netflix.ocelli.loadbalancer.ChoiceOfTwoLoadBalancer Maven / Gradle / Ivy
package netflix.ocelli.loadbalancer;
import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Random;
import java.util.concurrent.atomic.AtomicReference;
import netflix.ocelli.LoadBalancer;
import rx.Subscriber;
import rx.functions.Func2;
/**
* This selector chooses 2 random hosts and picks the host with the 'best'
* performance where that determination is deferred to a customizable function.
*
* This implementation is based on the paper 'The Power of Two Choices in
* Randomized Load Balancing' http://www.eecs.harvard.edu/~michaelm/postscripts/tpds2001.pdf
* This paper states that selecting the best of 2 random servers results in an
* exponential improvement over selecting a single random node (also includes
* round robin) but that adding a third (or more) servers does not yield a significant
* performance improvement.
*
* @author elandau
*
* @param
*/
public class ChoiceOfTwoLoadBalancer extends LoadBalancer {
public static ChoiceOfTwoLoadBalancer create(final Func2 func) {
return new ChoiceOfTwoLoadBalancer(func);
}
private final AtomicReference> clients;
ChoiceOfTwoLoadBalancer(final Func2 func) {
this(func, new AtomicReference>(new ArrayList()));
}
private ChoiceOfTwoLoadBalancer(final Func2 func, final AtomicReference> clients) {
super(new OnSubscribe() {
private final Random rand = new Random();
@Override
public void call(final Subscriber s) {
List local = clients.get();
if (local.isEmpty()) {
s.onError(new NoSuchElementException("No servers available in the load balancer"));
}
else if (local.size() == 1) {
s.onNext(local.get(0));
s.onCompleted();
}
else if (local.size() > 1){
int first = rand.nextInt(local.size());
int second = (rand.nextInt(local.size()-1) + first + 1) % local.size();
s.onNext(func.call(local.get(first), local.get(second)));
s.onCompleted();
}
}
});
this.clients = clients;
}
@Override
public void call(List t) {
this.clients.set(t);
}
}