact information
Web site: http://www.arlut.utexas.edu/gash2
Author Email: ganymede_author@arlut.utexas.edu
Email mailing list: ganymede@arlut.utexas.edu
US Mail:
Computer Science Division
Applied Research Laboratories
The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029
Telephone: (512) 835-3200
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package arlut.csd.ganymede.server;
import java.util.HashMap;
import java.util.ArrayList;
import java.util.List;
/*------------------------------------------------------------------------------
class
DBLockSync
------------------------------------------------------------------------------*/
/**
* <p>This class acts to provide a singleton object for interlock
* coordination. All global data required for coordinating DBLock
* lock activity is stored in the singleton object.</p>
*
* <p>Note that much code in the various {@link
* arlut.csd.ganymede.server.DBLock} subclasses, and in the rest of
* the Ganymede server, establishes external synchronization on the
* DBLockSync object referenced in {@link
* arlut.csd.ganymede.server.DBStore#lockSync DBStore.lockSync}, so
* certain methods in this class which do not appear synchronized may
* in fact be dependent on external synchronization.</p>
*/
public final class DBLockSync {
/**
* <p>Identifier keys for current {@link arlut.csd.ganymede.server.DBLock
* DBLocks}.</p>
*
* <p>This hash is used by the establish() method in various DBLock
* subclasses to guarantee that only one lock will established by
* a client at a time, to prevent any possibility of DBLock deadlock.</p>
*
* <p>The values in this hash may either be scalar DBLock objects, or
* in the case of readers (where it is permissible for a single client
* to have several distinct reader locks), a List of DBReadLocks.</p>
*/
private HashMap lockHash;
/**
* <p>A count of how many {@link arlut.csd.ganymede.server.DBLock
* DBLocks} are established on {@link
* arlut.csd.ganymede.server.DBObjectBase DBObjectBases} in this
* DBStore.</p>
*/
private int locksHeld = 0;
/**
* <p>A count of how many {@link arlut.csd.ganymede.server.DBLock
* DBLocks} are waiting to be established on {@link
* arlut.csd.ganymede.server.DBObjectBase DBObjectBases} in this
* DBStore.</p>
*/
private int locksWaiting = 0;
/* -- */
public DBLockSync()
{
resetLockHash(0);
}
/**
* <p>This method causes the DBLockSync object's lock owner HashMap
* to be reset. If count is not zero, that value will be used to
* set the initial capacity for the HashMap.</p>
*/
public synchronized void resetLockHash(int count)
{
if (count <= 0)
{
lockHash = new HashMap(20); // default value
}
else
{
lockHash = new HashMap(count);
}
locksHeld = 0;
locksWaiting = 0;
}
/**
* <p>This method associates a DBLock with the given key, making
* sure that there is no conflicting lock request for the key</p>
*
* @return True if the lock could be associated with key, False if
* there was a pre-existing conflicting association.
*/
public synchronized boolean claimLockKey(Object key, DBLock lock)
{
if (lock instanceof DBReadLock)
{
Object obj = lockHash.get(key);
if (obj != null && !(obj instanceof List))
{
return false;
}
List<DBLock> lockList = (List<DBLock>) obj;
if (lockList == null)
{
lockList = new ArrayList<DBLock>();
lockHash.put(key, lockList);
}
lockList.add(lock);
}
else
{
if (lockHash.containsKey(key))
{
return false;
}
lockHash.put(key, lock);
}
return true;
}
/**
* <p>This method disassociates a DBLock from the given key</p>
*
* <p>If the key was not previously claimed for the given lock, an
* IllegalStateException will be thrown.</p>
*/
public synchronized void unclaimLockKey(Object key, DBLock lock)
{
Object obj = lockHash.get(key);
if (obj == null)
{
throw new IllegalStateException("No such key");
}
if (lock instanceof DBReadLock)
{
if (!(obj instanceof List))
{
throw new IllegalStateException("Error, can't remove a read lock while there is a " +
obj +
" associated with key " + key +
".. there are no readlocks here.");
}
List<DBLock> lockList = (List<DBLock>) obj;
if (!lockList.contains(lock))
{
throw new IllegalStateException("Mismatched lock claim");
}
lockList.remove(lock);
if (lockList.size() == 0)
{
lockHash.remove(key); // that was the last read lock on this key
}
}
else
{
if (obj != lock)
{
throw new IllegalStateException("Mismatched lock claim");
}
lockHash.remove(key);
}
}
/**
* <p>This method returns a List of DBReadLock objects associated
* with key, if any. If there is no DBReadLock vector associated
* with the key, an IllegalStateException will be thrown.</p>
*
* <p>The List returned is part of DBLockSync's internal data
* structures, and should only be browsed in a block synchronized on
* this DBLockSync object.</p>
*
* <p>The List returned should not be modified by external code.</p>
*/
public synchronized List<DBReadLock> getReadLockList(Object key)
{
Object o = lockHash.get(key);
if (o == null)
{
return null;
}
if (o instanceof List)
{
return (List<DBReadLock>) o;
}
throw new IllegalStateException("DBLockSync does not contain a readlock list for key " + key);
}
/**
* <p>This method returns a DBLock associated with the given key, if
* any.</p>
*
* <p>This method will only ever return a DBWriteLock or a
* DBDumpLock. If the key is associated with a List of DBReadLocks,
* null will be returned.</p>
*/
public synchronized DBLock getLockHeld(Object key)
{
Object obj = lockHash.get(key);
if (obj == null || (obj instanceof DBLock))
{
return (DBLock) obj;
}
return null; // Can't return a single lock, this key has a readlock vector.
}
/**
* <p>Increments the count of locks waiting to be established.</p>
*/
public synchronized void incLocksWaitingCount()
{
locksWaiting++;
GanymedeAdmin.updateLocksHeld();
}
/**
* <p>Decrements the count of locks waiting to be established.</p>
*/
public synchronized void decLocksWaitingCount()
{
locksWaiting--;
GanymedeAdmin.updateLocksHeld();
}
/**
* <p>Increments the count of held locks for the admin consoles.</p>
*/
public synchronized void incLockCount()
{
locksHeld++;
GanymedeAdmin.updateLocksHeld();
}
/**
* <p>Decrements the count of held locks for the admin consoles.</p>
*/
public synchronized void decLockCount()
{
locksHeld--;
GanymedeAdmin.updateLocksHeld();
if (locksHeld < 0)
{
throw new RuntimeException("Locks held has gone negative");
}
}
/**
* <p>Returns the number of locks currently waiting to be established.</p>
*/
public int getLocksWaitingCount()
{
return locksWaiting;
}
/**
* <p>Returns the number of locks presently held in the database.</p>
*/
public int getLockCount()
{
return locksHeld;
}
}