edgechasing.java

用Java写的面相对象的数据库管理系统

// You can redistribute this software and/or modify it under the terms of
// the Ozone Core License version 1 published by ozone-db.org.
//
// The original code and portions created by SMB are
// Copyright (C) 1997-2000 by SMB GmbH. All rights reserved.
//
// $Id: EdgeChasing.java,v 1.5 2000/10/28 16:55:18 daniela Exp $

package org.ozoneDB.core.dr;

import org.ozoneDB.DxLib.*;
import org.ozoneDB.core.*;
import org.ozoneDB.util.*;


/**
 * @author <a href="http://www.softwarebuero.de/">SMB</a>
 * @version $Revision: 1.5 $Date: 2000/10/28 16:55:18 $
 */
public final class EdgeChasing extends DeadlockRecognition {
    
    protected Env env;
    
    
    public EdgeChasing( Env _env ) {
        env = _env;
    }
    
    
    public Locker detectDeadlock( Locker locker ) {
        env.logWriter.newEntry( this, "*** detectDeadlock(): " + locker, LogWriter.DEBUG );
        return sendLockMessage( locker, locker, true ) ? locker : null;
    } 
    
    
    /**
     * Recursive deadlock recognition.
     * @return True, if this transaction is blocked by root (?)
     */
    public boolean sendLockMessage( Locker root, Locker locker, boolean firstLevel ) {
        env.logWriter.newEntry( this, "*** sendLockMessage(): " + locker, LogWriter.DEBUG );
        Lockable blocker = locker.blockedBy();
        if (blocker == null) {
            return false;
        } 
        
        if (!firstLevel) {
            if (root == locker) {
                return true;
            } 
            
            if (blocker == null) {
                return false;
            } 
        } 
        
        //bin selbst beim warten auf blocker blockiert; wenn ich
        //lese-lock setzen moechte, muss ich die ta mit schreib-lock
        //auf blocker kontrollieren; wenn schreib-lock alle ta mit
        //lese-lock auf blocker - diese werden genau von allLockers()
        //geliefert
        
        //alle ta, die mich behindern; entweder ein schreibende oder
        //viele lesende transaktionen
        DxCollection lockers = blocker.allLockers();
        if (lockers != null) {
            DxIterator it = lockers.iterator();
            
            while (it.next() != null) {
                Locker nextLocker = (Locker)it.object();
                if (nextLocker != locker) {
                    //we are done if we have detected the first circle;
                    //go on with next candidate otherwise
                    if (sendLockMessage( root, nextLocker, false ) == true) {
                        return true;
                    } 
                } 
            } 
        } 
        return false;
    } 
    
}