Copyright 2018 Netflix, 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.
/**
 * Copyright 2018 Netflix, 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.netflix.concurrency.limits.limit;

import com.netflix.concurrency.limits.MetricIds;
import com.netflix.concurrency.limits.MetricRegistry;
import com.netflix.concurrency.limits.MetricRegistry.SampleListener;
import com.netflix.concurrency.limits.internal.EmptyMetricRegistry;
import com.netflix.concurrency.limits.internal.Preconditions;
import com.netflix.concurrency.limits.limit.functions.Log10RootFunction;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.function.Function;

Limiter based on TCP Vegas where the limit increases by alpha if the queue_use is small (< alpha) and decreases by alpha if the queue_use is large (> beta). Queue size is calculated using the formula, queue_use = limit − BWE×RTTnoLoad = limit × (1 − RTTnoLoad/RTTactual) For traditional TCP Vegas alpha is typically 2-3 and beta is typically 4-6. To allow for better growth and stability at higher limits we set alpha=Max(3, 10% of the current limit) and beta=Max(6, 20% of the current limit) /**
 * Limiter based on TCP Vegas where the limit increases by alpha if the queue_use is small ({@literal <} alpha)
 * and decreases by alpha if the queue_use is large ({@literal >} beta).
 * 
 * Queue size is calculated using the formula, 
 *  queue_use = limit − BWE×RTTnoLoad = limit × (1 − RTTnoLoad/RTTactual)
 *
 * For traditional TCP Vegas alpha is typically 2-3 and beta is typically 4-6.  To allow for better growth and 
 * stability at higher limits we set alpha=Max(3, 10% of the current limit) and beta=Max(6, 20% of the current limit)
 */
public class VegasLimit extends AbstractLimit {
    private static final Logger LOG = LoggerFactory.getLogger(VegasLimit.class);
    
    private static final Function<Integer, Integer> LOG10 = Log10RootFunction.create(0);

    public static class Builder {
        private int initialLimit = 20;
        private int maxConcurrency = 1000;
        private MetricRegistry registry = EmptyMetricRegistry.INSTANCE;
        private double smoothing = 1.0;
        
        private Function<Integer, Integer> alphaFunc = (limit) -> 3 * LOG10.apply(limit.intValue());
        private Function<Integer, Integer> betaFunc = (limit) -> 6 * LOG10.apply(limit.intValue());
        private Function<Integer, Integer> thresholdFunc = (limit) -> LOG10.apply(limit.intValue());
        private Function<Double, Double> increaseFunc = (limit) -> limit + LOG10.apply(limit.intValue());
        private Function<Double, Double> decreaseFunc = (limit) -> limit - LOG10.apply(limit.intValue());
        private int probeMultiplier = 30;
        
        private Builder() {
        }
        
        
The limiter will probe for a new noload RTT every probeMultiplier * current limit
iterations.  Default value is 30.  
Params:
probeMultiplier – 
Returns:
Chinable builder/**
         * The limiter will probe for a new noload RTT every probeMultiplier * current limit
         * iterations.  Default value is 30.  
         * @param probeMultiplier 
         * @return Chinable builder
         */
        public Builder probeMultiplier(int probeMultiplier) {
            this.probeMultiplier = probeMultiplier;
            return this;
        }
        
        public Builder alpha(int alpha) {
            this.alphaFunc = (ignore) -> alpha;
            return this;
        }
        
        public Builder threshold(Function<Integer, Integer> threshold) {
            this.thresholdFunc = threshold;
            return this;
        }
        
        public Builder alpha(Function<Integer, Integer> alpha) {
            this.alphaFunc = alpha;
            return this;
        }
        
        public Builder beta(int beta) {
            this.betaFunc = (ignore) -> beta;
            return this;
        }
        
        public Builder beta(Function<Integer, Integer> beta) {
            this.betaFunc = beta;
            return this;
        }
        
        public Builder increase(Function<Double, Double> increase) {
            this.increaseFunc = increase;
            return this;
        }
        
        public Builder decrease(Function<Double, Double> decrease) {
            this.decreaseFunc = decrease;
            return this;
        }
        
        public Builder smoothing(double smoothing) {
            this.smoothing = smoothing;
            return this;
        }
        
        public Builder initialLimit(int initialLimit) {
            this.initialLimit = initialLimit;
            return this;
        }
        
        @Deprecated
        public Builder tolerance(double tolerance) {
            return this;
        }
        
        public Builder maxConcurrency(int maxConcurrency) {
            this.maxConcurrency = maxConcurrency;
            return this;
        }
        
        @Deprecated
        public Builder backoffRatio(double ratio) {
            return this;
        }
        
        public Builder metricRegistry(MetricRegistry registry) {
            this.registry = registry;
            return this;
        }
        
        public VegasLimit build() {
            return new VegasLimit(this);
        }
    }
    
    public static Builder newBuilder() {
        return new Builder();
    }
    
    public static VegasLimit newDefault() {
        return newBuilder().build();
    }
    
    
Estimated concurrency limit based on our algorithm
/**
     * Estimated concurrency limit based on our algorithm
     */
    private volatile double estimatedLimit;
    
    private volatile long rtt_noload = 0;
    
    
Maximum allowed limit providing an upper bound failsafe
/**
     * Maximum allowed limit providing an upper bound failsafe
     */
    private final int maxLimit; 
    
    private final double smoothing;
    private final Function<Integer, Integer> alphaFunc;
    private final Function<Integer, Integer> betaFunc;
    private final Function<Integer, Integer> thresholdFunc;
    private final Function<Double, Double> increaseFunc;
    private final Function<Double, Double> decreaseFunc;
    private final SampleListener rttSampleListener;
    private final int probeMultiplier;
    private int probeCount = 0;
    private double probeJitter;

    private VegasLimit(Builder builder) {
        super(builder.initialLimit);
        this.estimatedLimit = builder.initialLimit;
        this.maxLimit = builder.maxConcurrency;
        this.alphaFunc = builder.alphaFunc;
        this.betaFunc = builder.betaFunc;
        this.increaseFunc = builder.increaseFunc;
        this.decreaseFunc = builder.decreaseFunc;
        this.thresholdFunc = builder.thresholdFunc;
        this.smoothing = builder.smoothing;
        this.probeMultiplier = builder.probeMultiplier;

        resetProbeJitter();

        this.rttSampleListener = builder.registry.distribution(MetricIds.MIN_RTT_NAME);
    }

    private void resetProbeJitter() {
        probeJitter = ThreadLocalRandom.current().nextDouble(0.5, 1);
    }

    private boolean shouldProbe() {
        return probeJitter * probeMultiplier * estimatedLimit <= probeCount;
    }

    @Override
    protected int _update(long startTime, long rtt, int inflight, boolean didDrop) {
        Preconditions.checkArgument(rtt > 0, "rtt must be >0 but got " + rtt);

        probeCount++;
        if (shouldProbe()) {
            LOG.debug("Probe MinRTT {}", TimeUnit.NANOSECONDS.toMicros(rtt) / 1000.0);
            resetProbeJitter();
            probeCount = 0;
            rtt_noload = rtt;
            return (int)estimatedLimit;
        }
        
        if (rtt_noload == 0 || rtt < rtt_noload) {
            LOG.debug("New MinRTT {}", TimeUnit.NANOSECONDS.toMicros(rtt) / 1000.0);
            rtt_noload = rtt;
            return (int)estimatedLimit;
        }
        
        rttSampleListener.addSample(rtt_noload);

        return updateEstimatedLimit(rtt, inflight, didDrop);
    }

    private int updateEstimatedLimit(long rtt, int inflight, boolean didDrop) {
        final int queueSize = (int) Math.ceil(estimatedLimit * (1 - (double)rtt_noload / rtt));

        double newLimit;
        // Treat any drop (i.e timeout) as needing to reduce the limit
        if (didDrop) {
            newLimit = decreaseFunc.apply(estimatedLimit);
        // Prevent upward drift if not close to the limit
        } else if (inflight * 2 < estimatedLimit) {
            return (int)estimatedLimit;
        } else {
            int alpha = alphaFunc.apply((int)estimatedLimit);
            int beta = betaFunc.apply((int)estimatedLimit);
            int threshold = this.thresholdFunc.apply((int)estimatedLimit);

            // Aggressive increase when no queuing
            if (queueSize <= threshold) {
                newLimit = estimatedLimit + beta;
            // Increase the limit if queue is still manageable
            } else if (queueSize < alpha) {
                newLimit = increaseFunc.apply(estimatedLimit);
            // Detecting latency so decrease
            } else if (queueSize > beta) {
                newLimit = decreaseFunc.apply(estimatedLimit);
            // We're within he sweet spot so nothing to do
            } else {
                return (int)estimatedLimit;
            }
        }

        newLimit = Math.max(1, Math.min(maxLimit, newLimit));
        newLimit = (1 - smoothing) * estimatedLimit + smoothing * newLimit;
        if ((int)newLimit != (int)estimatedLimit && LOG.isDebugEnabled()) {
            LOG.debug("New limit={} minRtt={} ms winRtt={} ms queueSize={}",
                    (int)newLimit,
                    TimeUnit.NANOSECONDS.toMicros(rtt_noload) / 1000.0,
                    TimeUnit.NANOSECONDS.toMicros(rtt) / 1000.0,
                    queueSize);
        }
        estimatedLimit = newLimit;
        return (int)estimatedLimit;
    }

    @Override
    public String toString() {
        return "VegasLimit [limit=" + getLimit() + 
                ", rtt_noload=" + TimeUnit.NANOSECONDS.toMicros(rtt_noload) / 1000.0 +
                " ms]";
    }
}