Github user aweisberg commented on a diff in the pull request:

    https://github.com/apache/cassandra/pull/283#discussion_r225335637
  
    --- Diff: src/java/org/apache/cassandra/locator/DynamicEndpointSnitchEMA.java ---
    @@ -0,0 +1,181 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you 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 org.apache.cassandra.locator;
    +
    +import java.net.UnknownHostException;
    +import java.util.ArrayList;
    +import java.util.HashMap;
    +import java.util.List;
    +import java.util.Map;
    +import java.util.Optional;
    +import java.util.concurrent.ConcurrentHashMap;
    +
    +import org.apache.cassandra.gms.Gossiper;
    +import org.apache.cassandra.metrics.ExponentialMovingAverage;
    +import org.apache.cassandra.net.LatencyMeasurementType;
    +
    +
    +/**
    + * A dynamic snitching implementation that uses Exponentially Decaying Histograms to prefer or
    + * de-prefer hosts
    + *
    + * This was the default implementation prior to Cassandra 4.0 and is being left as the default
    + * in 4.0
    + */
    +public class DynamicEndpointSnitchEMA extends DynamicEndpointSnitch
    +{
    +    // A ~10 sample EMA
    +    private static final double EMA_ALPHA = 0.10;
    +
    +    private final ConcurrentHashMap<InetAddressAndPort, AnnotatedEMA> samples = new ConcurrentHashMap<>();
    +
    +    /**
    +     * Adds two boolean markers to the ExponentialMovingAverage for telling if the data has been
    +     * updated or requested recently.
    +     *
    +     * recentlyMeasured is updated through {@link AnnotatedEMA#update(long, boolean)}
    +     * recentlyRequested is updated through {@link DynamicEndpointSnitch#markRequested}
    +     *
    +     * Both markers are periodically reset via {@link DynamicEndpointSnitch#latencyProbeNeeded(long)}
    +     */
    +    private static class AnnotatedEMA extends ExponentialMovingAverage
    --- End diff --
    
    You can use a common base with and compose instead of extending so that the base class can contain all the type specific functionality. I think you want to wrap anyways so we can adapt the methods from the underlying reservoir.
    
    Then the map can be in the base class since it's basically common anyways.


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