PessimisticLockManager.java

/**
* OLAT - Online Learning and Training<br>
* http://www.olat.org
* <p>
* Licensed under the Apache License, Version 2.0 (the "License"); <br>
* you may not use this file except in compliance with the License.<br>
* You may obtain a copy of the License at
* <p>
* http://www.apache.org/licenses/LICENSE-2.0
* <p>
* Unless required by applicable law or agreed to in writing,<br>
* software distributed under the License is distributed on an "AS IS" BASIS, <br>
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. <br>
* See the License for the specific language governing permissions and <br>
* limitations under the License.
* <p>
* Copyright (c) since 2004 at Multimedia- & E-Learning Services (MELS),<br>
* University of Zurich, Switzerland.
* <hr>
* <a href="http://www.openolat.org">
* OpenOLAT - Online Learning and Training</a><br>
* This file has been modified by the OpenOLAT community. Changes are licensed
* under the Apache 2.0 license as the original file.
*/
package org.olat.core.commons.services.lock.pessimistic;

import java.util.Date;
import java.util.List;

import javax.persistence.LockModeType;

import org.olat.core.commons.persistence.DB;
import org.olat.core.logging.AssertException;
import org.olat.core.logging.OLog;
import org.olat.core.logging.Tracing;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;


/**
 * 
 * Description:<br>
 * implementation for pessimistic locking.<br>
 * Do not use this class directly. please use Syncer or Locker via CoordinatorManager!
 * 
 * <P>
 * Initial Date:  25.10.2007 <br>
 * @author Felix Jost, http://www.goodsolutions.ch
 */
@Service("lockManager")
public class PessimisticLockManager implements InitializingBean {
	
	private static final OLog log = Tracing.createLoggerFor(PessimisticLockManager.class);
	

	private final String ASSET_INSERT_LOCK = "SYS_plock_global";
	private boolean initDone = false;
	
	private final DB dbInstance;
	
	@Autowired
	private PessimisticLockManager(DB dbInstance) {
		this.dbInstance = dbInstance;
	}
	
	@Override
	public void afterPropertiesSet() throws Exception {
		// make sure that the resource (= row in our table) to lock the creation of new assets exists
		PLock gLock = findPLock(ASSET_INSERT_LOCK);
		if (gLock == null) {
			// need to create it
			gLock = createPLock(ASSET_INSERT_LOCK);
			savePLock(gLock);
		}
		dbInstance.intermediateCommit();
		initDone = true;
	}
	
	private PLock findPLock(String asset) {	
		List<PLock> res = dbInstance.getCurrentEntityManager()
				.createNamedQuery("loadByPLockByAsset", PLock.class)
				.setParameter("asset", asset)
				.setLockMode(LockModeType.PESSIMISTIC_WRITE)
				.setHint("javax.persistence.lock.timeout", new Integer(30000))
				.getResultList();

		if (res.size() == 0) {
			return null; 
		} else {
			return res.get(0);
		}
	}
	
	private PLock createPLock(String asset) {
		PLockImpl lock = new PLockImpl();
		lock.setAsset(asset);
		lock.setCreationDate(new Date());
		return lock;
	}
	
	private void savePLock(PLock plock) {
		dbInstance.saveObject(plock);
	}
	
	/**
	 * do not use this class directly. please use Syncer or Locker via CoordinatorManager!
	 * @param asset
	 * @return
	 */
	public PLock findOrPersistPLock(String asset) {
		if (!initDone) throw new AssertException("init not called yet - make sure the ClusterModule is enabled in your olat.local.properties file");
		
		boolean debug = log.isDebug();
		if (debug) {
			log.debug("findOrPersistPLock START asset="+asset);
		}
		PLock plock = findPLock(asset);
		if (debug) {
			if (plock==null) {
				log.debug("findOrPersistPLock PLock not found");
			} else {
				log.debug("findOrPersistPLock found and locked PLock: "+plock);
			}
		}
		// if not found, persist it.
		if (plock == null ) {
			// synchronize the findOrCreate by using the special row with the global-lock-asset
			// locks the global lock - which is only used to sync creation of new resource entries, so that those can later be locked.
			findPLock(ASSET_INSERT_LOCK);
			if (debug) {
				log.debug("findOrPersistPLock global insert lock locked");
			}
			// need to read again within the protected region
			plock = findPLock(asset);
			if (plock == null) {
				if (debug) {
					log.debug("findOrPersistPLock creating new plock: "+asset);
				}
				plock = createPLock(asset);
				if (debug) {
					log.debug("findOrPersistPLock created new plock: "+asset);
				}
				savePLock(plock);
				if (debug) {
					log.debug("findOrPersistPLock saved new plock: "+asset);
				}
			} // else plock got created by another thread in the meantime

			// some notes:
			// takes advantage of the fact that the select for update blocks a transaction when a lock is already acquired.
			// 
			
			// since we have concurrent access here, we could have many threads which try to create
			// the entry to later lock upon.
			
			// we therefore could 
			// a) lock on a olat-wide lock
			// or b) catch the exception - and continue, since we know that the row already exists
			// even c) start a new connection and set serializable isolation level..
			
			// in a cluster, each vm syncs via synchronized() first, so that there is only one concurrent access to the PLockManager from one node at a given time.
			// -> we have maximal num-of-cluster concurrent accesses, e.g. 3-5
			
			// a: performance, should only occur once for a resource: the first time a lock for a certain resource is accessed.
			// b) is the transaction still safe to continue? what about hibernate first level cache etc. hibernate docs says in general we'd need to close the session.
			
			// -> go for solution a.
			
		} // else found
		return plock;
	}

	
}