GlobalLock.java

// Copyright 2012 Citrix Systems, Inc. Licensed under the
// Apache License, Version 2.0 (the "License"); you may not use this
// file except in compliance with the License.  Citrix Systems, Inc.
// reserves all rights not expressly granted by the License.
// You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 
// Automatically generated by addcopyright.py at 04/03/2012
package com.cloud.utils.db;

import java.util.HashMap;
import java.util.Map;

import org.apache.log4j.Logger;

import com.cloud.utils.Profiler;

//
// Wrapper class for global database lock to reduce contention for database connections from within process
//
// Example of using dynamic named locks
//
//		GlobalLock lock = GlobalLock.getInternLock("some table name" + rowId);
//		
//		if(lock.lock()) {
//			try {
//				do something
//			} finally {
//				lock.unlock();
//			}
//		}
//		lock.releaseRef();
//
public class GlobalLock {
    protected final static Logger s_logger = Logger.getLogger(GlobalLock.class);

	private String name;
	private int lockCount = 0;
	private Thread ownerThread = null;
	
	private int referenceCount = 0;
	private long holdingStartTick = 0;
	
	private static Map<String, GlobalLock> s_lockMap = new HashMap<String, GlobalLock>();
	
	private GlobalLock(String name) {
		this.name = name;
	}
	
	public int addRef() {
		synchronized(this) {
			referenceCount++;
			return referenceCount;
		}
	}
	
	public int releaseRef() {
		int refCount;
		
		boolean needToRemove = false;
		synchronized(this) {
			referenceCount--;
			refCount = referenceCount;
			
			if(referenceCount < 0)
				s_logger.warn("Unmatched Global lock " + name + " reference usage detected, check your code!");
			
			if(referenceCount == 0)
				needToRemove = true;
		}
		
		if(needToRemove)
			releaseInternLock(name);
		
		return refCount;
	}

	public static GlobalLock getInternLock(String name) {
		synchronized(s_lockMap) {
			if(s_lockMap.containsKey(name)) {
				GlobalLock lock = s_lockMap.get(name);
				lock.addRef();
				return lock;
			} else {
				GlobalLock lock = new GlobalLock(name);
				lock.addRef();
				s_lockMap.put(name, lock);
				return lock;
			}
		}
	}
	
	private static void releaseInternLock(String name) {
		synchronized(s_lockMap) {
			GlobalLock lock = s_lockMap.get(name);
			if(lock != null) {
				if(lock.referenceCount == 0)
					s_lockMap.remove(name);
			} else {
				s_logger.warn("Releasing " + name + ", but it is already released.");
			}
		}
	}
	
	public boolean lock(int timeoutSeconds) {
		int remainingMilliSeconds = timeoutSeconds*1000;
		Profiler profiler = new Profiler();
		boolean interrupted = false;
		try {
			while(true) {
				synchronized(this) {
					if(ownerThread != null && ownerThread == Thread.currentThread()) {
						s_logger.warn("Global lock re-entrance detected");
						
						lockCount++;
	
						if(s_logger.isTraceEnabled())
							s_logger.trace("lock " + name + " is acquired, lock count :" + lockCount);
						return true;
					}
					
					if(ownerThread != null) {
						profiler.start();
						try {
							wait(((long)timeoutSeconds)*1000L);
						} catch (InterruptedException e) {
							interrupted = true;
						}
						profiler.stop();
						
						remainingMilliSeconds -= profiler.getDuration();
						if(remainingMilliSeconds < 0)
							return false;
						
						continue;
					} else {
						// take ownership temporarily to prevent others enter into stage of acquiring DB lock
						ownerThread = Thread.currentThread();
						addRef();
					}
				}

				if(DbUtil.getGlobalLock(name, remainingMilliSeconds / 1000)) {
					synchronized(this) {
						lockCount++;
						holdingStartTick = System.currentTimeMillis();
						
						if(s_logger.isTraceEnabled())
							s_logger.trace("lock " + name + " is acquired, lock count :" + lockCount);
						return true;
					}
				} else {
					synchronized(this) {
						ownerThread = null;
						releaseRef();
						return false;
					}
				}
			}
		} finally {
			if(interrupted) {
				Thread.currentThread().interrupt();
			}
		}
	}
	
	public boolean unlock() {
		synchronized(this) {
			if(ownerThread != null && ownerThread == Thread.currentThread()) {
				lockCount--;
				if(lockCount == 0) {
					ownerThread = null;
					DbUtil.releaseGlobalLock(name);
					
					if(s_logger.isTraceEnabled())
						s_logger.trace("lock " + name + " is returned to free state, total holding time :" + 
							(System.currentTimeMillis() - holdingStartTick));
					holdingStartTick = 0;
					
					// release holding position in intern map when we released the DB connection
					releaseRef();
					notifyAll();
				}
				
				if(s_logger.isTraceEnabled())
					s_logger.trace("lock " + name + " is released, lock count :" + lockCount);
				return true;
			}
			return false;
		}
	}
	
	public String getName() {
		return name;
	}
}