curator-master/curator-x-discovery/src/main/java/org/apache/curator/x/discovery/strategies/RoundRobinStrategy.java
/*
 * 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 org.apache.curator.x.discovery.strategies;

import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import org.apache.curator.x.discovery.ProviderStrategy;
import org.apache.curator.x.discovery.ServiceInstance;
import org.apache.curator.x.discovery.details.InstanceProvider;

/**
 * This strategy rotates sequentially through the list of instances
 */
public class RoundRobinStrategy<T> implements ProviderStrategy<T> {
    private final AtomicInteger index = new AtomicInteger(0);

    @Override
    public ServiceInstance<T> getInstance(InstanceProvider<T> instanceProvider) throws Exception {
        List<ServiceInstance<T>> instances = instanceProvider.getInstances();
        if (instances.size() == 0) {
            return null;
        }
        int thisIndex = Math.abs(index.getAndIncrement());
        return instances.get(thisIndex % instances.size());
    }
}
