SequentialBalanceStrategy.java

/*
 Copyright (c) 2007, 2012, Oracle and/or its affiliates. All rights reserved.
 

  The MySQL Connector/J is licensed under the terms of the GPLv2
  <http://www.gnu.org/licenses/old-licenses/gpl-2.0.html>, like most MySQL Connectors.
  There are special exceptions to the terms and conditions of the GPLv2 as it is applied to
  this software, see the FLOSS License Exception
  <http://www.mysql.com/about/legal/licensing/foss-exception.html>.

  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; version 2
  of the License.

  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, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth
  Floor, Boston, MA 02110-1301  USA
 
 */
package com.mysql.jdbc;

import java.sql.SQLException;
import java.util.List;
import java.util.Map;
import java.util.Properties;

/**
 * A balancing strategy that starts at a random point, and then advances
 * in the list (wrapping around) for each new pickConnection() call.
 * 
 * The initial point selection, and subsequent point selections are
 * blacklist-aware.
 *
 */
public class SequentialBalanceStrategy implements BalanceStrategy {
	private int currentHostIndex = -1;
	
	public SequentialBalanceStrategy() {
	}

	public void destroy() {
		// we don't have anything to clean up
	}

	public void init(Connection conn, Properties props) throws SQLException {
		// we don't have anything to initialize
	}

	public ConnectionImpl pickConnection(LoadBalancingConnectionProxy proxy,
			List<String> configuredHosts, Map<String, ConnectionImpl> liveConnections, long[] responseTimes,
			int numRetries) throws SQLException {
		int numHosts = configuredHosts.size();

		SQLException ex = null;

		Map<String, Long> blackList = proxy.getGlobalBlacklist();

		for (int attempts = 0; attempts < numRetries;) {
			if (numHosts == 1) {
				currentHostIndex = 0; // pathological case
			} else  if (currentHostIndex == -1) {
				int random = (int) Math.floor((Math.random() * numHosts));
				
				for (int i = random; i < numHosts; i++) {
					if (!blackList.containsKey(configuredHosts.get(i))) {
						currentHostIndex = i; 
						break;
					}
				}
				
				if (currentHostIndex == -1) {
					for (int i = 0; i < random; i++) {
						if (!blackList.containsKey(configuredHosts.get(i))) {
							currentHostIndex = i; 
							break;
						}
					}
				}
				
				if (currentHostIndex == -1) {
					blackList = proxy.getGlobalBlacklist(); // it may have changed
								// and the proxy returns a copy
					
					try {
						Thread.sleep(250);
					} catch (InterruptedException e) {
					}

					continue; // retry
				}
			} else {

				
				int i = currentHostIndex + 1;
				boolean foundGoodHost = false;
				
				for (; i < numHosts; i++) {
					if (!blackList.containsKey(configuredHosts.get(i))) {
						currentHostIndex = i;
						foundGoodHost = true;
						break;
					}
				}

				if (!foundGoodHost) {
					for (i = 0; i < currentHostIndex; i++) {
						if (!blackList.containsKey(configuredHosts.get(i))) {
							currentHostIndex = i;
							foundGoodHost = true;
							break;
						}
					}
				}
			
				if (!foundGoodHost) {
					blackList = proxy.getGlobalBlacklist(); // it may have changed
					// and the proxy returns a copy
		
					try {
						Thread.sleep(250);
					} catch (InterruptedException e) {
					}
			
					continue; // retry
				}
			}

			String hostPortSpec = configuredHosts.get(currentHostIndex);

			ConnectionImpl conn = liveConnections.get(hostPortSpec);

			if (conn == null) {
				try {
					conn = proxy.createConnectionForHost(hostPortSpec);
				} catch (SQLException sqlEx) {
					ex = sqlEx;

					if (sqlEx instanceof CommunicationsException
							|| "08S01".equals(sqlEx.getSQLState())) {

						proxy.addToGlobalBlacklist( hostPortSpec );

						try {
							Thread.sleep(250);
						} catch (InterruptedException e) {
						}
						
						continue;
					}
					throw sqlEx;
				}
			}
			
			return conn;
		}

		if (ex != null) {
			throw ex;
		}

		return null; // we won't get here, compiler can't tell
	}

}