RoundRobinScheduler.java

/*
 * Licensed 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.facebook.presto.router.scheduler;

import com.facebook.airlift.log.Logger;
import com.facebook.presto.spi.router.RouterRequestInfo;
import com.facebook.presto.spi.router.Scheduler;

import javax.annotation.concurrent.GuardedBy;

import java.net.URI;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;

/**
* As the round-robin scheduler keeps the selected index as a state for scheduling,
* the candidates shall not be modified after they are assigned.
* This design indicates that the round-robin scheduler can only be used when
* the candidates are always consistent.
 */
public class RoundRobinScheduler
        implements Scheduler
{
    private List<URI> candidates;
    private static final Logger log = Logger.get(RoundRobinScheduler.class);

    @GuardedBy("this")
    private final Map<String, Integer> candidateIndexByGroup = new HashMap<>();

    private String candidateGroupName;

    @Override
    public Optional<URI> getDestination(RouterRequestInfo routerRequestInfo)
    {
        try {
            return Optional.of(candidates.get(candidateIndexByGroup.compute(candidateGroupName, (key, oldValue) -> {
                if (oldValue == null || oldValue + 1 >= candidates.size()) {
                    return 0;
                }
                return oldValue + 1;
            })));
        }
        catch (IllegalArgumentException e) {
            log.warn(e, "Error getting destination for user " + routerRequestInfo.getUser());
            return Optional.empty();
        }
    }

    @Override
    public void setCandidates(List<URI> candidates)
    {
        this.candidates = candidates;
    }

    @Override
    public void setCandidateGroupName(String candidateGroupName)
    {
        this.candidateGroupName = candidateGroupName;
    }
}