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From michaelandrepearce <...@git.apache.org>
Subject [GitHub] activemq-artemis pull request #2488: ARTEMIS-196 Implement Consumer Priority
Date Thu, 03 Jan 2019 13:59:18 GMT
Github user michaelandrepearce commented on a diff in the pull request:

    https://github.com/apache/activemq-artemis/pull/2488#discussion_r245007052
  
    --- Diff: artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/QueueConsumersImpl.java
---
    @@ -0,0 +1,648 @@
    +/*
    + * 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.activemq.artemis.core.server.impl;
    +
    +import org.apache.activemq.artemis.core.server.PriorityAware;
    +
    +import java.lang.reflect.Array;
    +import java.util.AbstractCollection;
    +import java.util.Arrays;
    +import java.util.Collection;
    +import java.util.Iterator;
    +import java.util.Objects;
    +import java.util.Set;
    +import java.util.concurrent.locks.ReentrantLock;
    +import java.util.function.Consumer;
    +import java.util.stream.Collectors;
    +
    +/**
    + * This class's purpose is to hold the consumers, it models around multi getPriority
(getPriority) varient of
    + * java.util.concurrent.CopyOnWriteArrayList, so that reads are concurrent safe and non
blocking.
    + *
    + * N.b. we could have made Level extend CopyOnWriteArrayList but due to the need to access
the internal Array structure,
    + * which is privileged to package java.util.concurrent. As such much of Level is is taken
from here.
    + *
    + * Modifications like in CopyOnWriteArrayList are single threaded via a single re-entrant
lock.
    + *
    + * Iterators iterate over a snapshot of the internal array structure, so will not see
mutations.
    + *
    + * There can only be one resettable iterable view, this is exposed at the top getPriority,
    + *     and is intended for use in QueueImpl only.
    + * All other iterators are not reset-able and are created on calling iterator().
    + *
    + * Methods getArray, setArray MUST never be exposed, and all array modifications must
go through these.
    + *
    + * @param <T> The type this class may hold, this is generic as can be anything
that extends PriorityAware,
    + *         but intent is this is the QueueImpl:ConsumerHolder.
    + */
    +public class QueueConsumersImpl<T extends PriorityAware> extends AbstractCollection<T>
implements QueueConsumers<T> {
    +
    +   private final QueueConsumersIterator<T> iterator = new QueueConsumersIterator<>(this,
true);
    +
    +   private volatile Level<T>[] levels;
    +   private volatile int size;
    +   private volatile T first;
    +
    +   private void setArray(Level<T>[] array) {
    +      this.levels = array;
    +   }
    +
    +   private Level<T>[] getArray() {
    +      return levels;
    +   }
    +
    +
    +   public QueueConsumersImpl() {
    +      levels = newLevelArrayInstance(0);
    +   }
    +
    +   @SuppressWarnings("unchecked")
    +   private static <T> Level<T>[] newLevelArrayInstance(int length) {
    +      return (Level<T>[]) Array.newInstance(Level.class, length);
    +   }
    +
    +   @Override
    +   public int size() {
    +      return size;
    +   }
    +
    +   @Override
    +   public boolean isEmpty() {
    +      return size() == 0;
    +   }
    +
    +   @Override
    +   public Set<Integer> getPriorites() {
    +      Level<T>[] levels = getArray();
    +      return Arrays.stream(levels).map(Level::level).collect(Collectors.toSet());
    +   }
    +
    +   @Override
    +   public Iterator<T> iterator() {
    +      return new QueueConsumersIterator<>(this, false);
    +   }
    +
    +   @Override
    +   public boolean hasNext() {
    +      return iterator.hasNext();
    +   }
    +
    +   @Override
    +   public T next() {
    +      return iterator.next();
    +   }
    +
    +   @Override
    +   public QueueConsumers<T> reset() {
    +      iterator.reset();
    +      return this;
    +   }
    +
    +   @Override
    +   public void forEach(Consumer<? super T> action) {
    +      Objects.requireNonNull(action);
    +      Level<T>[] current = getArray();
    +      int len = current.length;
    +      for (int i = 0; i < len; ++i) {
    +         current[i].forEach(action);
    +      }
    +   }
    +
    +   private Level<T> getLevel(int level, boolean createIfMissing) {
    +      Level<T>[] current = getArray();
    +      int low = 0;
    +      int high = current.length - 1;
    +
    +      while (low <= high) {
    +         int mid = (low + high) >>> 1;
    +         Level<T> midVal = current[mid];
    +
    +         if (midVal.level() > level)
    +            low = mid + 1;
    +         else if (midVal.level() < level)
    +            high = mid - 1;
    +         else
    +            return midVal; //key found
    +      }
    +
    +      if (createIfMissing) {
    +         Level<T>[] newLevels = newLevelArrayInstance(current.length + 1);
    +         if (low > 0) {
    +            System.arraycopy(current, 0, newLevels, 0, low);
    +         }
    +         if (current.length - low > 0) {
    +            System.arraycopy(current, low, newLevels, low + 1, current.length - low);
    +         }
    +         newLevels[low] = new Level<T>(level);
    +         setArray(newLevels);
    +         return newLevels[low];
    +      }
    +      return null;
    +   }
    +
    +   @Override
    +   public synchronized boolean add(T t) {
    --- End diff --
    
    Yes, its following the lines on design of CopyOnWriteArrayList, but using syncronized
methods over using Reentrant lock as concurrency expected is low.


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