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From GitBox <...@apache.org>
Subject [GitHub] michaelandrepearce commented on a change in pull request #2490: V2 196
Date Thu, 10 Jan 2019 08:34:45 GMT
michaelandrepearce commented on a change in pull request #2490: V2 196
URL: https://github.com/apache/activemq-artemis/pull/2490#discussion_r246670739
 
 

 ##########
 File path: artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/QueueConsumersImpl.java
 ##########
 @@ -0,0 +1,124 @@
+/*
+ * 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.PriorityAware;
+import org.apache.activemq.artemis.utils.collections.PriorityCollection;
+import org.apache.activemq.artemis.utils.collections.ResettableIterator;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Set;
+import java.util.Spliterator;
+import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
+import java.util.function.Consumer;
+
+/**
+ * This class's purpose is to hold the consumers.
+ *
+ * CopyOnWriteArraySet is used as the underlying collection to the PriorityCollection, as
it is concurrent safe,
+ * but also lock less for a read path, which is our HOT path.
+ * Also it was the underlying collection previously used in QueueImpl, before we abstracted
it out to support priority consumers.
+ *
+ * There can only be one resettable iterable view,
+ * A new iterable view is created on modification, this is to keep the read HOT path performent,
BUT
+ * the iterable view changes only after reset so changes in the underlying collection are
only seen after a reset,
+ *
+ * All other iterators created by iterators() method are not reset-able and are created on
delegating iterator().
+ *
+ * @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> implements QueueConsumers<T>
{
+
+   private final PriorityCollection<T> consumers = new PriorityCollection<>(CopyOnWriteArraySet::new);
+   private final Collection<T> unmodifiableConsumers = Collections.unmodifiableCollection(consumers);
+   private final AtomicReferenceFieldUpdater<QueueConsumersImpl, ResettableIterator>
changedIteratorFieldUpdater =  AtomicReferenceFieldUpdater.newUpdater(QueueConsumersImpl.class,
ResettableIterator.class, "changedIterator");
+   private volatile ResettableIterator<T> changedIterator;
+   private ResettableIterator<T> currentIterator = consumers.resettableIterator();
+
+   @Override
+   public Set<Integer> getPriorites() {
+      return consumers.getPriorites();
+   }
+
+   @Override
+   public boolean hasNext() {
+      return currentIterator.hasNext();
+   }
+
+   @Override
+   public T next() {
+      return currentIterator.next();
+   }
+
+   @Override
+   public QueueConsumers<T> reset() {
+      if (changedIterator != null) {
+         currentIterator = changedIterator;
+         changedIteratorFieldUpdater.compareAndSet(this, changedIterator, null);
 
 Review comment:
   ill mark comment resolved, if you think it needs further docs or could be further abstracted
to be more clearer let me know.

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