com.facebook.presto.router.scheduler.RoundRobinScheduler Maven / Gradle / Ivy
/*
* 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 java.net.URI;
import java.util.List;
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 candidates;
private static Integer candidateIndex = 0;
private static final Logger log = Logger.get(RoundRobinScheduler.class);
@Override
public Optional getDestination(String user)
{
try {
synchronized (candidateIndex) {
if (candidateIndex >= candidates.size()) {
candidateIndex = 0;
}
return Optional.of(candidates.get(candidateIndex++));
}
}
catch (IllegalArgumentException e) {
log.warn(e, "Error getting destination for user " + user);
return Optional.empty();
}
}
public void setCandidates(List candidates)
{
// Only keeps the first given `candidates` due to maintaining the
// selected index for round-robin.
if (this.candidates == null) {
this.candidates = candidates;
}
}
public List getCandidates()
{
return candidates;
}
}