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JDBC Type 4 driver for MySQL.
There is a newer version: 9.1.0
/*
 * Copyright (c) 2002, 2024, Oracle and/or its affiliates.
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License, version 2.0, as published by
 * the Free Software Foundation.
 *
 * This program is designed to work with certain software that is licensed under separate terms, as designated in a particular file or component or in
 * included license documentation. The authors of MySQL hereby grant you an additional permission to link the program and your derivative works with the
 * separately licensed software that they have either included with the program or referenced in the documentation.
 *
 * Without limiting anything contained in the foregoing, this file, which is part of MySQL Connector/J, is also subject to the Universal FOSS Exception,
 * version 1.0, a copy of which can be found at http://oss.oracle.com/licenses/universal-foss-exception.
 *
 * 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, version 2.0, 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.cj.jdbc.ha;

import java.lang.reflect.InvocationHandler;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import com.mysql.cj.jdbc.ConnectionImpl;
import com.mysql.cj.jdbc.JdbcConnection;

public class BestResponseTimeBalanceStrategy implements BalanceStrategy {

    public BestResponseTimeBalanceStrategy() {
    }

    @Override
    public ConnectionImpl pickConnection(InvocationHandler proxy, List configuredHosts, Map liveConnections,
            long[] responseTimes, int numRetries) throws SQLException {
        List allowList = new ArrayList<>(configuredHosts.size());
        allowList.addAll(configuredHosts);
        Map blockList = ((LoadBalancedConnectionProxy) proxy).getGlobalBlocklist();
        allowList.removeAll(blockList.keySet());

        SQLException ex = null;
        for (int attempts = 0; attempts < numRetries;) {
            long minResponseTime = Long.MAX_VALUE;
            int bestHostIndex = 0;

            // safety
            if (blockList.size() == configuredHosts.size()) {
                blockList = ((LoadBalancedConnectionProxy) proxy).getGlobalBlocklist();
            }

            for (int i = 0; i < responseTimes.length; i++) {
                long candidateResponseTime = responseTimes[i];

                if (candidateResponseTime < minResponseTime && !blockList.containsKey(allowList.get(i))) {
                    if (candidateResponseTime == 0) {
                        bestHostIndex = i;
                        break;
                    }

                    bestHostIndex = i;
                    minResponseTime = candidateResponseTime;
                }
            }

            String bestHost = allowList.get(bestHostIndex);
            ConnectionImpl conn = (ConnectionImpl) liveConnections.get(bestHost);
            if (conn == null) {
                try {
                    conn = ((LoadBalancedConnectionProxy) proxy).createConnectionForHost(bestHost);
                } catch (SQLException sqlEx) {
                    ex = sqlEx;

                    if (((LoadBalancedConnectionProxy) proxy).shouldExceptionTriggerConnectionSwitch(sqlEx)) {
                        ((LoadBalancedConnectionProxy) proxy).addToGlobalBlocklist(bestHost);
                        blockList.put(bestHost, null);

                        if (blockList.size() == configuredHosts.size()) {
                            attempts++;
                            try {
                                Thread.sleep(250);
                            } catch (InterruptedException e) {
                            }
                            blockList = ((LoadBalancedConnectionProxy) proxy).getGlobalBlocklist(); // try again after a little bit
                        }

                        continue;
                    }

                    throw sqlEx;
                }
            }

            return conn;
        }

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

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

}