com.datastax.drivers.jdbc.pool.cassandra.connection.RoundRobinBalancingPolicy.java Source code

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Here is the source code for com.datastax.drivers.jdbc.pool.cassandra.connection.RoundRobinBalancingPolicy.java

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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with 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.
 */
package com.datastax.drivers.jdbc.pool.cassandra.connection;

import java.sql.SQLException;
import java.util.Collection;
import java.util.Set;

import com.google.common.collect.Iterables;

/**
 * Implements a RoundRobin balancing policy based off the contents
 * of the active {@link HClientPool}. If a pool is shutdown by another 
 * thread in the midst of the selection process, we return the pool
 * at position 0
 *
 * @author zznate
 */
public class RoundRobinBalancingPolicy implements LoadBalancingPolicy {

    private static final long serialVersionUID = 1107204068032227079L;
    private int counter;

    public RoundRobinBalancingPolicy() {
        counter = 0;
    }

    @Override
    public HClientPool getPool(Collection<HClientPool> pools, Set<CassandraHost> excludeHosts) {
        HClientPool pool = getPoolSafely(pools);
        if (excludeHosts != null && excludeHosts.size() > 0) {
            while (excludeHosts.contains(pool.getCassandraHost())) {
                pool = getPoolSafely(pools);
                if (excludeHosts.size() >= pools.size())
                    break;
            }
        }
        return pool;
    }

    private HClientPool getPoolSafely(Collection<HClientPool> pools) {
        try {
            return Iterables.get(pools, getAndIncrement(pools.size()));
        } catch (IndexOutOfBoundsException e) {
            return pools.iterator().next();
        }
    }

    private int getAndIncrement(int size) {
        int counterToReturn;

        // There should not be that much of contention here as
        // the "if" statement plus the increment is executed real fast.
        synchronized (this) {
            if (counter >= 16384) {
                counter = 0;
            }
            counterToReturn = counter++;
        }

        return counterToReturn % size;
    }

    @Override
    public HClientPool createConnection(CassandraHost host) throws SQLException {
        return new ConcurrentHClientPool(host);
    }
}