DeviationShedder.java

/**
 * Copyright 2016 Yahoo Inc.
 *
 * 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.yahoo.pulsar.broker.loadbalance.impl;

import java.util.HashMap;
import java.util.Map;
import java.util.TreeSet;

import org.apache.commons.lang3.tuple.ImmutablePair;
import org.apache.commons.lang3.tuple.Pair;

import com.yahoo.pulsar.broker.BrokerData;
import com.yahoo.pulsar.broker.ServiceConfiguration;
import com.yahoo.pulsar.broker.loadbalance.LoadData;
import com.yahoo.pulsar.broker.loadbalance.LoadSheddingStrategy;

/**
 * An abstract class which makes a LoadSheddingStrategy which makes decisions based on standard deviation easier to
 * implement. Assuming there exists some real number metric which may estimate the load on a server, this load shedding
 * strategy calculates the standard deviation with respect to that metric and sheds load on brokers whose standard
 * deviation is above some threshold.
 */
public abstract class DeviationShedder implements LoadSheddingStrategy {
    // A Set of pairs is used in favor of a Multimap for simplicity.
    protected TreeSet<Pair<Double, String>> metricTreeSetCache;
    protected TreeSet<Pair<Double, String>> bundleTreeSetCache;

    /**
     * Initialize this DeviationShedder.
     */
    public DeviationShedder() {
        bundleTreeSetCache = new TreeSet<>();
        metricTreeSetCache = new TreeSet<>();
    }

    // Measure the load incurred by a bundle.
    protected abstract double bundleValue(String bundle, BrokerData brokerData, ServiceConfiguration conf);

    // Measure the load suffered by a broker.
    protected abstract double brokerValue(BrokerData brokerData, ServiceConfiguration conf);

    // Get the threshold above which the standard deviation of a broker is large
    // enough to warrant unloading bundles.
    protected abstract double getDeviationThreshold(ServiceConfiguration conf);

    /**
     * Recommend that all of the returned bundles be unloaded based on observing excessive standard deviations according
     * to some metric.
     * 
     * @param loadData
     *            The load data to used to make the unloading decision.
     * @param conf
     *            The service configuration.
     * @return A map from all selected bundles to the brokers on which they reside.
     */
    @Override
    public Map<String, String> findBundlesForUnloading(final LoadData loadData, final ServiceConfiguration conf) {
        final Map<String, String> result = new HashMap<>();
        bundleTreeSetCache.clear();
        metricTreeSetCache.clear();
        double sum = 0;
        double squareSum = 0;
        final Map<String, BrokerData> brokerDataMap = loadData.getBrokerData();

        // Treating each broker as a data point, calculate the sum and squared
        // sum of the evaluated broker metrics.
        // These may be used to calculate the standard deviation.
        for (Map.Entry<String, BrokerData> entry : brokerDataMap.entrySet()) {
            final double value = brokerValue(entry.getValue(), conf);
            sum += value;
            squareSum += value * value;
            metricTreeSetCache.add(new ImmutablePair<>(value, entry.getKey()));
        }
        // Mean cannot change by just moving around bundles.
        final double mean = sum / brokerDataMap.size();
        double standardDeviation = Math.sqrt(squareSum / brokerDataMap.size() - mean * mean);
        final double deviationThreshold = getDeviationThreshold(conf);
        String lastMostOverloaded = null;
        // While the most loaded broker is above the standard deviation
        // threshold, continue to move bundles.
        while ((metricTreeSetCache.last().getKey() - mean) / standardDeviation > deviationThreshold) {
            final Pair<Double, String> mostLoadedPair = metricTreeSetCache.last();
            final double highestValue = mostLoadedPair.getKey();
            final String mostLoaded = mostLoadedPair.getValue();

            final Pair<Double, String> leastLoadedPair = metricTreeSetCache.first();
            final double leastValue = leastLoadedPair.getKey();
            final String leastLoaded = metricTreeSetCache.first().getValue();

            if (!mostLoaded.equals(lastMostOverloaded)) {
                // Reset the bundle tree set now that a different broker is
                // being considered.
                bundleTreeSetCache.clear();
                for (String bundle : brokerDataMap.get(mostLoaded).getLocalData().getBundles()) {
                    if (!result.containsKey(bundle)) {
                        // Don't consider bundles that are already going to be
                        // moved.
                        bundleTreeSetCache.add(
                                new ImmutablePair<>(bundleValue(bundle, brokerDataMap.get(mostLoaded), conf), bundle));
                    }
                }
                lastMostOverloaded = mostLoaded;
            }
            boolean selected = false;
            while (!(bundleTreeSetCache.isEmpty() || selected)) {
                Pair<Double, String> mostExpensivePair = bundleTreeSetCache.pollLast();
                double loadIncurred = mostExpensivePair.getKey();
                // When the bundle is moved, we want the now least loaded server
                // to have lower overall load than the
                // most loaded server does not. Thus, we will only consider
                // moving the bundle if this condition
                // holds, and otherwise we will try the next bundle.
                if (loadIncurred + leastValue < highestValue) {
                    // Update the standard deviation and replace the old load
                    // values in the broker tree set with the
                    // load values assuming this move took place.
                    final String bundleToMove = mostExpensivePair.getValue();
                    result.put(bundleToMove, mostLoaded);
                    metricTreeSetCache.remove(mostLoadedPair);
                    metricTreeSetCache.remove(leastLoadedPair);
                    final double newHighLoad = highestValue - loadIncurred;
                    final double newLowLoad = leastValue - loadIncurred;
                    squareSum -= highestValue * highestValue + leastValue * leastValue;
                    squareSum += newHighLoad * newHighLoad + newLowLoad * newLowLoad;
                    standardDeviation = Math.sqrt(squareSum / brokerDataMap.size() - mean * mean);
                    metricTreeSetCache.add(new ImmutablePair<>(newLowLoad, leastLoaded));
                    metricTreeSetCache.add(new ImmutablePair<>(newHighLoad, mostLoaded));
                    selected = true;
                }
            }
            if (!selected) {
                // Move on to the next broker if no bundle could be moved.
                metricTreeSetCache.pollLast();
            }
        }
        return result;
    }
}