/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License, Version 1.0 only
 * (the "License").  You may not use this file except in compliance
 * with the License.
 *
 * You can obtain a copy of the license at legal-notices/CDDLv1_0.txt
 * or http://forgerock.org/license/CDDLv1.0.html.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at legal-notices/CDDLv1_0.txt.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
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 *      Portions Copyright [yyyy] [name of copyright owner]
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 * CDDL HEADER END
 *
 *
 *      Copyright 2010 Sun Microsystems, Inc.
 */

package org.forgerock.opendj.ldap;

import java.util.Collection;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A round robin load balancing algorithm distributes connection requests across
 * a list of connection factories one at a time. When the end of the list is
 * reached, the algorithm starts again from the beginning.
 * <p>
 * This algorithm is typically used for load-balancing <i>within</i> data
 * centers, where load must be distributed equally across multiple data sources.
 * This algorithm contrasts with the {@link FailoverLoadBalancingAlgorithm}
 * which is used for load-balancing <i>between</i> data centers.
 * <p>
 * If a problem occurs that temporarily prevents connections from being obtained
 * for one of the connection factories, then this algorithm automatically
 * "fails over" to the next operational connection factory in the list. If none
 * of the connection factories are operational then a
 * {@code ConnectionException} is returned to the client.
 * <p>
 * The implementation periodically attempts to connect to failed connection
 * factories in order to determine if they have become available again.
 *
 * @see FailoverLoadBalancingAlgorithm
 * @see Connections#newLoadBalancer(LoadBalancingAlgorithm)
 */
public final class RoundRobinLoadBalancingAlgorithm extends AbstractLoadBalancingAlgorithm {
    private final int maxIndex;

    private final AtomicInteger nextIndex = new AtomicInteger(-1);

    /**
     * Creates a new round robin load balancing algorithm which will monitor
     * offline connection factories every 1 second using the default scheduler.
     *
     * @param factories
     *            The ordered collection of connection factories.
     */
    public RoundRobinLoadBalancingAlgorithm(final Collection<ConnectionFactory> factories) {
        super(factories);
        this.maxIndex = factories.size();
    }

    /**
     * Creates a new round robin load balancing algorithm which will monitor
     * offline connection factories using the specified frequency using the
     * default scheduler.
     *
     * @param factories
     *            The connection factories.
     * @param interval
     *            The interval between attempts to poll offline factories.
     * @param unit
     *            The time unit for the interval between attempts to poll
     *            offline factories.
     */
    public RoundRobinLoadBalancingAlgorithm(final Collection<ConnectionFactory> factories,
            final long interval, final TimeUnit unit) {
        super(factories, interval, unit);
        this.maxIndex = factories.size();
    }

    /**
     * Creates a new round robin load balancing algorithm which will monitor
     * offline connection factories using the specified frequency and scheduler.
     *
     * @param factories
     *            The connection factories.
     * @param interval
     *            The interval between attempts to poll offline factories.
     * @param unit
     *            The time unit for the interval between attempts to poll
     *            offline factories.
     * @param scheduler
     *            The scheduler which should for periodically monitoring dead
     *            connection factories to see if they are usable again.
     */
    public RoundRobinLoadBalancingAlgorithm(final Collection<ConnectionFactory> factories,
            final long interval, final TimeUnit unit, final ScheduledExecutorService scheduler) {
        super(factories, interval, unit, scheduler);
        this.maxIndex = factories.size();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    String getAlgorithmName() {
        return "RoundRobin";
    }

    /**
     * {@inheritDoc}
     */
    @Override
    int getInitialConnectionFactoryIndex() {
        // A round robin pool of one connection factories is unlikely in
        // practice
        // and requires special treatment.
        if (maxIndex == 1) {
            return 0;
        }

        // Determine the next factory to use: avoid blocking algorithm.
        int oldNextIndex;
        int newNextIndex;
        do {
            oldNextIndex = nextIndex.get();
            newNextIndex = oldNextIndex + 1;
            if (newNextIndex == maxIndex) {
                newNextIndex = 0;
            }
        } while (!nextIndex.compareAndSet(oldNextIndex, newNextIndex));

        // There's a potential, but benign, race condition here: other threads
        // could
        // jump in and rotate through the list before we return the connection
        // factory.
        return newNextIndex;
    }

}