001/*
002 * HA-JDBC: High-Availability JDBC
003 * Copyright (c) 2004-2007 Paul Ferraro
004 * 
005 * This library is free software; you can redistribute it and/or modify it 
006 * under the terms of the GNU Lesser General Public License as published by the 
007 * Free Software Foundation; either version 2.1 of the License, or (at your 
008 * option) any later version.
009 * 
010 * This library is distributed in the hope that it will be useful, but WITHOUT
011 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
012 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License 
013 * for more details.
014 * 
015 * You should have received a copy of the GNU Lesser General Public License
016 * along with this library; if not, write to the Free Software Foundation, 
017 * Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
018 * 
019 * Contact: ferraro@users.sourceforge.net
020 */
021package net.sf.hajdbc.balancer;
022
023import java.util.LinkedList;
024import java.util.Queue;
025
026import net.sf.hajdbc.Database;
027
028/**
029 * Balancer implementation whose {@link #next()} implementation uses a circular FIFO queue.
030 * 
031 * @author  Paul Ferraro
032 * @param <D> either java.sql.Driver or javax.sql.DataSource
033 */
034public class RoundRobinBalancer<D> extends AbstractBalancer<D>
035{
036        private Queue<Database<D>> databaseQueue = new LinkedList<Database<D>>();
037
038        /**
039         * @see net.sf.hajdbc.balancer.AbstractBalancer#added(net.sf.hajdbc.Database)
040         */
041        @Override
042        protected void added(Database<D> database)
043        {
044                int weight = database.getWeight();
045                
046                for (int i = 0; i < weight; ++i)
047                {
048                        this.databaseQueue.add(database);
049                }
050        }
051
052        /**
053         * @see net.sf.hajdbc.balancer.AbstractBalancer#removed(net.sf.hajdbc.Database)
054         */
055        @Override
056        protected void removed(Database<D> database)
057        {
058                int weight = database.getWeight();
059                
060                for (int i = 0; i < weight; ++i)
061                {
062                        this.databaseQueue.remove(database);
063                }
064        }
065        
066        /**
067         * @see net.sf.hajdbc.Balancer#next()
068         */
069        @Override
070        public Database<D> next()
071        {
072                this.lock.lock();
073                
074                try
075                {
076                        if (this.databaseQueue.isEmpty())
077                        {
078                                return this.databaseSet.first();
079                        }
080                        
081                        if (this.databaseQueue.size() == 1)
082                        {
083                                return this.databaseQueue.element();
084                        }
085                        
086                        Database<D> database = this.databaseQueue.remove();
087                        
088                        this.databaseQueue.add(database);
089                        
090                        return database;
091                }
092                finally
093                {
094                        this.lock.unlock();
095                }
096        }
097
098        /**
099         * @see net.sf.hajdbc.balancer.AbstractBalancer#cleared()
100         */
101        @Override
102        protected void cleared()
103        {
104                this.databaseQueue.clear();
105        }
106}